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|10.0 RESEARCH & DEVELOPMENT
The DOE complex engages
in a variety of research & development (R&D) activities that often incorporate the design and use of special or unusual apparatus and
equipment in its facilities.
Requirements of existing electrical codes, recognized industry standards, and DOE Orders
not specifically address these types of apparatus. Even with these specialized R&D needs, the
workplace must be maintained free
of known hazards that cause, or are likely to cause, death or
serious injury. Special efforts must therefore be made to ensure adequate electrical
beginning with design and continuing through development, fabrication and construction,
modification, installation, inspection,
testing, operation, and maintenance of R&D electrical
apparatus and facilities. This section provides guidelines to complement existing
codes and recognized industry standards in conformance with DOE Orders and OSHA
Because of the differences
in R&D program requirements in the DOE complex and the
unpredictability of R&D activities, it is impractical to establish a single
set of electrical safety
requirements to be applied uniformly. General electrical safety guidelines, however, apply
across the DOE complex.
This section contains safety criteria for the DOE complex in the design, development, fabrication
and construction, modification, installation,
inspection, testing, operation, and maintenance of
R&D electrical apparatus and facilities. Personnel safety shall be the primary consideration.
When conflicts between electrical codes, recognized industry standards, DOE Orders, or
regulations arise, the requirement that addresses
the particular hazard and provides the greater
personnel safety protection shall govern.
This section addresses
R&D electrical systems which are not specifically addressed elsewhere
in the Electrical Safety Handbook. The electrical environment of
the DOE complex is extremely
varied, ranging from low-voltage electronic circuits to common office and industrial electrical
large, high-voltage power distribution systems to high-voltage/low-current and low voltage/
high current systems associated with R&D
programs. Electrical systems of all types are
an integral part of R&D operations and associated support work.
It is important to note that special types of work on R&D electrical systems (e.g., electronic
circuits) are considered
electrical work, and therefore the work shall follow electrical safety
Consistent with other sections of this document, electrical systems and equipment and all
design, development, fabrication and
construction, modification, installation, inspection, testing,
operation, and maintenance shall be in accordance with applicable electrical
Specific attention shall be focused on the electrical regulations of OSHA, including:
1. 29 CFR 1910.137
2. 29 CFR
3. 29 CFR 1910.269
4. 29 CFR 1910.301-399
5. 29 CFR 1926.401-449.
10.4 STANDARDIZED SAFETY PRACTICES AND PROCEDURES
Standardized safety practices shall be developed for performing electrical work. These practices
should be consistent with the other
electrical safety-related work practices noted elsewhere in
10.5 EQUIPMENT NOT LISTED BY A NATIONALLY RECOGNIZED TESTING
Electrical equipment is considered to be acceptable either by being listed by an NRTL,
manufactured, and tested according to nationally recognized standards, or approved
by AHJ-determined criteria. Refer to Section 2.5, Approval
of Electrical Equipment.
Procurement and use of equipment not listed by an NRTL should be reviewed by the AHJ. The
extensive testing involved
in the listing process usually cannot be duplicated at the user facility,
and many of the tests are destructive in nature. The AHJ should
develop an examination
acceptance process to ensure appropriate confidence in the safety of the product.
See Section 9.0, Enclosed Electrical/Electronic
Equipment, for additional guidance. Also see UL
508 and applicable ANSI and IEEE documents.
10.5.2 DESIGN AND CONSTRUCTION
Equipment should be constructed such that:
1. There is adequate protection from fire, electric shock, or injury to personnel during normal
use or servicing.
2. Normal use or servicing will not cause the components or materials to exceed electrical,
mechanical, or temperature
3. The components, wiring, and other internal parts are protected from being displaced or
NRTL-listed parts and UL-recognized components should be used wherever possible. An
assembly of recognized components is not equal
to a listed product, but more readily enables
an independent evaluation of the assembly.
All equipment not listed by a NRTL should
be constructed according to applicable standards,
such as UL, ISA, ANSI, and IEEE. Equipment for which specific standards are unavailable
should be constructed according to the principles of established standards, as determined by
Equipment should be examined
for safety as extensively as possible. Areas of consideration
include but are not limited to:
1. Failure modes
2. Heat effects
3. Magnetic effects
4. Grounding and bonding
5. Guarding of live parts
6. Leakage currents
7. Dielectric testing
8. Access to
9. Overcurrent and over-temperature protection
10. Clearances and spacing
12. Design and procedural
13. Signage, labels, and administrative controls
14. Mechanical motion
15. Stored energy
Documentation should be developed to substantiate the acceptance of any equipment. Such
documentation should include but not be limited
1. Tests performed
2. Conditions of acceptability
3. Applicable standards to which the equipment was evaluated
of approved use, if any.
10.6 OPERATION AND MAINTENANCE
Maintenance procedures and schedules should be developed for R&D equipment.
equipment shall be checked, cleaned, and maintained on a schedule and in a manner based on
its application and use. Additional
information is referenced in Section 3.0, Electrical Preventive
10.7 EMPLOYEE QUALIFICATIONS
This section provides
guidance for determining the qualification process for persons involved
with specialized electrical equipment, configurations or work tasks
associated with experiments.
The guidance provided in this section is in addition to the minimum qualifications described in
Training and Qualifications of Qualified Workers.
The hazards associated with R&D equipment are sometimes unique
because the equipment
itself is unique. These hazards are sometimes made worse because of an uncommon design or
the fact that it may be
one of a kind. Special efforts are thus necessary to identify all the
potential hazards that may be present in a specific unique design.
These hazards should be
identified and a plan developed to mitigate the associated risk. Personnel working on R&D
be qualified to work on this equipment, depending on its unique safety
10.7.2 ADDITIONAL QUALIFICATIONS
assigned to tasks involving R&D equipment shall be apprised of the hazards
identified in Section 10.7.1. It is suggested that they participate
in developing mitigation plans to
reduce the risks associated with the hazards.
A list of additional experience qualifications should
be developed by the appropriate personnel
including the workers. This list should identify specific training requirements necessary for
unusual equipment or tasks.
10.8 GENERIC R&D EQUIPMENT
There are many possible types of electrical ac and dc power source
hazards in complex R&D
systems and the various design philosophies preclude establishing hazard classifications based
on voltage alone.
10.8.1 POWER SOURCES
1. Internal component failure can cause excessive voltages. Internal component open-circuit
failure in capacitor banks and Marx generators can result in full voltages across components
that may not be appropriately discharged in
the usual manner.
2. Internal component shorts in capacitor banks and Marx generators can result in excessive
fault current, causing
extreme heat, over-pressurization of capacitor enclosures, and rupture.
3. Overloading or improper cooling of power supplies can cause
excessive temperature rise.
4. Output circuits and components can remain energized after input power is interrupted.
and control power circuits can remain energized after the main power circuit is
6. When power supplies serve more than
one experiment, errors made when switching
between experiments may create hazards to personnel.
7. R&D electrical apparatus may
contain large amounts of stored energy, requiring fault
8. Liquid coolant leaking from R&D electrical equipment may
pose an electrical hazard to
10.8.1.2 DESIGN AND CONSTRUCTION
In design and construction of R&D equipment, it
is important to remember the following
1. Install only components essential to the power supply within the power-supply
2. Provide appropriate separation between high-voltage components and low-voltage supply
and/or control circuits.
3. Provide to personnel a visible indicator that the power supply is energized.
4. Minimize the number of control stations and provide
an emergency shutdown switch where
5. Where possible, avoid multiple-input power sources.
6. Apply a label containing
emergency shutdown instructions to equipment that is remotely
controlled or unattended while energized.
10.8.1.3 OPERATION AND MAINTENANCE
Before working in a power-supply enclosure or an associated equipment enclosure, see
2 and 7. Personnel should take the following precautions:
1. Implement lockout/tagout.
2. Check for auxiliary power circuits that
could still be energized.
3. Inspect automatic shorting devices to verify proper operation.
4. Short the power supply from terminal to
terminal and terminal to ground with grounding hooks.
10.8.2 CONDITIONS OF LOW VOLTAGE AND HIGH CURRENT
It is usual for R&D facilities to have equipment that operates at less than 50 V. Although this
equipment is generally regarded as
nonhazardous, it is considered hazardous when high
currents are involved. Examples of such equipment are a power supply rated 3 kA at 25
magnet power supply with rated output of 200 A at 40 V, and a bus bar carrying 1 kA at 5 V.
Though there is a low probability of
electric shock at voltages less than 50 V (See Figure 10-1),
there is a hazard due to arcing and heating in case of an accidental fault.
For example, a tool
could drop onto the terminals and initiate an arc, causing severe burns.
Figure 10-1. Process for the analysis of circuit hazards.
10.8.2.2 DESIGN AND CONSTRUCTION
A circuit operating at 50 V or less shall be treated as a hazardous circuit if the power in
create electrical shocks, burns, or an explosion due to electric arcs. Observe all of the following
rules for such circuits:
1. Provide protective covers and/or barriers over terminals and other live parts to protect
2. By suitable marking,
identify the hazard at the power source and at appropriate places.
Figure 10-1. Process for the analysis of circuit hazards.
3. Consider magnetic forces in both normal-operation and short-circuit conditions. Use
conductors that have appropriate physical
strength and are adequately braced and
supported to prevent hazardous movement.
4. Inductive circuits may create high-voltage hazards
when interrupted. Careful circuit design
will include a method to bleed off power safely should an interruption occur.
Follow these guidelines for working on circuits operating at 50 V or less that are treated as
Work on such circuits when they are de-energized.
2. If it is essential to work on or near energized low-voltage, high-current circuits,
safety rules as if the circuits were operating at more than 50 V. Refer to Section 2.1.2,
"Considerations for Working on Energized
Systems and Equipment" and 2.13.4, "Safe
Energized Work (Hot Work)."
10.8.3 CONDITIONS OF HIGH VOLTAGE AND LOW CURRENT
When the output current of high-voltage supplies is below 5 mA, the shock hazard to personnel
is low. Where combustible atmospheres
or mixtures exists, the hazard of ignition from a spark
may exist. High-voltage supplies (ac or dc) can present the following hazards:
1. Faults, lightning, or switching transients can cause voltage surges in excess of the normal
2. Internal component
failure can cause excessive voltages on external metering circuits and
low-voltage auxiliary control circuits.
3. Overcurrent protective
devices such as fuses and circuit breakers for conventional
applications may not adequately limit or interrupt the total inductive energy
and fault currents
in highly inductive dc systems.
4. Stored energy in long cable runs can be an unexpected hazard. Safety instructions
be in place to ensure proper discharge of this energy.
5. Secondary hazards such as startle or involuntary reactions from contact
low-current systems may result in a fall or entanglement with equipment.
10.8.3.2 DESIGN CONSIDERATIONS
in R&D labs may encounter energized parts in a variety of configurations, locations,
and under environmental conditions that are not
usual for most electrical power personnel.
Sometimes the equipment can be designed to incorporate mitigation of the hazards associated
with working on such equipment. If not, then safe operating procedures must be developed and
10.8.3.3 SAFETY PRACTICES
An analysis of high-voltage circuits should be performed by a qualified person before work
unless all exposed energized parts are guarded as required for high-voltage work. The
analysis must include fault conditions where circuit
current could rise above the nominal rated
value as explained here and shown graphically in Figure 10-1. Depending on the results of the
analysis, any of the following may apply:
1. If the analysis concludes that the current is above 5 mA or energy is above 10 joules,
then the work is considered to be energized work and should be performed in
accordance with Section 2, "General Requirements" and/or Section
7, "Work In Excess
of 600 Volts."
2. If the analysis concludes that the current is between 0.5 mA and 5 mA and between 0.25
10 joules, then the worker may be exposed to a secondary hazard (e.g., startle
reaction) that must be mitigated.
3. If the analysis
concludes that the current is below 0.5 mA and below 0.25 joules, then
the worker exposure is minimal and no special precautions are required.
High-voltage supplies that use rated connectors and cables where there are no exposed
energized parts are not considered hazards. Connections
shall not be made or broken with the
power supply energized unless they are designed and rated for this type of duty (e.g., load-break
elbows). Inspect cables and connectors for damage and do not use if they are damaged.
Exposed high-voltage parts must be guarded to avoid
10.8.4 RADIO-FREQUENCY/ MICROWAVE RADIATION AND FIELDS
The DOE complex conducts R&D programs that involve
sources of radio-frequency/microwave
(RFMW) non-ionizing electromagnetic radiation. Devices that may produce RFMW radiation
and radar equipment, industrial equipment such as radio-frequency
heaters, and scientific and medical equipment such as magnetic resonance
klystron tubes. The nationally recognized consensus standard for personnel exposure to radiofrequency
radiation is ANSI/IEEE
C95.1(1999), Electromagnetic Fields, Safety Levels with
Respect to Human Exposure to Radio Frequency.
amplifiers frequently use dc high-voltage power sources.
2. There may be x-ray hazards (when supply voltage exceeds 10 kV and there are
3. Currents maybe induced in conductive objects or metal structures that are not part of the
4. RF currents can cause severe burns.
5. Falls from towers may result from RF burns from antennas.
6. Electromagnetic interference
may cause equipment to malfunction.
7. Electromagnetic fields may cause unintended ignition of explosives, fuel, and ordnance.
8. Grounding and bonding conductors that
are adequate for dc and power frequencies may
develop substantial voltage when fast pulses and radio frequency currents are present,
to inductance and skin effect.
10.8.4.2 DESIGN AND CONSTRUCTION
Engineering control in accordance with ANS/IEEE C95.1 (1999) should
be the primary method
used to restrict exposure whenever practical. If engineering controls are not practical, work-time
on the averaging intervals and other work-practice and administrative controls,
must be used.
1. Warning Signs. Signs commensurate
with the RFMW level must be used to warn
personnel of RFMW hazards. These signs must be posted on access panels of RFMW
at entrances to and inside regulated areas.
2. Access Limitation. Access can be limited by controls such as barriers, interlocks,
administrative controls or other means. The operation supervisor controls access to
regulated areas and must approve non-routine entry of
personnel into these places.
When practical, sources of RFMW radiation should be switched off when not in use.
3. Shielding. Shielding
that encloses the radiating equipment or provides a barrier between
the equipment and the worker may be used to protect personnel; the shielding
must account for the frequency and strength of the field.
4. Interlocks. Chamber or oven-type equipment that uses microwave
radiation must have
interlocks designed to (1) prevent generation of the radiation unless the chamber is
sealed and (2) shut off such
equipment if the door is opened.
5. Lockout/Tagout. The design shall incorporate features that allow the equipment to be
and tagged out for servicing.
6. PPE. PPE such as eyewear is not readily available and is generally not a useful option
against RFMW radiation and fields. Protection must therefore be achieved
by other means.
10.8.4.2.1 EXEMPTIONS FROM RFMW EXPOSURE
The following items are exempt from the RFMW exposure limits; however, their manufacture is
subject to Federal RFMW emission
1. Cellular phones and two-way pagers and PDAs
2. Two-way, hand-held radios and walkie-talkies that broadcast between
10 kHz and 1
GHz and emit less than 7 W
3. Microwave ovens used for heating food
4. Video display terminals.
10.8.4.2.2 EXPOSURE CRITERIA FOR PULSED RFMW RADIATION
The basic considerations for peak-power exposure limits are consistent with
(1999) as follows:
1. For more than five pulses in the averaging time and for pulse durations exceeding 100
normal time averaging applies and the time-averaged power densities
should not exceed the Maximum Permissible Exposure (MPE) given in Table
controlled and Table 10-2 for uncontrolled environments, per IEEE/ANSI C95.1 (1999).
2. For intermittent pulse sources with
no more than five pulses during the averaging time,
the peak power density for any of the pulses should not exceed the limit given by the
This limits the specific absorption (SA) of each pulse to SA=28.8 joules/kg (whole-body
or spatial average), or SA=144 joules/kg
for 5 pulses.
For intermittent pulse sources with no more than five pulses during the averaging time,
the single-pulse SA of <
28.8 joules/kg, though higher than the threshold for auditory
effect (clicking), is three orders of magnitude lower than the SAs that produce
3. Maximum E field for any of the pulses should be no more than 100 kV/m. This peak E-field
prescribed to eliminate the possibility of air breakdown or spark discharges,
which occur at 2,900 V/m. A large safety factor is applied
to account for local field
enhancements where nominally lower fields may result in arcing discharges.
Table 10-1. Controlled Environment Exposure Limits
a. The exposure values in terms of electric and magnetic field strength are the values
obtained by spatially averaging values over
an area equivalent to the vertical cross
section of the human body (projected area).
b. These plane-wave equivalent power density
values, although not appropriate for
near-field conditions, are commonly used as a convenient comparison with MPEs
at higher frequencies
and are displayed on some instruments in use.
c. The ƒ = frequency in MHz.
d. It should be noted that the current limits given
in this table may not adequately
protect against startle reactions and burns caused by transient discharges when
contacting an energized
NEC Article 527, Temporary Installations, requires removal of temporary wiring upon
completion of the experiment for which
it was installed.
An overly strict interpretation can obstruct the scientific objectives of the R&D configuration,
but if a liberal
interpretation is allowed, there is a possibility that unsafe wiring methods will be
used. The AHJ must consider programmatic needs without
sacrificing personnel safety.
Table 10-2. Uncontrolled Environment Exposure Limits
a. The exposure values in terms of electric and magnetic field strength are the
values obtained by spatially averaging values over
an area equivalent to the
vertical cross section of the human body (projected area).
b. These plane-wave equivalent power density
values, although not appropriate for
near-field conditions, are commonly used as a convenient comparison with MPEs
at higher frequencies
and are displayed on some instruments in use.
c. The ƒ = frequency in MHz.
d. It should be noted that the current limits given
in this table may not adequately
protect against startle reactions and burns caused by transient discharges when
contacting an energized
10.9.1 WIRING METHODS
Unsafe wiring methods can cause electrical injury or
R&D work may require the use of wiring methods that are not anticipated in the NEC. These
methods may not be consistent with
normal commercial and industrial wiring methods, and
should be reviewed by the AHJ for approval.
10.9.1.2 DESIGN AND CONSTRUCTION
10.9.1.2.1 DESIGN AND CONSTRUCTION AS AN INTEGRAL PART OF EQUIPMENT
If the AHJ determines that wiring is an integral part of an apparatus
interconnections), then the wiring methods used should be evaluated by the AHJ as
providing safe operating conditions.
This evaluation may be based on a combination of
standards and engineering documentation where appropriate. Such an evaluation should
consist of an analysis of all stresses imposed on any electrical conductive elements,
including, but not limited to electrical, magnetic,
heating, and physical damage potential.
The wiring methods selected must mitigate to the greatest practical extent any undesired
of a failure sequence.
If cable trays are used as mechanical support for experimental circuits, they should be solely
this use and appropriately labeled. Any such use must be analyzed for detrimental
heating effects of the proposed configuration.
POWER SUPPLY INTERFACE BETWEEN UTILITY SYSTEMS AND R&D EQUIPMENT
Utility supply voltages should be brought as near to the utilization
equipment as possible using
NEC-compliant wiring methods.
Any temporary wiring methods used (e.g., extension cords) should be approved
by the AHJ for
a specified limited time.
Flexible cords and cables should be routed in a manner to minimize tripping hazards.
The conventional use of cable trays is defined in NEC Article 392. If power cables are placed in
a cable tray used for control and signal
cables, separation is advised but not always required.
According to NEC Article 392.6(E), multiconductor cables rated at 600 volts or less
permitted to be installed in the same cable tray. This presumes the cables are listed, having a
minimum rating of 300 volts. However,
cables rated over 600 volts require separation from
those rated at 600 volts or less, per Article 392.6(F). Communications cables are required
separated from light or power conductors by at least 2 inches, in accordance with NEC Article
configurations or physical constraints may require the unconventional
application of cable trays. Only the AHJ may approve these unconventional
deemed necessary, enhanced fire protection or other safety measures shall be used to ensure
safety to personnel and equipment.
For coaxial, heliax, and specialty cables used for experimental R&D equipment, where NEC
tray-rated cable types are not available
which meet the technical requirements of the
installation, the non-tray-rated cables shall be permitted with the approval of the AHJ. If
necessary, enhanced fire protection or other safety measures shall be used to ensure safety to
personnel and equipment.
When metallic cable tray is being used, it shall be bonded to the equipment grounding system,
but should not be relied upon to provide
the equipment ground. The experimental equipment
must be appropriately grounded by some other method.
10.9.1.3 OPERATION AND MAINTENANCE
The operation and maintenance of R&D systems which use wiring methods that are not
anticipated by the NEC require special considerations
from all personnel. The AHJ evaluation
for safe operating conditions must include a review of unique features in the engineering
10.9.2 UNCONVENTIONAL PRACTICES
R&D performed by DOE contractors often incorporates the design of specialized equipment
in the need for specialized grounding and the use of materials and components in an
unconventional manner. Even with these experimental needs
and special design
considerations, the maximum safety of personnel and equipment still needs to be ensured. The
practice of using materials
or components for purposes other than originally designed needs
special consideration in their use, identification, personnel protection,
and equipment protection.
The lack of proper grounding can cause electrical shock
and/or burns to personnel. The NEC
and NESC define legally-required grounding. To mitigate potential hazards, grounding shall be
in accordance with the NEC and NESC.
10.9.2.1.2 DESIGN AND CONSTRUCTION
NEC, Article 250, "Grounding" notes that grounds also
1. Voltage limitation in case of lightning, line surges, or unintentional contact with higher
of voltage to ground under normal operation
3. Facilitated overcurrent device operation in case of ground faults
4. A path to
conductive objects that will limit the voltage to ground.
In R&D work there is one additional function for grounds: a common reference
plane or system
ground return for electronic devices, circuits, and systems. (See Section 9.3) It is recognized
that such grounds are
essential in some cases to control:
1. Noise associated with the primary power system:
a. Incoming on the line
b. Outgoing from local equipment
2. Ground wire noise
3. Circuit coupling
a. Ground loop (shared circuit return)
b. Magnetic, capacitive, or electro-magnetic.
If system return impedances are low enough, then simple radio-frequency chokes can be used
to limit this noise with no effect on the
A 50-microhenry choke will add 1/50 of an ohm at 60 Hz, but will look like 2 ohms at 7.5 kHz
and 30 ohms at 100 kHz.
Such an RF choke will serve to discriminate against noise on the
An inexpensive RF choke may be installed in the safety
1. Pulling the green ground wire 20 feet longer than required.
2. Coiling the extra length on a 6-inch diameter (about 12 turns).
3. Securing it tightly wound with cable ties.
4. Connecting it into the circuit.
These actions satisfy the NEC requirement for a continuous
ground and noise isolation is also
Whatever scheme is used, the ground of experimental equipment shall be connected to the
same ground as the facilities' electrical system to ensure equal potential.
For practices involving hazardous materials, such as explosives,
the grounding shall also
comply with the requirements of Section 5.0, Special Occupancies.
10.9.2.1.3 NOISE COUPLING MECHANISMS.
Grounding can reduce the interference in the five types of coupling mechanisms listed here.
1. Conductive Coupling. (Source and load
wired together) It is sometimes practical to provide a
separate return path for both the source and the load. If the system layout allows
conductive coupling cannot occur between these two, as is shown in Figures 10-2 and 10-3.
2. Capacitive Coupling. (High-impedance
proximity coupling) The technique for increasing
resistance to capacitive coupling among cables is to ground one end of the shield to produce
the shortest, most direct shunt path back to the source of the coupled current as is shown in
Figures 10-4 and 10-5.
Caution: It is
possible to inadvertently increase coupling between source and load if the shield
ground does not properly shunt the current coupled onto
Figure 10-2. Arc currents through the process power supply return (existing ground) develop
a voltage that appears in series with
the process controls because they share that return.
Figure 10-3. It has been possible to install a separate return conductor for the power supply. The arc
currents no longer appear
in series with the process controls. There is no conductive coupling.
Capacitive Coupling Situation
Figure 10-4. Capacitance between circuit 1 and circuit 2 is allowing current to be transferred into
circuit 2 via an electric field. This current flows through Z2, the impedance of circuit 2, and develops an interfering voltage.
Capacitive Coupling Shield
Figure 10-5. A shield has been interposed between circuit 1 and circuit 2. The coupling will be reduced
as it shunts the coupled current around Z2 instead of through it. The interfering voltage could be increased instead of decreased if not properly
3. Inductive Coupling. (Near-field, low-impedance loop-to-loop coupling) The technique for
increasing resistance to magnetic coupling
in shielded cables is to ground both ends of the
shield to an effective signal return ground as is shown in Figures 10-6 and 10-7.
4. System Signal Returns. Each installation will require individual analysis and treatment. A
single ground poses no problem, but multiple
grounds can result in a ground loop. These can
upset the proper functioning of instruments. A signal isolator offers a way of overcoming
5. Instrumentation Grounding2. Equipment that is used to implement a control instrumentation
Figure 10-8) makes use of a common signal ground as a reference for analog
signals. Any additional grounds that are introduced into the control
circuit will almost
certainly cause ground loops to occur.
A typical process instrumentation loop is shown in Figure 10-8. It is a
DC system that operates
at a specific voltage (24 volts in this case) to a master ground reference called a signal ground.
signal varies within the range of 4-20 mA, depending upon the value of the
variable (pressure, temperature, etc.) seen by the sensor. A precisely
calibrated circuit takes
this mA signal and converts it into a form that can be used by a process-control computer, PLC,
2 The information in this section and figures 10-8 and 10-9 is reprinted with permission from the September
of EC&M magazine © 1991 Intertec Publishing Corporation. All rights reserved.
Figure 10-6. The pulse power supply, its cable, load, and return form a transmitting loop
which couples into the loop formed by the
controller, its multiconductor cabling, 1/0 and
return. Note that in actual installations these loops can be very large and very close.
dedicated instrument, or whatever controller that supervises the system. In this example, the
mA signal is converted to a 1-5 V signal
for a chart recorder. At 4 mA, the voltage measured by
the recorder is 250 x.004 = 1 V. At 20 mA, the measured voltage is 5 V. Normally,
scale is calibrated so the voltage reads directly in °F, psi, etc.
In order to minimize the danger of introducing ground
loops into this complicated network of
sensitive equipment, a dedicated instrumentation system ground bus is usually employed. This
ultimately receives grounds from the signal common, the do power supply common, the
cabinet ground, and the instrumentation ac power ground.
The bus is tied to earth via the
building ground and the plant ground grid. Figure 10-9 shows the typical way in which
of these various grounds is accomplished.
The cabinet ground is a safety ground that protects equipment and personnel from accidental
shock hazards while providing a direct drain line for any static charges or electromagnetic
interference (EMI) that may affect the cabinets.
The cabinet ground remains separate from the do
signal ground until it terminates at the master ground bus.
Figure 10-7. Intentional returns have been installed for both the pulse power supply and
the controller right in the trays for the cables.
Both loops have been reduced to small
cross sections, reducing inductive coupling. Any electromagnetic (far field) radiation
by the pulse power supply and its cabling will also be reduced.
Eliminating grounds is not feasible for some instruments, such as
thermocouples and some
analyzers, because they require a ground to obtain accurate measurements. Also, some
instruments must be grounded
to ensure personnel safety.
When grounds cannot be eliminated, the solution to instrumentation ground loops lies in signal
These devices break the galvanic path (dc continuity) between all grounds while
allowing the analog signal to continue throughout the loop.
An isolator also eliminates the noise of
ac continuity (common-mode voltage)3.
10.9.2.1.4 OPERATION AND MAINTENANCE
Before starting each operation (experiment, test, etc.) the exposed portions of the grounding
system should be visually checked for any
damage and to determine that all necessary
connections have been made. If more than one operation is conducted every day, then visual
checks should be performed only at the beginning of each shift, if the grounding system will be
needed during that shift. The adequacy of
the grounding system should be verified annually. It is
recommended that the grounding impedance within the equipment be maintained at 0.25
less. (See IEEE 1100).
3 Much of the information above came
from the article entitled Causes and Cures of Instrumentation Ground
Loops, by Pat Power, Moore Industries, Houston, TX.
Figure 10-8. Typical Process Instrument Loop.
10.9.2.2 MATERIALS USED IN AN UNCONVENTIONAL MANNER
The practice of using
materials or components for purposes other than originally designed needs
special safety considerations in use, identification, personnel
protection, and equipment
The use of materials for something other than their original design
criteria has the potential for
providing an additional hazard, especially to personnel unfamiliar with the research apparatus.
may assume that the material is used as originally designed and can unknowingly
expose themselves to hazards unless special precautions are
Some examples of items used in an unconventional manner are:
1. Copper pipe used as an electrical conductor
flexible copper pipe used as an electrical conductor
3. Specially designed high-voltage or high-current connectors
4. Specially designed
high-voltage or high-current switches
Figure 10-9. Typical control instrumentation ground system.
5. Water column used as a high-voltage resistor
6. Standard coax
cable used in special high-voltage pulsed circuits
7. Water column used as a charged-particle beam attenuator
8. Commercial cable tray
used as a mechanical support for experimental apparatus.
10.9.2.2.2 DESIGN AND CONSTRUCTION
During design, special consideration
should be given to installing interlocks and protective
barriers. Signs warning of the hazards should be posted to help prevent unsuspecting
from being injured.
10.9.2.2.3 OPERATION AND MAINTENANCE
Appropriate safety procedures and training should be part
of the process to qualify personnel.
The procedures should describe the methods used to promote safe work practices relating to
energized circuits in accordance with Section 2.1.2, Considerations for Working on
Energized Systems and Equipment, Section 2.13, Work Practices,
and 29 CFR 1910.331-335.
10.9.3 WORK ON ENERGIZED OR DE-ENERGIZED ELECTRICAL EQUIPMENT
Unless explicitly stated otherwise in this section, all work
on energized/de-energized equipment
will conform to Section 2.0, "General Requirements."
10.10 REQUIREMENTS FOR SPECIFIC R&D EQUIPMENT
Electrical equipment and components used in research may pose hazards not commonly found
in industrial or commercial facilities. Special
precautions are required to design, operate, repair,
and maintain such equipment. Electrical safety and personnel safety circuits (e.g.,
are covered in this section as a guide to reduce or eliminate associated hazards. Training and
experience in the specialized
equipment are necessary to maintain a safe workplace.
All personnel involved with research electrical equipment should be trained and
be familiar with
the hazards they may encounter in the workplace. Only qualified electrical personnel should
design, install, repair,
or maintain electrical research equipment or components. Safety-related
design, operation, and maintenance techniques should be incorporated
into all new or modified
equipment. Existing equipment should be modified when necessary to ensure safety.
Equipment for which specific
standards are not available should be constructed according to the
principles of established standards, as determined by the AHJ.
Capacitors and inductors are used in research apparatus in special configurations as well as in
their standard configurations. The design,
operation, and maintenance of research apparatus
using capacitors and inductors in these special configurations require that special consideration
be given to the safety of both personnel and equipment.
This section covers capacitors that are used in the
following typical R&D applications:
1. Energy storage
2. Voltage multipliers
Examples of capacitor hazards include:
1. Capacitors may store and accumulate a dangerous residual charge after the equipment has
been de-energized. Grounding capacitors in series may transfer rather than discharge the
2. A hazard exists when a
capacitor is subjected to high currents that may cause heating and
3. When capacitors are used to store large amounts of energy, internal failure of one capacitor
in a bank frequently results
in explosion when all other capacitors in the bank discharge into
the fault. Approximately 104 joules is the threshold energy for explosive
failure of metal cans.
4. High-voltage cables should be treated as capacitors since they have the capability to store
5. The liquid dielectric and combustion products of liquid dielectric in capacitors may be toxic.
6. Because of the phenomenon of
"dielectric absorption," not all the charge in a capacitor is
dissipated when it is short-circuited for a short time.
7. A dangerously
high voltage can exist across the impedance of a few feet of grounding cable
at the moment of contact with a charged capacitor.
Discharging a capacitor by means of a grounding hook can cause an electric arc at the point
of contact. (See 10.10.1.2.3).
faults may rupture capacitor containers. Rupture of a capacitor can create a fire
hazard. PCB dielectric fluids may release toxic gases when
decomposed by fire or the heat
of an electric arc.
10. Fuses are generally used to preclude the discharge of energy from a capacitor
bank into a
faulted individual capacitor. Improperly sized fuses for this application may explode.
10.10.1.2 DESIGN AND CONSTRUCTION
The following cautions in design and construction should be considered:
1. Isolate capacitor banks by elevation, barriers, or enclosures
to preclude accidental contact
with charged terminals, conductors, cases, or support structures.
2. Interlock the circuit
breakers or switches used to connect power to capacitors.
3. Provide capacitors with current-limiting devices.
4. Design safety devices
to withstand the mechanical forces caused by the large currents.
5. Provide bleeder resistors on all capacitors not having discharge devices.
6. Design the discharge-time-constant of current-limited shorting and grounding devices to be
as small as practicable.
7. Provide suitable grounding.
10.10.1.2.1 AUTOMATIC DISCHARGE DEVICES
1. Use permanently connected bleeder resistors when practical.
2. Have separate bleeders when capacitors are in series.
3. Use automatic shorting devices that operate when the equipment is de-energized or when
the enclosure is opened, which discharges
the capacitor to safe voltage (50 V or less) in
less time than is needed for personnel to gain access to the voltage terminals. It must never
be longer than 5 minutes.
4. For Class C equipment with stored energy greater than 10 J, provide an automatic,
device that functions when normal access ports are opened.
5. Ensure that discharge devices are contained locally within protective barriers
wiring integrity. They should be in plain view of the person entering the protective barrier so
that the individual can verify
proper functioning of the devices.
6. Provide protection against the hazard of the discharge itself.
10.10.1.2.2 SAFETY GROUNDING
1. Fully visible, manual grounding devices should be provided to render capacitors safe while
work is being performed.
2. Grounding points must be clearly marked.
3. Prevent transferring charges to other capacitors.
10.10.1.2.3 GROUND HOOKS
Conductor terminations should be soldered or terminated in an approved crimped lug. All
conductor terminations must be strain-relieved within
2. Ground hooks must be grounded and impedance should be less than 0.1 ohms to ground.
3. The cable conductor must be clearly
visible through its insulation.
4. A cable conductor size of at least #2 AWG should be used, and the conductor shall be
carrying the available fault current of the system.
5. Ground hooks shall be used in sufficient number to adequately ground all designated
6. Permanently installed ground hooks must be permanently grounded and stored in a manner
to ensure that they are used.
10.10.1.2.4 DISCHARGE EQUIPMENT WITH STORED ENERGY IN EXCESS OF 10 JOULES
1. A discharge point with an impedance capable of limiting
the current to 500A or less should
2. The discharge point must be identified with a unique marker (example: yellow circular
marker with a red slash), and should be labeled "HI Z PT" in large legible letters.
3. A properly installed grounding hook should first
be connected to the current-limiting
discharge point and then to a low-impedance discharge point (< 0.1 ohm) that is identified
a unique marker (example: yellow circular marker).
4. The grounding hooks should be left on all of these low-impedance points during the time of
5. The low-impedance
points shall be provided whether or not the HI-Z current-limiting points
6. Voltage indicators that are visible from all
normal entry points should be provided.
1. Capacitors connected in parallel should be individually fused, when
2. Caution must be used in the placement of automatic discharge safety devices with respect
to fuses. If the discharge will
flow through the fuses, a prominent warning sign should be
placed at each entry indicating that each capacitor must be manually grounded
3. Special knowledge is required for high-voltage and high-energy fusing.
10.10.1.3 OPERATION AND MAINTENANCE
1. The protective devices (interlocks) shall not be bypassed unless by qualified electrical
personnel when inspecting, adjusting, or
working on the equipment. Proper procedures
need to be followed when bypassing interlocks.
2. Procedures should be established for
tagging the interlock and logging its location and
the time when bypassed and restored. Written approval shall be obtained from an
authority before bypassing an interlock .
3. Only qualified electrical personnel (those trained in the proper handling and storage of
power capacitors and hazard recognition) shall be assigned the task of
servicing/installing such units.
4. Proper PPE shall be used
when working with capacitors.
5. Access to capacitor areas shall be restricted until all capacitors have been discharged,
6. Any residual charge from capacitors shall be removed by grounding the terminals before
servicing or removal.
7. Automatic discharge and grounding devices should not be relied upon.
8. Grounding hooks shall be inspected before each use.
9. Capacitor cases should be considered "charged."
10. Protective devices should be tested periodically.
11. All uninstalled
capacitors capable of storing 5 joules or greater shall be short-circuited
with a conductor no smaller than #14 AWG.
12. A capacitor that develops an internal open circuit may retain substantial charge internally
even though the terminals are short-circuited.
Such a capacitor can be hazardous to
transport, because the damaged internal wiring may reconnect and discharge the
the short-circuiting wires. Any capacitor that shows a significant
change in capacitance after a fault may have this problem. Action should
be taken to
minimize this hazard when it is discovered.
This section covers inductors as well as electromagnets
and coils that are used in the following
1. Energy storage
2. Inductors used as impedance devices in a pulsed
system with capacitors
3. Electromagnets and coils that produce magnetic fields to guide or confine charged particles
4. Inductors used
in do power supplies
5. Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR), and
Examples of Inductor hazards include:
1. Overheating due to overloads, insufficient cooling,
or failure of the cooling system could
cause damage to the inductor and possible rupture of the cooling system.
and superconductive magnets may produce large external force fields that
may affect the proper operation of the protective instrumentation
3. Magnetic fields could attract nearby magnetic material, including tools and surgical implants,
causing injury or
damage by impact.
4. Whenever a magnet is suddenly de-energized, production of large eddy currents in adjacent
can cause excessive heating and hazardous voltages. This state may
cause the release or ejection of magnetic objects.
5. The worker
should be cognizant of potential health hazards.
6. Interruption of current in a magnet can cause uncontrolled release of stored energy.
Engineered safety systems may be required to safely dissipate stored energy. Large
amounts of stored energy can be released in the event
of a "quench" in a superconducting
10.10.2.2 DESIGN AND CONSTRUCTION
The following should be considered:
1. Provide sensing devices (temperature, coolant-flow) that are interlocked with the power
2. Fabricate protective
enclosures from materials not adversely affected by external
electromagnetic fields. Researchers should consider building a nonferrous barrier
to prevent accidental attraction of iron objects and prevent damage to the cryostat. This is
especially important for superconducting
3. Provide equipment supports and bracing adequate to withstand the forces generated during
4. Appropriately ground electrical supply circuits and magnetic cores and provide adequate
5. Provide means
for safely dissipating stored energy when excitation is interrupted or a fault
6. Provide appropriate warning signs to prevent
persons with pacemakers or similar devices
from entering areas with fields of greater than 0.001 Tesla.
7. Personnel exposure to magnetic
fields of greater than 0.1 Tesla should be restricted.
8. When a magnet circuit includes switching devices that may not be able to interrupt
magnet current and safely dissipate the stored energy, provide a dump resistor connected
directly across the magnet terminals that
is sized to limit the voltage to a safe level during
the discharge and safely dissipate the stored energy.
10.10.2.3 OPERATION AND
Verify that any inductor is de-energized before disconnecting the leads or checking continuity or
ELECTRICAL CONDUCTORS AND CONNECTORS
The conductors and connectors covered here are only those used in unconventional
Examples of hazards are as follows:
1. Metallic cooling-water pipes that are also used as electrical conductors
hazards (i.e., they may not be readily recognizable as electrical conductors).
2. Improper application or installation
of connectors can result in overheating, arcing, and
3. Hazardous induced voltages and arcing can result from inadequate
high- and low-voltage cables.
4. Use of an improper cable for a given type of installation (routing) can result
in a fire hazard.
10.10.3.2 DESIGN AND CONSTRUCTION
When working with special conductors and connectors for R&D applications, the following
guidelines shall be implemented for design and construction:
1. Select cables that are listed by an NRTL for a given type of installation
(such as in conduits,
trays, underground, or in an enclosure) whenever possible. Since cables used for R&D are
sometimes unique (such
as some coaxial cables), they may not be available as NRTL listed.
In that case, obtain AHJ approval.
2. Where liquid- or gas-cooled
conductors are used, sensing devices (temperature or coolant flow)
shall be provided for alarm purposes or equipment shutdown if the cooling
malfunctions. Provide adequate labeling, insulation, or other protection for metallic cooling water
piping used as electrical conductors.
3. Provide engineering calculations to support overrating of conductors for any application.
Avoid conductor loops (wide spacing) between
high-current supply and return conductors to
prevent voltage and current induction in adjacent circuits or structural members.
Ground coaxial cable shielding when possible. If test conditions require an ungrounded
shield, provide barriers and warning signs to notify
personnel that the shield is ungrounded
and should be assumed to be energized. Provide suitable routing and additional protection
coaxial cables used in pulsed-power applications where the braid of the coaxial cable
rises to high voltage levels.
Cable connectors and connections should be checked after installation, periodically, and should
be tightened as necessary.
Special attention should be given to aluminum cable connections.
Ensure that charges are not built up on equipment that has been disconnected,
such as vacuum
feed through systems.
10.10.4 INDUCTION AND DIELECTRIC HEATING EQUIPMENT
This section describes electrical hazards
associated with induction heating, RF equipment, and
microwave equipment used in research. The hazards are mainly associated with high power/
high-frequency RF generators, waveguides and conductors, and the working coils
producing high temperatures.
1. RF power as high as 50 kW and frequency in the tens of kHz range to hundreds of MHz is
supplied from the RF and microwave generators.
Being close to or making contact with an
unprotected coil, conductors or waveguide opening may result in severe body burns.
voltages are present inside the power generators.
3. Dangerous levels of RF energy may be present in the laboratory.
10.10.4.2 DESIGN AND CONSTRUCTION
1. The heating coils, sources of high-frequency energy, and other live parts outside the
generator cabinet must be shielded or guarded to prevent access or contact.
2. The heating coil should have its cold (outside) lead properly
3. A coaxial cable of correct impedance and adequate construction may be desirable to deliver
the RF power to the coil in
order to prevent the leakage of the RF energy in the laboratory.
10.10.4.3 OPERATION AND MAINTENANCE
1. Shielding must be maintained
to minimize RFMW radiation.
2. Wearing metallic objects when operating or maintaining the induction heating system is
3. Posting suitable warnings to indicate equipment hazards.
10.10.5 LASERS AND X-RAY EQUIPMENT
This section is applicable
to laser systems and x-ray equipment used in research. Both fixed
and portable equipment are covered regardless of input voltage. Only electrical
addressed in this subsection. Refer to ANSI Z136.1 for laser hazards and 29 CFR 1910.306 (f)
for x-ray hazards.
1. Dangerous voltages are present inside the equipment.
2. Implosion hazards may exist with the covers removed.
3. Energy storage devices may present a hazard due to a residual charge even when the
system is de-energized
4. Dangerous voltages
can exist across the impedance of the grounding conductor during
5. Failure of interlocks and safety devices may allow
access to energized
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