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 do
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 safety
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 electrical
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.
contains safety criteria for the DOE complex in the design, development, fabrication
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
systems to large, high-voltage power distribution systems to high-voltage/low-current and
high current systems associated with R&D programs. Electrical systems of all types are
integral part of R&D operations and associated support work.
10.3 COMPLIANCE WITH OSHA
important to note that special types of work on R&D electrical systems (e.g., electronic
considered electrical work, and therefore the work shall follow electrical safety
Consistent with other sections of this document, electrical systems and equipment and all
development, fabrication and construction, modification, installation, inspection, testing,
maintenance shall be in accordance with applicable electrical requirements.
Specific attention shall be focused on the electrical regulations of OSHA, including:
1. 29 CFR 1910.137
2. 29 CFR 1910.147
3. 29 CFR 1910.269
4. 29 CFR 1910.301-399
5. 29 CFR 1926.401-449.
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 LABORATORY
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
acceptance process to ensure appropriate confidence in the safety of the product.
9.0, Enclosed Electrical/Electronic Equipment, for additional guidance. Also see UL
508 and applicable ANSI and
10.5.2 DESIGN AND CONSTRUCTION
Equipment should be constructed such that:
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,
3. The components, wiring, and other internal parts are protected from being displaced
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
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 serviceable parts
9. Overcurrent and over-temperature protection
10. Clearances and spacing
12. Design and procedural documentation
13. Signage, labels,
and administrative controls
14. Mechanical motion
15. Stored energy
Documentation should be developed to substantiate the acceptance of any equipment. Such
include but not be limited to:
1. Tests performed
2. Conditions of acceptability
3. Applicable standards to which the equipment was evaluated
4. Limitations 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
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
Section 2.8, 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
identified and a plan developed to mitigate the associated risk. Personnel working on R&D
equipment shall be qualified to work on this equipment, depending on its unique safety
10.7.2 ADDITIONAL QUALIFICATIONS
Personnel 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
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
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.
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.
5. Auxiliary 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
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 enclosure.
2. Provide appropriate separation between high-voltage components and
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
Sections 2 and 7. Personnel should take the following precautions:
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 V, a
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
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 it can
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
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.
10.8.2.3 OPERATION AND
Follow these guidelines for working on circuits operating at 50 V or less that are treated as
1. 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, observe the
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
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
2. Internal component failure can cause excessive voltages on external metering
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 should
be in place to ensure proper discharge of this energy.
Secondary hazards such as startle or involuntary reactions from contact with high-voltage
may result in a fall or entanglement with equipment.
10.8.3.2 DESIGN CONSIDERATIONS
Personnel in R&D labs may encounter energized parts in a variety of
and under environmental conditions that are not usual for most electrical power
Sometimes the equipment can be designed to incorporate mitigation of the hazards associated
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
begins 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
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
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
and 10 joules, then the worker may be exposed to a secondary hazard (e.g., startle
reaction) that must be
3. If the analysis concludes that the current is below 0.5 mA and below 0.25 joules, then
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 accidental contact.
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
telecommunications and radar equipment, industrial equipment such as radio-frequency
heaters, and scientific
and medical equipment such as magnetic resonance imagers and
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
voltage when fast pulses and radio frequency currents are present,
due 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
used to restrict exposure whenever practical. If engineering controls are not practical,
limits, based on the averaging intervals and other work-practice and administrative controls,
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
enclosures and at entrances to and inside
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
the equipment and the worker may be used to protect personnel; the shielding design
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
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
locked out and tagged
out for servicing.
6. PPE. PPE such as eyewear is not readily available and is generally not a useful
as protection against RFMW radiation and fields. Protection must therefore be achieved
10.8.4.2.1 EXEMPTIONS FROM RFMW EXPOSURE LIMITS
The following items are exempt from the RFMW
exposure limits; however, their manufacture is
subject to Federal RFMW emission standards:
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
10.8.4.2.2 EXPOSURE CRITERIA FOR PULSED RFMW RADIATION
The basic considerations for peak-power exposure
limits are consistent with ANSI/IEEE C95.1
(1999) as follows:
1. For more than five pulses in the
averaging time and for pulse durations exceeding 100
milliseconds, normal time averaging applies and the
time-averaged power densities
should not exceed the Maximum Permissible Exposure (MPE) given in Table 10-1 for
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
limit is 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
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
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 object.
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
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
b. These plane-wave equivalent power density values, although not appropriate for
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
contacting an energized object.
10.9.1 WIRING METHODS
Unsafe wiring methods can cause electrical injury or fire hazards.
R&D work may require the use of wiring methods that are not anticipated in the NEC. These
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 (e.g., instrumentation
interconnections), then the wiring methods used should be evaluated by the AHJ
providing safe operating conditions. This evaluation may be based on a combination of
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
effects of a failure sequence.
If cable trays are used as mechanical support for experimental
circuits, they should be solely
dedicated to this use and appropriately labeled. Any such use must be analyzed
heating effects of the proposed configuration.
10.9.1.2.2 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
Flexible cords and cables should be routed in a manner to minimize tripping hazards.
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 are
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 to be
separated from light or power conductors by at least 2 inches, in accordance with NEC Article
Certain experimental configurations or physical constraints may require the unconventional
application of cable trays. Only the AHJ may approve these unconventional applications. If
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
non-tray-rated cables shall be permitted with the approval of the AHJ. If deemed
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,
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
resulting in the need for specialized grounding and the use of materials and components
unconventional manner. Even with these experimental needs and special design
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
NESC define legally-required grounding. To mitigate potential hazards, grounding shall be
accordance with the NEC and NESC.
10.9.2.1.2 DESIGN AND CONSTRUCTION
NEC, Article 250, "Grounding"
notes that grounds also provide:
1. Voltage limitation in case of lightning, line surges, or unintentional
contact with higher
2. Stability of voltage to ground under normal operation
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
1. Noise associated with the primary power system:
a. Incoming on the line
Outgoing from local equipment
2. Ground wire noise
3. Circuit coupling
a. Ground loop (shared circuit return)
b. Magnetic, capacitive, or
If system return impedances are low enough, then simple radio-frequency chokes can be
to limit this noise with no effect on the safety function.
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
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
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
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 this, then
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
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 the shield.
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 shunted.
3. Inductive Coupling. (Near-field, low-impedance loop-to-loop coupling) The technique for
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
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 the
5. Instrumentation Grounding2
. Equipment that is used to implement a control
strategy (see 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
at a specific voltage (24 volts in this case) to a master ground reference called a signal ground.
The instrumentation signal varies within the range of 4-20 mA, depending upon the value of the
(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
1991 issue 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, the recorder
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
equipment, a dedicated instrumentation system ground bus is usually employed. This
bus 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
interconnection 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
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
reducing inductive coupling. Any electromagnetic (far field) radiation
being generated 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,
instruments must be grounded to ensure personnel safety.
When grounds cannot be eliminated, the
solution to instrumentation ground loops lies in signal
isolators. 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
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 ohms or
less. (See IEEE 1100).
3 Much of the information above came from the article entitled Causes and Cures of
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
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.
assume that the material is used as originally designed and can unknowingly
expose themselves to hazards unless
special precautions are followed.
Some examples of items used in an unconventional manner are:
Copper pipe used as an electrical conductor
2. Insulated 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
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 personnel
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
work on energized circuits in accordance with Section 2.1.2, Considerations for
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
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., interlocks)
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
All personnel involved with research electrical equipment should be trained and be familiar with
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
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
Examples of capacitor hazards include:
1. Capacitors may store and accumulate a dangerous residual charge after the equipment has
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
a bank frequently results in explosion when all other capacitors in the bank discharge into
Approximately 104 joules is the threshold energy for explosive failure of metal cans.
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.
8. 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).
Internal faults may rupture capacitor containers. Rupture of a capacitor can create a fire
dielectric fluids may release toxic gases when decomposed by fire or the heat
of an electric arc.
Fuses are generally used to preclude the discharge of energy from a capacitor bank into a
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
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
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.
Class C equipment with stored energy greater than 10 J, provide an automatic,
mechanical discharging device
that functions when normal access ports are opened.
5. Ensure that discharge devices are contained locally
within protective barriers to ensure
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
2. Grounding points must be clearly marked.
3. Prevent transferring charges to other capacitors.
10.10.1.2.3 GROUND HOOKS
1. Conductor terminations should be soldered or terminated in an approved crimped lug. All
conductor terminations must be strain-relieved within 15 cm.
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
4. A cable conductor size of at least #2 AWG should be used, and the conductor shall be
capable of carrying the available fault current of the system.
5. Ground hooks shall be used in sufficient number to adequately ground all designated points.
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
by 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 possible.
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 before work
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
2. Procedures should be established for tagging the interlock and logging its
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,
shorted, and grounded.
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.
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
capacitor through the short-circuiting wires. Any
capacitor that shows a significant
change in capacitance after a fault may have this problem. Action should be
minimize this hazard when it is discovered.
This section covers inductors as well as electromagnets and coils that are used in the
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
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.
2. Electromagnets and superconductive magnets may produce large external force fields that
may affect the proper operation of the protective instrumentation and controls.
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
material can cause excessive heating and hazardous voltages. This state may
cause the release or ejection of
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
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
Researchers should consider building a nonferrous barrier designed
to prevent accidental attraction of iron
objects and prevent damage to the cryostat. This is
especially important for superconducting magnet systems.
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
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 the
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 MAINTENANCE
Verify that any inductor is de-energized
before disconnecting the leads or checking continuity or
10.10.3 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 present shock
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 separation between
high- and low-voltage
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:
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.
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 system
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.
4. 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
for coaxial cables used in pulsed-power applications where the braid of the coaxial cable
rises to high voltage levels.
10.10.3.3 OPERATION AND MAINTENANCE
Cable connectors and connections should be checked after
installation, periodically, and should
be tightened as necessary. Special attention should be given to aluminum
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.
Dangerous 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.
The heating coil should have its cold (outside) lead properly grounded.
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
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 hazards
addressed in this subsection. Refer to ANSI Z136.1 for laser hazards and 29 CFR 1910.306 (f)
1. Dangerous voltages are present inside the equipment.
2. Implosion hazards may exist with the covers
3. Energy storage devices may present a hazard due to a residual charge even when the
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