 This
presentation was generated by Flour Hanford the U.S. Department of Energy.
The main point of this
presentation is that the neutral wire (typically white in color) is unfortunately named in a way that implies it
is harmless. Most people know that if you touch the black (or red or blue) wire, chances are you are going to
receive an electrical shock. They also believe that it is safe to touch the white (neutral wire).
Under
most conditions, this is true. However, that is only so if the path your body makes to ground is a significantly
higher resistance to current flow than what the neutral wire path provides. This requires the neutral wire to have
a complete path back to the service panel. If the neutral wire somehow gets disconnected from the neutral bar in
the panel, your body becomes the only return path to ground for the electrical current.
So, the cardinal
law of working around live circuits is to assume that the neutral conductor is at the same potential as the hot
wire. I have worked on many live circuits in my life and have been bitten a couple times by open neutrals - it is
not a good feeling. Just for the record, the highest AC voltage I have been hit by was a 3-phase 480 V supply to a
commercial welder. The highest DC was from a 2 kV radar CRT display. I have been lucky - no permanent damage...
other than mental :-)
Potential Hazards
with Neutral Conductors
Neutrals Are Current Carrying Conductors
- Neutrals are grounded but carry current under load.
- The source of neutral current cannot always be identified.
- Breaking a neutral under load could create a shock hazard.
- Individuals contacting a lifted neutral potentially provide an alternate path to ground.
- A broken neutral or lifted neutral could result in a shock or an arc.
Energized Neutral Examples
- A neutral was misidentified and inadvertently opened creating an arc (ORPS EM-SR-WSRC-FTANK-2005-0009)
- A circuit was moved to a different distribution panel, but the neutral was spliced in the original panel
(ORPS EM-RL-PHMC-PFP-2005-0011)
- An electrician received a shock after lifting a neutral from its bus bar. The neutral received its power
through an emergency light that received power from another distribution panel. (ORPS
SC-PNSO-PNNL=PNNLBOPER-2005-0018)
Configuration That Requires Additional Precautions:
Component Labels
 |
Circuit Breaker |
|
 |
Circuit A Ungrounded Conductor |
 |
Circuit B Ungrounded Conductor |
 |
Grounded Conductor (Neutral Conductor) |
 |
Ground |
Notes:
- 3 current carrying conductors in a raceway.
- Copper ground wire omitted for simplicity.
- Highlighted in yellow indicates energized.
|
 |
Ballast / Fluorescent Light |
 |
Ballast / Fluorescent Light enclosure |
The Multi-Wire Branch Circuit is an acceptable configuration according to the
National Electrical Code (NEC) Section 210.4.
The
Multi-Wire Branch Circuit
This circuit has also been referred to as:
- The Edison Circuit
- Common Neutral Circuit
- Shared Neutral Circuit
- These circuits are typically found on 120 / 240 volt single phase systems, but can be found on 208Y /120 and
277Y / 480 volt systems as well.
- The Grounded Neutral wire carries the unbalanced load current. (“Grounded Conductor”)
Use the following guidance when the neutral conductor must be interrupted:
- Treat the neutral as energized even though the circuit is locked out at the source. (Use PPE that is
appropriate for the hazard, i.e. gloves and eye protection)
- Measure absence of voltage to ground immediately after lifting leads when more than one neutral is lifted
from a device or when a splice is broken.
- If known, Lock out both / all load breakers.
- If both circuit breakers in a multi-wire branch circuit are not known?
- Test the neutral circuit with a clamp-on type current detector to identify if the neutral is carrying
current before lifting neutral leads or breaking a neutral connection.
NOTE: Current will exist only if one or more circuits sharing the neutral have a load energized at the
time of measurement.
When a common neutral hazard is discovered at a device:
- Stop work and remove the hazardous condition or plan a new work package considering known energized
conductors.
- It should be corrected by installation of pigtails or other means, to maintain continuity of the neutral
wiring in accordance with NFPA 70 National Electrical Code.
  Other
general precautions include:
- Label the doors of lighting and power panels where common neutrals are known to exist.
- This will alert LOTO planners and workers of potential problems.
- Provide instructions in work packages where common neutrals are known to exist to remind workers to be alert
to wiring that may indicate a common neutral and to maintain neutral circuit continuity.
- Suspect a multi-wire branch circuit when three or more neutral conductors are spliced together in a junction
box, outlet box, or lighting fixture.
Note: Some of the original PowerPoint slides contained animations, which have been
omitted here. No important information is lost in the process.
|
Kirt
