In order to prevent an overcurrent situation, when common neutrals are used, the loads must be on different phases.  If the same phase is used, the current on the neutral wire will be equal to load-1 plus load-2, possibly doubling the current and creating an unsafe over current situation.   Remember, there is no circuit protection for the neutral wire.  When overloaded, it can only heat until it burns open.  The verification of “phase alignment” is critical in order to prevent overloads.

Many problems exist with when common or shared neutrals are used in an electrical system:

1. Possibility of over amperage if phases are not aligned properly
2. Cannot adjust breakers at later dates due to phase alignment issues
3. Safety hazard for personnel when servicing the equipment as neutral wires may still be “hot” even after the proper lockout/tagout procedures have been implemented
4. Electrical noise is increased
5. Possibility that electrical faults occurring on a shared circuit could damage equipment on the other shared circuits
6. GFCI circuits are nearly impossible to install

Additional regulatory information:

1. Special tied breakers are required to ensure all breakers feeding the neutral wire are disconnected (required under 29 CFR 1910.137 & 1910.301, and DOE/ID-10600)
2. Although technically acceptable under the NEC, it is considered “bad practice” by industrial standards
3. Industry standards consider it especially bad practice to share neutrals for circuits intended for computer systems
4. Best-practice recommends that shared or common neutrals be sized up one gage of wire to reduce electrical noise and help prevent overcurrent scenarios.
5. Based on interpretation, the practice of sharing neutrals may be considered illegal under certain 29 CFR 1910 regulations
6. Certain DOE – Operation Office Architectural Engineering Standards, dated November 1994, no longer permit the use of “Edison Circuits” in new construction
7. The Department of Energy has released ESH Bulletin 98-2 which is entitled “EDISON CIRCUITS POSE SAFETY HAZARD” which sites ORPS Report ID-LTC-WASTEMNGT-1997-0013, INEEL Lessons Learned #97283 in its requirement for immediate corrective action in removing all Edison Circuits (shared or common neutrals) from their facilities.

In order to avoid the danger of having breakers later re-arranged onto the same leg (which would create a potential overcurrent hazard), shared neutral circuits should be connected to multi-gang breakers rather than to separate single gang breakers.

When an Edison circuit is discovered, it is recommended that electricians perform one of the following modifications:

• Install clips on the affected circuit breakers that will open when either “hot” leg of the Edison circuit is opened.
• Post warning signs on every electrical panel in the facility indicating that Edison circuits are installed in the associated electrical systems.
• Train all electrical workers about the hazards of “Edison Circuits”.

Your company’s standard lockout/tagout procedures should be modified to include a requirement to verify zero energy on the neutral line.  Additional dangers to electricians are posed by circuits with common neutrals that are controlled by switches, such as thermostats, that may close unexpectedly.  Therefore, it is important to consider the potential presence of Edison circuits.

Electricians working on sites with Edison Circuits should be trained, warning signs posted, and should consider the possibility that every circuit may be Edison circuits, and take the necessary safety precautions.