Transfer Switches for Emergency Lighting: Three-phase systems during single-phase conditions

Shutterstock / Net Vector
Shutterstock / Net Vector
Published On
Nov 15, 2022

I was on a phone call with two extremely knowledgeable city electrical plan reviewers addressing a question about lighting circuits for emergency power in 700.16 and how 700.17 and 700.18 apply to emergency lighting for the 2020 National Electrical Code .

Emergency illumination is required by 700.16(A) to “include means of egress lighting, illuminated exit signs, and all other luminaires specified as necessary to provide the required illumination.” Emergency lighting system reliability is covered in 700.16(B) and states these systems must “be designed and installed so that the failure of any illumination source cannot leave in total darkness any space that requires emergency illumination.” There wasn’t any question about that statement in 700.16(B), since it is fairly straightforward.

Lighting, interrupted

Section 700.17 states that branch circuits that supply emergency lighting must be installed to provide service from a source complying with 700.12 when the normal supply for lighting is interrupted, in one of two ways. The first way is by providing “an emergency lighting supply, independent of the normal lighting supply, with provisions for automatically transferring the emergency lights upon event of failure of the normal lighting branch circuit.”

The second way is by providing “two or more branch circuits (hopefully on separate phases) supplied from separate and complete systems with independent power sources. One of the two power sources and systems must be part of the emergency system, and the other is permitted to be part of the normal power source and system. Each system must provide sufficient power for emergency lighting purposes. Unless both systems are used for regular lighting and are kept lighted, means must be provided for automatically energizing either system upon failure of the other. Either or both systems are permitted to be part of the general lighting of the protected occupancy if circuits supplying lights for emergency illumination are installed in accordance with other sections of Article 700.”

What that is really saying is that emergency lighting is required to be independent of the normal lighting supply and must be automatically operated, such as with a transfer switch and generator, when there is a failure of the branch circuit or circuits supplying normal lighting. In addition, 700.18 further states that, “for branch circuits that supply equipment classed as emergency, there must be an emergency supply source to which the load will be transferred automatically upon failure of the normal supply.”

All three of us agreed that using a transfer switch with a generator, based on 700.12, supplied the appropriate emergency supply source, where any failure in the normal supply coming from the utility company power would transfer over and start the generator.

A puzzling question

I then asked a question that got us scratching our heads. We had no good answer. If the utility company had a fault on one phase of their three-phase transformer, and the supply to the building service was single-phasing, would the transfer switch work properly to switch over to generator power, thus restoring power to the emergency lighting?

I told them I would pose that question to some of the various transfer switch manufacturers to see if they had an answer. Would it transfer to the generator power, or would the transfer switch continue to supply “normal” power as a single-phase source?

I spoke to one manufacturer by phone and located a technical brief from another, resulting in an answer: The single-phasing condition is generally picked up by the transfer switch as a failure, resulting in the switch transferring the building loads to the emergency power source, the generator.

In some cases, the single-phasing could go unnoticed. The technical brief and manufacturer agreed this could happen. Where 50% or more of the building load was motors, “phantom” voltage could be generated through the windings of the motors and fed back to the “missing phase” at the transfer switch sensing terminals.

If the phantom voltages are within the broad parameters of “normal” conditions, the transfer switch may not recognize the single-phasing and not switch over or may switch back and forth. If the three-phase motors do not have single-phasing protection, they could be damaged, and the emergency lighting may not operate if supplied by that one phase.

The solution may be the simple addition of a phase balance protection logic system in the transfer switch that monitors each phase voltage, as the voltage relates to the three-phase parameter. If any one phase varies from a set limit, the condition is recognized as single-phasing and the load is transferred by the transfer switch to the generator power. Electrical contractors and engineers should verify that the transfer switches have this capability and set it for this condition.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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