Prove It! All you need to know about the witness test

National Electrical Code

The National Electrical Code has several requirements applicable to emergency systems, legally required standby systems, essential electrical systems and optional standby systems. From a basic performance standpoint, the standby power system is intended to supply power when the normal electric service is interrupted. In many occupancies, the emergency system must illuminate an egress path for a period of time long enough that occupants can safely exit the building or structure.

Some essential electrical systems, in hospitals for example, are required to supply power for significantly long periods of time while the occupants remain in a defend-in-place state. The NEC is primarily a prescriptive-based set of installation rules, with very few performance-based requirements. The required witness testing for emergency systems is one such performance-based requirement that is a responsibility of the authority having jurisdiction. A written record of the test is required.

This article reviews some important elements of conducting and witnessing performance tests of an emergency system, a legally required standby system, and the life safety branch of the essential electrical system in a health care facility. It is important to understand that the purpose of a standby system witness test is to verify the system will successfully perform as intended during loss of normal power and for the duration of time required by the Code for the applicable system being tested. The best way to verify this performance is to simulate a power failure.

Sections 700.3 and 701.3 provide the minimum requirements that must be tested and witnessed by the authority having jurisdiction. These rules are the same for each type of system. Although the title of these rules is “Test and Maintenance,” this article will focus on the witness testing requirements. These two directly relate to one another, but “maintenance” will be covered in more detail in a subsequent writing.

Now on to some requirements for conducting and witnessing a performance test of the system. First, it is important to know what type of equipment is installed and how it works. As an example, a battery-backup emergency lighting system (unit equipment) will perform differently thatn an emergency system that has a combustion engine generator as the emergency source. The specific requirements, including performance requirements, are provided in Sections 700.12 and 701.12, respectively.

The basic requirements for witness testing include verifying that the complete system operates, and it is tested under anticipated load conditions. These tests are often done in darkness so proper minimum foot-candle levels are illuminating the floor and egress paths can be verified.

Another part of the witness test verifies capacity, and if a generator and transfer equipment are part of the system, the starting of the generator and transfer to the emergency source must occur within a 10-second limit in accordance with 700.12. For legally required standby systems, the time limit for generator start and transfer to the standby source must not exceed 60 seconds. Once again, this is verified by witnessing.

Some other requirements to verify during the witness test are provided in Sections 700.6 and 701.6. They are the required audible and visual “signals” related to the emergency source. These signals indicate at least four key statuses: derangement of the emergency source, whether or the not the emergency source is carrying the load, any malfunction of the battery charger and whether the source has a ground fault on the system. See NFPA 110-2019, Standard for Emergency and Standby Power Systems.

A basic witness test of emergency lighting from unit equipment is required to be performance tested. There are a few important items to look for during this test. The first thing to verify is that the unit equipment for emergency lighting is connected to the same branch circuit as normal lighting for the area in accordance with Section 700.12(I) and 701.12(J). Verify that the charging light on the unit is on.

To simulate power failure, the area lighting branch circuit can be shut off at a minimum or the main could be opened; either operation should cause the unit equipment to function. The exit luminaires are often also “unit equipment” with batteries and chargers. This type of standby illumination must remain operational for 1.5 hours and not fall below 60% of the initial illumination.

Emergency lighting systems must be designed and installed so that if an individual lighting element fails, it does not leave in total darkness any space that requires emergency illumination. Where high-intensity discharge lighting such as high- and low-pressure sodium, mercury vapor and metal halide is the sole source of normal illumination, the emergency lighting system must operate until normal illumination has been restored, which often includes time-delay features that are verified during the witness testing. The reason relates to the cool-down time for these types of luminaires.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Correlating Committee; chair of the NFPA Electrical Section; and a member of the IBEW, NFPA Education Section and the UL Electrical Council. Reach him at mj...

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