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Use of interlock between LV CB for GF, ST and Instantaneous protection

Use of interlock between LV CB for GF, ST and Instantaneous protection

Use of interlock between LV CB for GF, ST and Instantaneous protection

(OP)
thread238-120722: Unrestricted earth fault protection

In a post by mc5w

"(Electrical) 10 Apr 05 2:07  
U.S. rules REQUIRE feeder ground fault protection immediately downstream of the service in a hospital. There must be a restraint wire running from the feeder circuit breakers to the service circuit breakers to prevent the service from tripping while a feeder circuit breaker detects a ground fault.

Is would not be a bad thing to copy this idea"
 
mc5w is referring to a US National Electrical Code (NFPA 70, aka NEC, published every 3 years by NFPA[National Fire Protection Association]) requirement in article 517. He  states that an interlock signal between feeder protection and the main incomer protection is required. That is not the case. He is correct in that two levels of GF protection are required on the portion of the electrical system fed by the normal source (typically the Utility Company).  The interlock signal, however, is not required, but it is often specified as well. The interlock signal, part of the "Zone-Selective-Interlocking system", also known as "ZSI", is an enhancement to speed up the protection at the main incoming main device.  ZSI can also be used for short time delay acceleration of the main incomer CB.  Recently at least one global manufacturer has introduced in the US and other markets a CB able to also, and separately, zone interlock the instantaneous protection function.  This provides the ability provide selective (i.e. discriminating) protection and instantaneous protection at the same time. In the case of instantaneous ZSI, ZSI is able to improve protection and discrimination simultaneously. In the case of short-time and ground-fault ZSI is able to improve protection but the discrimination is equal to what would be possible without ZSI.
To repeat, it is true that 2 levels of GF protection is required by code in the US, but the interlock is an optional enhancement.
From the perspective of enhancing protection, and in the case of instantaneous ZSI enhancing both protection and discrimination, particularly with respect to "Arc Flash Hazard", I endorse mc5w's statement about adopting this practice with proper consideration of the wiring requirements and limitations of the scheme in multiple source systems.
By way of disclosure I am in the USA and I am an application Engineer for a large Electrical Equipment manufacturer.
 

RE: Use of interlock between LV CB for GF, ST and Instantaneous protection

Welcome to the Forum. Nice job for a first post. thumbsup2


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