Zone Selective Interlocking

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JoeStillman said:
I had my first experience with it this week and it has me scratching my head. What's the difference between ZSI and a proper coordination study?
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The type of ZSI I am familiar with, Digitrip,FP5000,EDR5000,etc does not replace a proper coordination study. It is used as a substitute for bus differential protection. If the individual OCPD is not set correctly thirty feet of copper can disappear very quickly.
 
A ZSI system requires the upstream and downstream circuit breakers to communicate with each other. In a fault condition, if the downstream circuit breaker sees the fault, it will prohibit the upstream circuit breaker from opening. Without this prohibiting signal, the upstream circuit breaker will open as expected. A coordination study will allow circuit breakers to be selected so that their TCCs do not cross, meaning that the immediate upstream circuit breaker will trip during a fault condition. This delay can greatly increase the arc flash hazard since the upstream breaker may never trip in the instantaneous range. ZSI is a way to provide a coordinated system without driving up the arc flash hazard. As SG-1 mentioned, the same protection can be provided with bus differential relaying.
 
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