Coordination with ZSI

Grouch

Grouch

Senior Member
Location
New York, NY
Can Zone Selective Interlocking (ZSI) be used to meet selective coordination requirements between an upstream and a downstream breaker?

Or do the breakers have to coordinate, regardless of ZSI?
 
I don't see why not - that's the entire point of zone selective interlocking. Once it is installed and tested, it is part of the protection system, just like the overcurrent elements in the breakers.

It might help to know what "requirements" you are talking about. In general, there is no NEC requirement that electrical systems be selectively coordinated and in many cases it is not possible to achieve this. Hospital life safety systems are the main exception to this.

ZSI is commonly used to reduce arc flash incident energy.
 
D. Castor said:
It might help to know what "requirements" you are talking about.
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The requirements are in the NYC amendments to the NEC:

SECTION 240.12
Subsection 240.12(A)
(A) Service Overcurrent Protective Device: Where the service overcurrent protective device (OCPD) rating or setting is above 601 Amps, such device shall be selectively coordinated with the next downstream OCPD.

I have a project where my service switch is 3000 amps, 277/480 volts, and requires GFP. I haven't sat down to thoroughly analyze the curves, but I can see already the curve of the GFP relay will overlap with a downstream device. Any ground fault magnitude in that region will cause the service switch to trip before the downstream device.

ZSI I believe can prevent this, since the downstream breaker will be forced to trip immediately, and prevent coordination issues with the GFP relay. And yes, ZSI is an option available on both upstream (service switch) and downstream breakers.
 
Grouch said:
Can Zone Selective Interlocking (ZSI) be used to meet selective coordination requirements between an upstream and a downstream breaker?

Or do the breakers have to coordinate, regardless of ZSI?
Click to expand...
This might be a question for... the AHJ 😭... which means I can reach out to the EPR review team (formerly advisory board) and see what they say, since they are the ones who approve the drawings and any coordination studies.
 
I have always considered selective coordination, sych as life safety systems, to be independent of external communications requirements that do not have an automatic integrity management.

I have used ZSI regularly for coordinating protection between OCPD levels/zones in process applications and for arc flash reduction with regular maintenance and testing.
 
jim dungar said:
I have always considered selective coordination, sych as life safety systems, to be independent of external communications requirements that do not have an automatic integrity management.
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So you're saying that selective coordination should be accomplished without the use of external communications... aka ZSI (the control wiring between the OCPD's)... unless the ZSI has automatic integrity management?
 
You would need ground fault protection on the downstream breakers and ground fault ZSI to make this work. The advantage of ZSI is that it allows the upstream devices to operate more quickly if the fault is not detected by the downstream breakers. The normal "slow" curve for the upstream breaker should still coordinate with the downstream breakers.

ZSI systems are generally designed to be fail-safe - if communications is lost, the upstream breaker will trip on its unrestrained "fast" curve. Protection is prioritized over coordination.
 
D. Castor said:
You would need ground fault protection on the downstream breakers and ground fault ZSI to make this work. The advantage of ZSI is that it allows the upstream devices to operate more quickly if the fault is not detected by the downstream breakers. The normal "slow" curve for the upstream breaker should still coordinate with the downstream breakers.

ZSI systems are generally designed to be fail-safe - if communications is lost, the upstream breaker will trip on its unrestrained "fast" curve. Protection is prioritized over coordination.
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Why is GFP needed on the downstream breakers?
 
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