Coordination with ZSI

[Grouch]

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?

 

[Grouch]

I assume the answer is the latter.

 

[D. Castor]

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.

 

[Grouch]

It might help to know what "requirements" you are talking about.

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]

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?

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.

 

[jim dungar]

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.

 

[Grouch]

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.

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?

 

[D. Castor]

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.

 

[Grouch]

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.

Why is GFP needed on the downstream breakers?

 

[jim dungar]

Protection is prioritized over coordination.

Which would be a problem in the specific areas where the NEC requires Selective Coordination.

 

[D. Castor]

I think that would be up to the AHJ. How many failure modes are we required to design for? It's quite possible that a standalone breaker will fail to operate at all, regardless of the settings.

 

[D. Castor]

Why is GFP needed on the downstream breakers?

ZSI simply allows more sensitive settings to be used - it doesn't provide any additional protection - it just trips the breaker faster. To coordinate, the downstream breaker's ZSI has to send a signal indicating it has detected a fault, so without GF, the only way this works is if the phase trip setting on the downstream breaker is lower than the ground fault setting on the main breaker. This might work if the feeder breakers are small enough, but in general, you should have GF on the feeders that signal the main breaker ZSI when a ground fault is detected, since the GF can be set below load current. If the phase trip on the downstream breaker coordinates with the ground fault on the upstream breaker, then it's already coordinated, even without ZSI. For a fault downstream of the feeder breaker, ZSI doesn't really change anything. Both devices will operate on their normal "slow" curves.

Also, ZSI only works for low voltage power circuit breakers, not molded case.

 

[jim dungar]

Also, ZSI only works for low voltage power circuit breakers, not molded case.

That is manufacturer and type dependent.

I have used ZSI with the Schneider Electric Square D industrial breakers, like I-Line, for decades.

 

[D. Castor]

I have used ZSI with the Schneider Electric Square D industrial breakers, like I-Line, for decades.

Interesting, thanks. I've never seen that done in 50 years. I've lived a sheltered life, apparently. I don't see how a MCCB could be used as the upstream device since the mandatory instantaneous trip would make it difficult to coordinate when it's restrained. Some Square D breakers can be rated as either MCCB or LVPCB depending on how they are ordered, but I don't think that includes I-line. I'm always learning something.

 

[jim dungar]

I don't see how a MCCB

Electronic trip units with ZSI have been available in I-Line for +40 years.
The instantaneous trip point can be delayed up to a self-protection point required to pass the UL tests. Yes, the ability to selectively coordinate breaker is dependent on the amount of available short circuit circuit current. Luckily branches usually have lower SCA than their larger upstream feeders.

Very few MEP's specify these for ZSI because it limilts 'competitive bidding'.

 

[ATSman]

Although the info provided does not specifically address the OPs question on ZSI, I think it is valuable enough to note and should be discussed. I did not know how to reference a past post so I copied it to the attached file.