ZSI Trip units

[sahelrahman]

I am currently working on coordination study of a project. The curves of few breakers are overlapped. So it is tough to say which one will trip first. But what I understood is that, since the breaker trip units used has the characteristic of Zone selective Interlocking (ZSI) , the breakers communicate with each other and it makes sure that the downstream breaker trips before the upstream breakers. Am I right?

 

[alehman]

ZSI main function is to allow the circuit breaker closest to the fault to operate at minimum delay, while signaling the upstream breaker to operate with delay per it's setting. It does not guarantee coordination.

ZSI normally functions only for the short-time delay band and ground fault. Obviously the communication must be wired to work.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[sahelrahman]

Hi Alan,
Thanks for the valuable info. IS ZSI a function of short time delay? We need to specify the delay for each breaker right? So if we specify a minimum delay for the downstream and more delay for the upstream breaker, I assume that the downstream breaker trips before the upstream breaker even if the curves are overlapped. Correct me if I am wrong.

Thanks

 

[jghrist]

You specify only enough delay in the upstream breaker to ensure that the signal from the downstream breaker is received. The downstream breaker is set normally with inverse time-overcurrent and/or short time and instantaneous overcurrent.

If the downstream breaker senses fault current (above pickup value) then it sends a blocking signal to the upstream breaker. If the upstream breaker senses a fault but does not receive a blocking signal, then the fault is assumed to be on the bus and the breaker trips with a very short time delay.

If a blocking signal is received by the upstream breaker, then it trips with inverse time-overcurrent, set slower than the time-overcurrent of the downstream breaker.

 

[dpmac]

Alehmans comment about the ZSI being wired should not be taken lightly. Just because the breaker trip unit has the capability for ZSI does not mean the switchgear is wired to use it. Have seen it happen a number of times.

 

[alehman]

sahelrahman,
If you use one manufacturer's product line, you may not need to specify the minimum delay on the upstream device. This should be coordinated by the manufacturer.

You should review the manufacturer's literature carefully. These functions are usually described in detail.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Zogzog]

Most of the ZSI capable trip units I have seen in service were never set up to communicate. I recomend testing the system before making any assumptions.

 

[sahelrahman]

Thank you guys for responding to my thread. That was informative...