Use of shunt trip for earth fault protection 3

[paliha]

I have come across many earth fault protection arrangements, where the shunt trip of the MCCB along with an earth fault relay is used to isolate a circuit with earth fault. This arrangement is not fail safe for the following,

1) Power supply for the micro relay / earth fault / shunt trip is obatained through a fuse. Opening of the fuse will will not be noticeable, but earth fault protection is out of operation.

2) Failure of the shunt trip will also lead to the same situation.

Is there a better way of interfacing the earth fault relay for fail safe operation?

 

[rbulsara]

There are always better ways to do everything. You also get what you play for and there has to be justifications for going with more expensive schemes.

To answer your question, you can provide a more reliable power source for the shut trip and relay. Such as a battery back up, etc.

Any component can fail any time, but that is why regular testing is recommended.

You can also use the MCCB with electronic trip units with built-in EF function in it.

Just remember that EFP is not required by Code for all circuits, but if you application dictates it, its fine. EF protection can also be source of nuisance trips or miscoordination.

 

[davidbeach]

You could also use an undervoltage release rather than a shunt trip so that loss of control voltage results in a trip. If the application truly requires fail safe you need redundancy.

 

[rbulsara]

Typo correction: play=pay.

 

[dpc]

As David Beach points out, as long as you are depending on a single circuit breaker to clear the fault, the system is vulnerable to a single point failure. In many cases, there will be another circuit breaker upstream that will clear the fault (more slowly) if the downstream breaker fails to operate for any reason.

In the US, energize-to-trip is by far the most common approach, even for critical applications. For improved reliability, dual battery systems, dual trip coils, dual relays etc are often used, in conjunction with some type of breaker failure logic.

 

[paliha]

I wish to thank you for all the information. As dpc pointed out, in most cases there will be protections upstream. Undervoltage suggested by davidbeach is fail safe, but will need resetting after power outage.(i think) So it is a matter of choice considering the circumstances.

 

[alehman]

This is a very common scenario. Typically the control power source is tapped upstream of the device that is being tripped.