Methods to Reduce Clearing Time

[SonicEE]

Hello All,

This is with regards to 480V Switchgear applications. There are two(2) methods that I am looking into to reduce the clearing time. One(1) is Zone Selective Interlocking and the other is using the Maintenance Mode Switch.

I would need to propose one(1) method but not sure which is typically more common or if there are any drawback of one or the other. Any comments will be appreciated.

Thanks,

SonicEE

 

[RRaghunath]

Zone Selective Interlocking (ZSI) feature has been available with circuit breakers for long. The demerit with this (and why many clients hesitate to accept to implement) is that it depends on communication from downstream breakers to upstream breakers. Which means if the communication monitoring system is not fool proof, ZSI can become a cause for disaster.
On the other hand, Maintenance mode switch is simpler and more reliable.

 

[SonicEE]

Hi Raghunath,

Thanks. With regards to communication between the upstream and downstream relays, would that be a hard-wired connections?

Regards,

SonicEE

 

[dpc]

ZSI systems rely on communications between feeder, tie and main breaker trip units. Generally this has been some kind of twisted-pair communications cable or similar. However they are always configured as fail-safe, so if communications are lost, the breakers will trip on the fast curve since they would not receive a restraining signal. So communications failure may cause mis-coordination, but the arc-flash energy level would not change. I don't see how that could be termed a "disaster". Other than the communications circuit, the ZSI logic in the trip unit should be just as reliable as the trip unit and breaker. I recommend ZSI be included for any new low-voltage switchgear. However, ZSI and Maintenance Mode settings are not mutually-exclusive. Both methods can be provided on the same equipment at little increase in cost.

 

[RRaghunath]

dpc,
Thanks for your comment.
Agree with you. the ZSI logic is fail safe and safety of equipment and personnel is ensured when the communication is lost.
But, the loss of incomer & bus tie and thus loss of one bus section could mean loss of production in a process industry. That's what I meant by 'Disaster'.

 

[RRaghunath]

SonicEE,
Hard-wired as well as Soft communication options exist. You may check your circuit breaker trip unit / protective relay manuals.

 

[unclebob]

An high impedance GF protection would help too. As a large number of 3phase faults begin with a ground fault.