Tripping speed of a relay and how it compares to safety

[rockman7892]

In our plant we have a 5kV SHD-GC trailing cable feeding a piece of equipment in our mining operation. Because of recent copper thefts in the area management wants to de-energize this cable when not being used (nights and weekends)to avioid somebody cutting into a live cable and thus possibly having a lawsuit on our hands.

I agree with managments decision to de-energize this cable but was just curious, could a relay be set quick enough to de-energize the cable if someone cut into it before they were electrocuted or injured. In other words even with the most sensetive settings, what would happen first the breaker tripping or someone being injured. I guess this depends on how they cut into it, weather it is a line-line or line to ground.

As I mentioned the cable is a 5kV 4/0 SHD-GC cable which is proteced by a 5kV ABB breaker with a powerline electromechanical relay. The breaker/relay combo uses a zero sequence CT for ground sensing, and we are resistance grounded here limiting our ground current to 400A. Upstream of the breaker there is 5kV load break switch with type E fuses.

When we do decide to open this circuit every night, is it better to operate the breaker each time, or open the load break swtich evertime. Which is better to put the wear and tear on?

 

[Zogzog]

Not sure if you can be sued by a copper thief trespassing on your property getting hurt, thats up to legal I guess but dosent seem right. Maybe just add a sign like the one below.

The quickest relay in the world still wont stop someone from getting injured cutting into a live cable, the fault current needs to get high enough to trip the relay and by then it is too late. Is this a 5HK breaker? THey are built pretty well and would be the better choice of the 2 to operate daily, but you will need to decrease your PM periodicty.

 

[rockman7892]

Zogzog

I believe this is a 5HK breaker but I'm going to check. I agree with you that it would be easier and better to operate it daily. I just found out there is actually a second ufused disconnect that is right before the breaker and obviously after the fused disconnect. Is there any truth to a chance of having a fuse blow as a result of operating a switch frequently.

Also with the switch open the breaker looses power and should trip anyway thus having to reset it as well.

I also forgot that this breaker has a groud check monitor on it.

Lets say someone cut into on leg and caused a L-G fault, even with a ground pickup of 5A I guess it would probalby be too late?

 

[cranky108]

Just consitering the speed of a typical breaker even a 5a setting would happen to late to prevent someone from getting hurt.