We recently had a situation where a water line burst and some of the falling water made it's way to above one of our MCCs.
I shut off the 600 amp main breaker, in order to de-energize the MCC. Typically our load is 200-300 amps (@ 480Y/277). Someone suggested that I should have shut off the individual breakers prior to opening the main. I see no reason do this, particularly in this situation, however I would like opinions on opening the main (not necessarily regularly) under it's rated load.

I agree.
If I felt there was time, I would probably dump some branch loads first, not for the sake of the main CB, but for the sake of sensitive equipment out there somewhere that might not like being hammered.

With larger loads, dumping some stuff makes it more likely that certain types of switches will open and reduces the arc intensity when they are opened.

Should be no problem, after all, that breaker should be able to interupt a fault several times higher than it's rating without doing damage.

Tom

Wow, where i'm from I'd never even think about turning on a main without shedding the load first. You can break down a large inrush current by bringing on the loads a little bit at a time, especially with motor loads.

>turning on a main
He was talking about killing the main before it got wet.

I understand your point and agree fully. Inrush can be minimized by bringing the load on gradually. I was discussing turning the power off, though.

I believe you would have to make a really
Quick judgement call on that situation.
Hope no one was injured in this incident!

Luckily, nobody was hurt and equipment damage was minimal.
I would have preferred to shut the feeder off at the substation supplying our building. However, up until this incident, we did not have a key. I had to decide quickly what to do.

Isn't that one of the purposes the main was installed in the first place? To provide a means of disconnecting power quickly.
I see nothng wrong with it, because it is designed for that purpose.

The fire dept or the utility is not going to take time to go around and find other disconnects to releive load!

You did the right thing. Don't fret over it.

If the accusers were there then why didn't they do it? Or were they there and didn't know quite what to do? DUH DUH

Hind sight is 20/20 and talk is sometimes cheap.

[This message has been edited by glenn35 (edited 07-25-2001).]

gelnn35 is right and i belive pissing around de-energizing the load side of the MCC while water is leaking into the enclosure would be the worst course of action.ABOVE ALL THE MAIN CIRCUIT BREAKER BY FAR IS THE SAFEST PLACE TO DE-ENERGIZE OR ENERGIZE ANY LOAD THAT COULD COME UNDER A FAULT CURRENT CONDITION.The load at risk included the line side of the MCC.


[This message has been edited by frank (edited 07-26-2001).]

Redsy, I also think you did the right thing. If you had not the water would have tripped the main. You just beat it to the punch!

Shutting down in an emergency is one thing, but starting with a full load is not a good idea. Arc, even arc supressors are for light loads not for peaks.

[This message has been edited by Resistor (edited 07-26-2001).]

I received a message from Bill today. I may owe a lot of people an apology. But this is one thread where I have regretted that I failed to quote the person to whom I was replying. Since my reply was the first, I thought it was obvious when I said that I agreed, that I was agreeing with Redsy.
Much to my horror, I later realized that it could be ambiguous as to what I was agreeing with.

Redsy had said, "I see no reason do this," and I said that I agree.

I know there have been other threads where someone said that he disagreed or that something was wrong, but we could only guess with whom he might be disagreeing or what he thought was wrong.

As for me, when I quote something, it is so the readers can tell to whom I am replying. It is a crude attempt to turn posts into a conversation. I meant no harm. I'm not the only person who quotes. And I try to make any quotes pertinent, not just a complete copy of a preceding post.

I apologize to anyone who is offended by this style. It's a technique that works for me. I find that it makes it easier to get to the heart of a matter by identifying exactly where we agree and where we don't.

Naturally I'm interested in hearing from folks who agree as well as those who don't.
If everyone thought the same as I do, I wouldn't learn much at all.