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Opening breaker under rated load. #2769
07/24/01 10:52 AM
07/24/01 10:52 AM
R
Redsy  Offline OP
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
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.

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Re: Opening breaker under rated load. #2770
07/24/01 02:56 PM
07/24/01 02:56 PM
A
Anonymous
Unregistered

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.

Re: Opening breaker under rated load. #2771
07/24/01 03:37 PM
07/24/01 03:37 PM
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
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


Few things are harder to put up with than the annoyance of a good example.
Re: Opening breaker under rated load. #2772
07/24/01 07:05 PM
07/24/01 07:05 PM
C
CanadianSparky  Offline
Member
Joined: Mar 2001
Posts: 85
Alberta Canada
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.

Re: Opening breaker under rated load. #2773
07/24/01 07:14 PM
07/24/01 07:14 PM
A
Anonymous
Unregistered

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

Re: Opening breaker under rated load. #2774
07/24/01 07:37 PM
07/24/01 07:37 PM
R
Redsy  Offline OP
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Quote
Originally posted by CanadianSparky:
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.


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.

Re: Opening breaker under rated load. #2775
07/24/01 10:39 PM
07/24/01 10:39 PM
S
sam  Offline
Member
Joined: Jan 2001
Posts: 29
I believe you would have to make a really
Quick judgement call on that situation.
Hope no one was injured in this incident!

Re: Opening breaker under rated load. #2776
07/25/01 05:49 AM
07/25/01 05:49 AM
R
Redsy  Offline OP
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Quote
Originally posted by sam:
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.

Re: Opening breaker under rated load. #2777
07/25/01 01:16 PM
07/25/01 01:16 PM
G
glenn35  Offline
Member
Joined: Jul 2001
Posts: 40
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. [Linked Image]

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 [Linked Image]

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

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

Re: Opening breaker under rated load. #2778
07/25/01 11:19 PM
07/25/01 11:19 PM
F
frank  Offline
Member
Joined: May 2001
Posts: 376
windsor ontario canada
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).]

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