JUST SHUT DOWN A 10 Story Office Building with a 20A Circuit - Electrician Talk - Professional Electrical Contractors Forum
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Old 02-19-2020, 10:49 PM   #1
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Default JUST SHUT DOWN A 10 Story Office Building with a 20A Circuit

Short circuit on a 20A lighting circuit and it shut down the whole building!!
What would have caused this to happen? Why did none of the other breakers in the series trip?
Guessing the trip settings on the existing main were dialed down way too low?

Needless to say, my guys had a stressful morning/
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Old 02-19-2020, 11:27 PM   #2
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Phase to phase, or to ground? Spiked the CT's in the main sw. gear? But at the least I'd get the distribution, and main breaker's tested. What kind of building management system, or energy efficiency (load shed) scheme do they have?
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Old 02-19-2020, 11:37 PM   #3
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Not certain, it’s an older building. I don’t believe they have any type of management system. I am flying over Friday to take a look. The short was to ground...
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Old 02-20-2020, 12:09 AM   #4
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perhaps under normal circumstances the main breaker is already running close to the edge, and the lighting circuit short was just the straw that broke the camels back or so to speak !
so check current pull on your main.
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Old 02-20-2020, 12:24 AM   #5
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Main breaker is a ground fault type that needs calibration ?
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Old 02-20-2020, 12:29 AM   #6
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If the 20 amp circuit was 277 volts and the service is 1000 amps or more then it's pretty normal to have a simple ground fault trip the main. I've seen it happen more than once.........
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Old 02-20-2020, 01:04 AM   #7
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I’ve done it a time or two working with 277.

Didn’t @MDShunk trip the main in a mall once?
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Old 02-20-2020, 01:14 AM   #8
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But why wouldn’t the branch circuit breaker trip first? Or the breaker feeding the lighting panel?
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Old 02-20-2020, 03:03 AM   #9
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A friend of mine did that at the County courthouse, took out a number of courtrooms.
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Old 02-20-2020, 05:48 AM   #10
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Quote:
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But why wouldn’t the branch circuit breaker trip first? Or the breaker feeding the lighting panel?

Well if the system is not coordinated, a fault on a branch circuit will take out an upstream Main...
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Old 02-20-2020, 06:14 AM   #11
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I agree a coordinated study may not have been done and the setting was too low on the main.

I took out a 400 amp breaker and a 7 pole contactor with a short on a 20 amp circuit at an IBM plant in ny
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Old 02-20-2020, 06:17 AM   #12
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Quote:
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I agree a coordinated study may not have been done and the setting was too low on the main.

I took out a 400 amp breaker and a 7 pole contactor with a short on a 20 amp circuit at an IBM plant in ny
LOL at an IBM facility!?! That's priceless, they were known for running a ridiculously tight ship and letting you know about it constantly. I wonder how many of their engineers had eyes on this and it still made it past the goalie...
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Old 02-20-2020, 06:51 AM   #13
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When we do new construction...typically the engineer will specify the settings on the main breaker or on the Protective relays.....this often gets forgotten by the EC.

The instantaneous trip or the GFCI settings are probably at the minimum
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Old 02-20-2020, 07:33 AM   #14
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Quote:
Originally Posted by Hawaii Electrical View Post
Short circuit on a 20A lighting circuit and it shut down the whole building!!
What would have caused this to happen? Why did none of the other breakers in the series trip?
Guessing the trip settings on the existing main were dialed down way too low?

Needless to say, my guys had a stressful morning/
They certainly have a coordination issue.
My conversations begin with something like:
"I want this event to be my responsibility but, this simple lighting short has exposed a deficiency in your circuit breaker coordination all the way to the main breaker. This will keep happening until it gets resolved"
Would you like us to commission a coordination study and present you with the remediation plan or do you have an engineer that can help you with it?
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Old 02-20-2020, 07:43 AM   #15
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Fastest trip curve wins.
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Old 02-20-2020, 08:04 AM   #16
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Fastest trip curve wins.
Just like sperm.
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Old 02-20-2020, 09:55 AM   #17
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Coordination study....
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Old 02-20-2020, 10:15 AM   #18
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But why wouldn’t the branch circuit breaker trip first? Or the breaker feeding the lighting panel?
No coordination. Upstream fuses?
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Old 02-20-2020, 11:08 AM   #19
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But why wouldn’t the branch circuit breaker trip first? Or the breaker feeding the lighting panel?
The branch breaker and the panel feeder breaker are still thermal/magnetic. The main is GFP and set wrong.
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Old 02-20-2020, 11:11 AM   #20
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But why wouldn’t the branch circuit breaker trip first? Or the breaker feeding the lighting panel?

I’m not entirely sure the mechanics of why, I was probably not paying attention at that point of my apprenticeship but I do know that a 20A phase to phase short has the potential to drop an entire building GFI.
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