Strange subpanel behaviour

Old 03-30-03, 05:12 AM
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Location: Central NJ
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Strange subpanel behaviour

I have a 50 A subpanel in my workshop that powers both workshop receptacles and workshop lights that are on separate subpanel breaker. When I once accidentally shortcut the lights circuit the lights breaker triggered AND subpanel 50 A double pole breaker in main panel also triggered. I didn't expect this to happen. I thought that subpanel breaker should never trigger when any subpanel internal breaker triggers. When I use my table saw connected to workshop receptacle and the blade stalls this triggers 20 A subpanel breaker but main panel 50 A double pole sub panel breaker never triggers. Could anyone explain to me why in the case of shortcut both breakers trigger.
Old 03-30-03, 05:55 AM
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There is some good news here. Everything is wired correctly and its working. It happens all the time and I have had it happen from a single strand on a number 16 extension cord. During a short, or a fault your electric service can deliver thousands of amps and both breakers tripped simaltaneously. When you tripped a breaker during an overload condition it did so because it continiously exceeded a draw of 20 amps or whatever the rating of the breaker was and it went off because of overheating. Nothing to worry about, infact quite the opposite really,,, should give you a warm fuzzy feeling all over.
Old 03-30-03, 06:11 PM
P Michael
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I've seen this happen.
Jason told Brian to drill a hole for the anchor bolt in the concrete floor exactly where it happened to be. Nobody knew it but there was a conduit feeding the water heater [270/480] exactly there. So Brian started drilling and suddenly there was Poof!
This blew the breaker in the panel. It also blew the breaker in the main panel for the building. It also blew two breakers in the central electrical room putting four college classrooms out of comission for a while.
I think it might have something to do with the precise timing of the event. That is, with AC current you have the current going up and down 60 times a second. If you short it when the voltage is crossing the neutral, you won't get much POOF! If you short it when it's at the maximum, you get a big POOF.
Old 03-31-03, 03:11 PM
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This phenomina is mainly related to the rating of the panel. Most likely these were series rated panels. In a series rated panel during an extremely heavy fault the main as well as the branch circuit breakers will open nearly simultaneously. The purpose is simple, opening the circuit in 2 places reduces the fault current that the breakers actually see.

A good example is a Square D panel with a 20000 AIC main breaker and 10000 AIC branch circuit breaker. When the fault current exceeds 10000 amps on the branch circuit the main will open as well to reduce the fault current present at the branch breaker. This will basically keep the branch breaker from exploding.

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