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Old 07-06-2010, 09:57 AM   #1
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How to test that a breaker trips on short


My wife replaced the lamp socket on an old lamp and mixed up the terminals and created a short. When she plugged in the lamp it caught on fire. I don't understand why the breaker did not trip (unless is it defective). My understanding is that the magnetic trip on a short is almost a sure thing. Also if the individual circuit breaker was bad the main breaker for the panel should have tripped which it did not. I have two questions.

First how can I safely test the other circuits for this same problem?

Second, assuming that the breaker is not defective, any ideas on why this would happen? The circuits in question are on a subpanel in our pool house. The subpanel was added by a previous owner and all work they did is suspect so I assume this will end up being an instillation problem.

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Old 07-06-2010, 11:36 AM   #2
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How to test that a breaker trips on short


well, generally swapping the hot and neutral on a light fixture won't actually cause any problems in operation. The screw shell of a fixture is required by code to be connected to the neutral but that is a safety factor. A neutral can generally be touched by a person and not shock them. The shell is somewhat exposed, even when there is a lamp in place so it makes sense to connect the neutral to the screw shell.


for a breaker to trip on a short, there has to be enough current flow to trip the breaker. That will be many times the rating of the breaker. If there is a short but it is of high resistance, while the short will heat up as current flows, it might not allow enough current to flow to trip the breaker on the instantaneous trip portion of the breaker.

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First how can I safely test the other circuits for this same problem?
You really can't. Breakers are a lot like religion. You have to have faith they will do what they are designed to do.

There are certain brands of breakers that are known be have been problematic. Many of them are no longer made due to the problems.

You should also exercise your breakers annually. That simple means you flip the breaker on and off several times. A breaker can get stiff as they set basically unused for years. The exercising helps them remain properly functional.

why it didn't trip the breaker? I would have to guess the short she created was not low enough resistance to allow enough current to flow to trip the breaker. Without knowing actually what she did, there is no way to really know.

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Old 07-06-2010, 12:03 PM   #3
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How to test that a breaker trips on short


It was not an issue of swapping the hot and neutral. She had a single bulb lamp but the new socket she got supported a secondary socket. As in the first diagram here: http://www.do-it-yourself-help.com/wiring_diagrams_lamp_switches.html

She correctly connected the hot to the brass terminal but did not connect neutral to the silver terminal but to the black terminal instead. This created the short.

Why not just momentarily short the hot to neutral or to ground with a screw driver? I do this as a double check before I start working with any exposed wires just to make sure I have the correct breaker off.
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Old 07-06-2010, 12:09 PM   #4
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How to test that a breaker trips on short


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My wife replaced the lamp socket on an old lamp and mixed up the terminals and created a short. When she plugged in the lamp it caught on fire. I don't understand why the breaker did not trip (unless is it defective).
When was the last time you toasted a slice of bread? Notice how a toaster can glow red hot without tripping a breaker? Something probably went wrong and created a resistive connection that just plain got hot without exceeding the capacity or the breaker.
Were you able to identify where the fire actually started by looking at the leftover pieces?
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Old 07-06-2010, 12:21 PM   #5
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How to test that a breaker trips on short


Yes, it started inside the socket.
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Old 07-06-2010, 12:27 PM   #6
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How to test that a breaker trips on short


i would like to strongly advise you to test for voltage using a much safer method than creating a dead short. Use a voltage meter instead.


If you need to see proof just search "arc blast" or "arc flash". Warning some of these will be extremely graphic.
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Old 07-06-2010, 12:52 PM   #7
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How to test that a breaker trips on short


as well, not that anybody does this but every breaker manufacturer I have researched this for recommends that you replace a breaker once it has tripped on a short circuit fault. They will not warranty the functionality after such an event.

as I said, nobody follows this rule and I believe it is simply to relieve the manufacturer of liability should a subsequent short circuit fault not cause the breaker to trip but in their defense, the breaker can be subjected to thousands of amps in that instant. As such, the dependability of the breaker should be suspect.

as to the connections: which neutral did she connect to the black terminal and was anything else also connected to the black terminal.

explain exactly what was connect to what. When you say neutral, there are 2 neutral wires in the diagram and I don't know which one was connected where. I would make a difference to what would happen.
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Old 07-06-2010, 01:12 PM   #8
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How to test that a breaker trips on short


Ignore that second little socket/bulb pictured inside the lamp "B". Her lamp does not have that feature. It is your typical single socket/bulb lamp. When she got the new socket she got the wrong type she got the "2 circuit lamp switch" which is socket A in the diagram. The nutral that was connected to the black terminal was the nutral from the plug. Nothing else was connected. Just nutral from the plug to the black terminal, hot from the plug to the brass terminal. Only 2 wires involved. A CFL bulb was in the socket and she pluged it in, then the problems started. I assume that the socket switch must have been in one of the two positions that lights up the optional secondary bulb.
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Old 07-06-2010, 10:10 PM   #9
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How to test that a breaker trips on short


To trip a 15 amp breaker, current in excess of 15 amps must be drawn.
To carry 15 amps safely, 14 guage wire is required by most electrical codes.
Lamp cord could be as little as 20 guage and if current of 15 amps is carried on 20 guage the conductor will start to heat.
If it heats up enough, the insulation will melt and even begin to flame.
Breakers are designed to prevent fires from ocurring in building structures, not in the various devices that may be connected to the circuit.
They do not even prevent electrocution. If full contact is made to a 15 amp circuit, a person could die, and the breaker would never trip.
Grounding, that we are all familiar with, is used to trip a breaker, incase of an appliance fault, before a human comes in contact. If the grounding fails, a user would be at serious risk.
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Old 07-07-2010, 07:39 AM   #10
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How to test that a breaker trips on short


Do not try to test a circuit breaker for short circuit performance or short circuit purposes.

In addition to possibly damaging the breaker as described above, you will create sparks that may scare the living daylights out of you.
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Old 07-07-2010, 03:45 PM   #11
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How to test that a breaker trips on short


two rules i like to live by.
1. the more times a breaker trips the more likely it is to fail
2. If you suspect a breaker of being bad...don't test(refer to rule one), replace it.
is your life worth less than the 10 bucks for a new breaker?
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Old 07-07-2010, 04:15 PM   #12
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How to test that a breaker trips on short


I was never too concerned that the breaker was bad, only that somehow the panel and or breaker was installed incorrectly. I called Schneider Electric and they assured me that this is not the case. The tech agreed with some of other posts here that the lamp socket internal wiring or the lamp cord itself were not of low enough gauge to allow enough current flow to trip the breaker.
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Old 07-07-2010, 05:15 PM   #13
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How to test that a breaker trips on short


FWIW,

To test 15A and 20A breakers I put a 50A spare breaker across the line and switch it on. I have an LED wired in to tell me when the voltage disappears, which has always been 'instantly'. If the lower amp breaker doesn't trip the 50A will, and if the LED doesn't go out I switch off the 50A breaker after one second or so.
I do not do this in houses not my own.

The resi short circuit at an outlet is limited to typically 400A. At my load center the measured short circuit was 11,000 A. If you want personal protection level 2 arc flash gear is supposed to be sufficient for residential.

You can check your breaker and wiring system at lower amperages, say 30A, by plugging in 3 hair dryers and checking the breaker's online trip curve for how long it should take to trip. Sometimes 10 seconds to 60 seconds is an acceptable interval for this kind of overload, so apparently house wiring and its insulation can easily tolerate this I-squared-T energy dump.

If you put it across a car battery it should trip instantly. Wear face protection in this case.

"the lamp cord itself were not of low enough gauge to allow enough current flow to trip the breaker." I disagree. The current will be about 400A.

#16 copper wire in free air should take 5 seconds to melt at 180A.
http://www.home.earthlink.net/~jimlux/hv/fuses.htm


This breaker testing issue comes up occasionally on forums and it is always controversial.

Last edited by Yoyizit; 07-07-2010 at 05:37 PM.
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Old 07-08-2010, 08:45 AM   #14
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How to test that a breaker trips on short


Q to a CB manuf:
For residential circuit breakers, what is the minimum and typical design lifetime, in number of trips or years?

A:they are tested per UL at 6000 times full load, and 4000 times no load.

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