If I remove panel cover to check circuit breaker and using a
screwdriver type of tester see if light goes on when the breaker
switch is on and it goes off when the switch is off...Does that mean
the breaker is working or is there more involved? Thanks.
Yes. There is more. You are only testing that the breaker can be
turned on and off. You did not test to assure that it will turn itself off
when it gets and overload, nor did you test to see if it might turn off at
the proper overload. Old breakers often start triggering early so they shut
down the circuit under normal load.
BTW a breaker that fails to shout down the circuit with an overload is
There are circuit breaker testers made for every manufacture that I am
Problem is that they start out at about $5k and go up from there. When I was
a field service engineer the company owned about everything that was
available. The smaller test sets for plug-in breakers, residential types,
were the least accurate.
Check out NEMA for specs on testing breakers. That is the most stringent
spec that I know.
Buying a new breaker will be easier and cheaper than testing molded case
breakers below 100 amps.
Assuming this is a 15 or 20 amp breaker, you could cause a deliberate
short, and see if it trips.
I wouldn't do this on a bigger breaker, too much current there.
Shut off the breaker, connect a short piece of wire to it, turn back on,
touch wire to ground (metal box), breaker should blow.
Cost $0.20 (2 ft of wire at 10/c a foot). Watch out for flying sparks.
If the withstand rating of the breaker is inadequate for the fault
current that the fault you are creating then the breaker may fail
explosively. If you look at the breaker you will find the withstand
rating expressed as X AIC were X is the number of amperes of fault
current that the breaker has been tested to interrupt without failure.
If the transformer that supplies your home has been replaced with a
larger unit to keep pace with neighborhood demand the available fault
current may exceed the rating of your panel or the breakers that are
installed in it. My last service upgrade was from a relatively small
transformer only one span away from the drop and the available fault
current was 8800 amperes. If the transformer had been on the same pole
as the drop the withstand would have needed to be upgraded above the
10,000 ampere value of the stock breakers.
In other words if you don't know what the available fault current is
then dead faulting a breaker is a DAMMED STUPID THING TO DO!
A normal breaker has a maximum amount of fault current it can interrupt
without damage. This is called it's withstand rating. The current it
is capable of withstanding without damage is labeled right on the
breaker. If there is more fault current available then the breaker is
built to withstand then the breaker will fail. If the fault current is
high enough it will fail explosively and bury parts of itself inside the
eyes of the idiot who dead faulted it. You are welcome to continue to
test your own breakers in the way you have recommended but to advise
others to do so is negligent. Many breakers in homes have had there
withstand ratings surpassed by the increase in available fault current
caused by changes in the supply network brought on by increased demand.
Faulting a breaker without knowing the available fault current is like
pulling the pin on a grenade without having anywhere to throw it. You
want to do that then fine but kindly stop being so reckless with the
safety of others by offering such dangerous advice to people who have
not the training to evaluate your competence.
Just plug in TWO 1500watt electric space heaters, turned on HIGH. A
15A breaker should trip in a few seconds. A 20A one should trip
shortly, but it may take several seconds, up to one minute. That is
the easiest way to test breakers without sparks. This method is ONLY
for 15A and 20A 120V breakers. Dont mess with any larger ones, or
I'm not advocating shorting out the breaker, but how would one
determine the available fault current at a service.
Also, if the available fault current exceeds the rated maximum fault
current of the existing breakers, how important is it to upgrade them?
Mr. Fixit eh
To test a fifteen or twenty ampere breaker you detach the house wiring,
attach a multi outlet assembly temporarily, plug in two 1800 watt hair
dryers, turn the first one on all the way, turn the second one on to
succesively higher settings until the breaker trips. The breaker should
trip in less than a minute at 3600 watts or 30 amperes of current flow.
If the withstand of the breakers is exceeded by say ten percent I
wouldn't be in a rush to change them. If the available fault current is
more than twenty percent over the breakers withstand rating I would have
them changed fairly soon. If the available fault current exceeded say
150% of the withstand rating I would change them as soon as the
replacements could be obtained. If breakers with a high enough
withstand are not available for your equipment then a fused disconnect
with appropriate fuses for the fault current could be installed ahead of
it but the calculation of the series rating of such equipment
combinations is beyond the skill of most electricians. In one case I
asked the power company to run a new service drop from a pole on the
other side of the house. The addition of over one hundred wire feet into
the supply wiring from the transformer reduced the available fault
current to below the rating of the equipment.
I used a heat gun, toaster oven, vacuum cleaner, and a lamp. I know
the lamp doesn't use much electricity, but I wanted one just so I'd
have something to see. And all my old Federal Pacific breakers worked
Load testing the breakers makes much more sense then the person who
said to short them out with a screwdriver. Safer too.
The failure mode of FPE breakers is unique enough that tripping them out
on overload will not assure that they will trip under fault conditions.
If you do a search of the Consumer Product Safety commission's site
you can find more information.
scott firstname.lastname@example.org (Childfree Scott) wrote in message
Here's a scary thing, I have the Federal Pacific breakers in my house
(built in '72 so the vintage puts them suspect) and I'm having trouble
with my computer's UPS beeping (low voltage?) if I turn on my laserjet
(heater inside). If I turn on the iron on that circuit it beeps
steady even if the computer isn't on. Since my breakers are this
goofy shape I thought I'd look on the net and see what my options were
and now I'm finding out people recommend replacing the whle panel.
Such a problem is extremely unlikely to be the panel or the breakers
themselves. If it was, they'd have burned out by now.
Irons and laserjets have pretty steep current requirements. If they're
on the same circuit as the UPS, and your steady-state voltage is
a little on the low side, an iron or laser (during periodic warm
up cycles) can "draw down" the voltage on the circuit far enough
for the UPS voltage alarm to go off.
Solution? Move the those things off the UPS circuit or vice-versa.
Or, report it to your power company, and they can recheck the supply
There's a more dangerous possibility - do your lights flicker throughout
the house when the iron goes on? Even more compelling, do any lights
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
This hairdryer setup would be a safe way to test the withstand rating
of the circuit breaker. How would you go about determining what the
current would be at a service location?
Mr Fixit eh
Hair dryers can be used to load test a circuit breaker but that test
will tell you nothing about the withstand rating. The withstand rating
is determined by testing to destruction at the electrical testing
laboratory. The resultant rating is on the breakers labeling.
The available fault current is determined by calculation. You need the
rating and internal impedance of the transformer as well as the type
size and length of the conductors between the transformer and the
terminals of the device that you are concerned about.
"Calculations can be based on the source fault current at the
transformer. Unless the user has fault data for the exact circuit of
interest, a conservative estimate can be achieved by starting with the
fault current at the site transformer. If desired, further refinement
can be done by calculating current reductions due to conductor
impedance. This method is most useful for smaller electrical facilities;
use at larger facilities may provide overly conservative values.
The maximum fault current available at the transformer terminals is
determined by the following formula, using data from the transformer
nameplate, which can be supplied by the power company or a site engineer.
Secondary full load current = Single phase transformer kVA
Secondary full load current = Three phase transformer kVA
Secondary fault current = Secondary full load current x 100
Percent transformer impedance
This is the maximum fault current at the transformer, which will be
reduced by conductor impedance. This reduction can be estimated using
wire characteristics for various cross-sections and lengths."
The calculation of series ratings of breakers or fuses that are down
stream from other Over Current Protective Devices is the kind of task
that electrical engineers are paid to do.
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