Short-fault Vs Overcurrent Vs Arc-fault Devices

Mark Twenhafel

Kinda' a newbie question, here:

Was reading the Industrial Control Panel article in Ch 4 of the NEC today, and something stuck out at me--one of the subsections talking about overcurrent-protection devices being required seemed to draw a distinction as if short-fault protective devices, arc-fault protective devices and overcurrent protective devices are all distinct "flavors" of devices. I.e., as if you can go down to the supply house and get one of each.

I'm limited in my experience to pulling romex in residential, so sometimes I have to try to picture this stuff in the code that's outside of my experience.

I'm familiar with overcurrent protection devices--the familiar breakers that we put in the panelboard--but in my mind, breakers also serve as short-fault protection devices. And, if you order them GFCI, they are also ground-fault protection devices.

So, why does the code seem to imply that they are discrete, distinct devices--can they be?

Thanks,
Mark

Magnettica

An overcurrent protective device (circuit breaker), does three things:

1) Provides overload protection
2) Provides short-circuit protection
3) Provides ground fault protection

And then there's all kinds of different fuses (non-time delay, dual element, instantaneous trip, inverse time).

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Timster

How an afci basically works: iaei.org/subscriber/magazine/00_d/gregory.htm

I did a service call today where a heater in the living room arc faulted. Nothing happened except for disintegrating the wire nut.

Glad we put them in bedrooms now...

I guess I am not allowed to post a helpful link here, so paste the above into your browser; just add all that h t t p : / / w w w . stuff...

Speedy Petey

Tim, you can post links.
You just can't post an advertising link.

HERE

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Mark Twenhafel

Thanks, Timster, for the link. I had forgotten how informative that magazine is. I put it on my toolbar to keep it in clear view.

Electric_Light

They have to put everything into one, since you can't cascade different breakers in the panel box realistically.

The wall outlet GFCI is actually GFCI only. It's basically a relay that is triggered by leakage current. It will not respond to over-current/shorts, this is acceptable, because it's cascaded with short/over-current protector upstream in the breaker panel.

The most basic breaker has a heat-activated bi-metallic material that responds to overheating from overload and it's affected by temperature extremes. It also has a coil inside. When there's a short, the very high current causes the coil to generate enough magnetic field to pull the circuit open. This is the magnetic portion of the breaker that responds instantaneously. The breaker itself will probably have a IR of 10,000A or so and should have the ability to handle the short circuit current of the circuit.

Fuses are disposable, but they can be made to have interruption rating that do not even compare to breakers. A Buss 1200A 600v fuse weighs around 4 lbs each and is rated to interrupt 200,000A @ 600v.

If you're not familiar with the idea of interruption rating, consider this. You've got a 12v electronic device that only uses 0.1A.

You can power it with 8 AA batteries or power it from a big car battery and the device would still run. If there's a short inside, the amount of current that's allowed to flow until the fuse can break the circuit is considerably different between the two. When the interruption rating is exceeded, the fuse/breaker will "break" but circuit continues to live through the arc and melt wires, blow up the fuse/breaker violently and start a fire.