Circuit Breakers

sometimes!

According to a Busman Seminar I attended, circuit breakers are only good for one tripping due to a short circuit.

Steven,

If you get false trips or cannot reset the breaker, then replace it. A breaker is a thermal device; current causes a bimetallic strip to bend. Time won't affect it.

A short circuit may destroy it but a normal trip should not affect it. A hundred trips might but not one or two.

Jay

Hmmm you mean the Bussman that sells fuses? Do you think there may some bias in that assessment?

This depends on the prospective short circuit current. And ratting of breaker. In a little square there is a number like 6000 or 10,000 this is the prospective short circuit current maximum. With larger breaker sometimes two figures one causes damage the other not. When the electrical system is checked every 10 years or with change of use or ownership then this is measured together with the earth loop impedance. The breakers are then checked for burning and ratings together with the size of cables etc. If they do not comply with the regulations of the year then a note should detail any points not conforming. Since the regs change year to year there are normally some no conformity although this does not mean there is any danger. Needs £1000 worth of test equipment so not really within the scope of a DIY man.

All best Eric

The most valuable tools are the ones God gave us. A failing breaker will usually buzz, pop, smoke, smell bad or run hot. You will probably hear noise on an AM radio and see white dots on your TV too.

OK, but seldom does a home breaker see more than on short circuit. They will see the odd overcurrent, and of course the GFCI breakers see ground faults more frequently, but the garden variety of breaker won't see a a short circuit. (Industry is a whole different breed of cat, but the original post was about household breakers.)

In this country, homes have had circuit breakers in general usage for over 40 years. I've heard nothing about replacement of home breakers. (Getting a new breaker for an old panel is a challenge, but that would be about the only reason I've heaard of to replace them.)

HR.

Except for the inconvenience, fuses are vastly superior protection because of their near transient response.

Dave M.

Use once and throw them away.

-- If it is not broken, I cannot fix it. If I screw with it, it will kill me. Payday is Friday.

It's Volts that jolts, but Mills that kills!

OK, but the downside is that users (like residential folks who don''t really understand this) will replace a 20 Amp fuse if a 15 Amp fuse opens to clear a fault. People have a much greater reluctance to replace a breaker. Also, they don't have to spend money every time the fool thing operates and causes the whoozit to stop working...

Balance the certainty of control of a breaker against the incremental superiority of a fuse - you decide what T.C. Mits should be using.

What?

HR.

That is why the NEC requires "type S adapters" in fuse panels these days. A 20 won't fit in a 15 socket. (although a 30 will fit in a 20 socket). Bussman doesn't explain that one.

Well, that's an oversimplification, though it expresses the right idea. IIRC, the NEMA (US) standard for molded-case circuit breakers includes a short-circuit test that goes something like:

1. Close the short circuit. The breaker under test trips.

1. 10 seconds (IIRC) after step 1, reset and close the breaker under test.

2. The breaker under test trips again.

The test only requires that the breaker interrupt the fault twice, with some conditions about emitting fire, molten metal, etc. The standard gives a list of conditions like broken arc chutes and other signs of destruction that are _not_ considered failures, if the breaker performed the two interruptions successfully and wouldn't have started a fire or something in the process. Needless to say, this is a design test, and a breaker that has been tested this way is _not_ then packaged up and sold to the public.

And yes, the standard does recommend replacing any breaker that has interrupted a fault at a current close to its short-circuit rating. That said, it generally requires a solid connection physically close to the breaker to allow anything like the calculated short-circuit current to flow. At low voltage (

And how are we to know the what the fault current was? We generally don't, and that is why Bussman says replace the breaker. Hmmmm...I wonder if fuses are less costly?

In Europe, breakers have two tripping components -- a thermal trip for overload protection (slow acting) , and a magnetic trip for fault current (short circuit) protection (very fast acting).

Breakers are marked something like C32 M6000.

The 32 is the steady current the breaker will pass without tripping. As you start to exceed that, the thermal trip will operate -- it depends how far over you go just how long it takes to trip, but at twice the current, a typical operating time would be ~100 seconds for a EN60898 European breaker, and 10 seconds at 4 times the rated current.

The 'C' is the rating of the magnetic trip, which can be B, C, or D. ('A' is not used to avoid confusion with A for Amps.) The magnetic trip provides the fast breaking for a fault current, and it operates in 10ms (half a mains cycle). This must be rated higher than the inrush current of the appliances on the circuit or the breaker will trip when applianced switched on. Type B will trip between 3 and 5 times the steading current rating, type C will trip between 5 and 10 times the steading current rating, type D will trip between 10 and 50 times the steading current rating. Type B is the type most commonly used in domestic situations, and sometimes type C on lighting circuits to reduce the chance of a filament lamp which arcs over from tripping it. Type C would be used for appliances on dedicated circuits with large motors such as aircon units. Type D is only used in commercial/industrial premises and requires special consideration for the cable safely handling fault currents. The older breakers to UK standards used Type 1, 2, 3, or 4 for the the rating of the magnetic trip, with

2, 3, and 4 being very roughly equivalent to B, C, and D. Type 1 was in practice too sensitive to find many uses, and does not have an equivalent in the European replacement standards.

The M6000 is the fault current the breaker can safely interrupt without damaging itself, 6000A in this case. M10000 is another common rating. If you exceed this, the breaker might be damaged interrupting the fault current. If you grossly exceed this, the breaker may be unable to interrupt the fault current, relying on the upstream protective device and likely causing considerable damage to itself and the surrounding area and the downstream cable carrying the fault current. Some manufacturers allow an M6000 breaker to be uprated to M10000 if the upstream protective device is a BS88 100A cartridge fuse, which is what would be used in the UK in cases where a very high prospective short circuit current is available at the domestic premises supply.

Bussman is in the fuse manufacturing business. There main competition is the manufacturers of breakers. It is in there best interest to make the cost of fuses appear to be lower than the cost of breakers. We can know the maximum fault that a breaker has been subjected to by doing the withstand calculation for that point in the system and assuming a bolted fault.

-- Tom

Well, if we're complying with code, we know the calculated short-circuit current is at least equal or less than the short-circuit rating of the breaker ;-)

But IIRC, there is a method described in IEEE somewhere for calculating the expected short-circuit current based on the kva of the line transformer, the length/size of feeder to breaker and from breaker to fault.

I think they are if you're going to replace the breaker each time it clears a fault. But many faults aren't direct shorts. I do know that in industrial settings, we require a breaker to be racked out and inspected if it opens under fault. But don't know how you can do this with molded case units.

And a downside of fuses is I wouldn't try to remove them under load. This means a separate disconnect adding to the total cost.

daestrom

Ah, but in distribution schemes, breaker trip settings can be coordinated to clear faults and minimize affected area.

daestrom

'use'??? You must have a deep pocket to replace breakers every time you open one ;-)

daestrom

In addition to what you described, the utility needs to advise the available short-circuit amps at the transformer primary. The calculations are routine for anyone who designs electrical systems, and very straightforward for the typical residence. However, the average homeowner doesn't know how to do this, or even that it should be done.

Also, when was the last time an electrical utility advised a homeowner that new distribution transformers or feeders had been installed that might increase the available short-circuit amps? I suspect there is more than one home where the panel is under-rated, although it might have been fine when originally installed.

Ben Miller