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dodgy mordaunt short speakers

Nah, you just need a nice low supply impedance to see it. With a fault current of, say, 250,000A the difference between a circuit breaker and a fuse is extremely obvious (if, of course, you have one hell of a good CT). I've seen the aftermath of a dead short applied to such a system whose CB wasnt rated for that level of fault current (and vaporised). oops.

its a bit harder to see in a house, where the fault current is 1,000 -

6,000A (depending on supply impedance, wire size etc). But I'm sure you've seen many a blown fuse whose rupture current was well below that (eg every 20mm glass fuse in existence). usually characterised by a totally destroyed fuse.

have a read of some Gould-Shawmut (bought by Ferraz IIRC) app notes, or maybe "Applying low-voltage fuses - classes & characteristics", H.W. Reichenstein, McGraw-Hill, ISBN 07-606577-4

Cheers Terry

Reply to
Terry Given
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** How funking smartarse.

You need to read the title of this NG.

** The ACTUAL short circuit current of a domestic AC outlet is rarely any more than 250 amps rms.

Go measure the voltage regulation of one with a known load as see how asinine your numbers are.

** More asinine tripe.

Most glass equipment fuses *can* break 250amps of current and do not need to break any more since a domestic circuit breaker WILL act instantly at that current level.

.......... Phil

Reply to
Phil Allison

there are electronics companies in Australasia that make 1MW 3-phase light dimmers, drives etc.

depends on how close you are to the switchboard, and the nearest distribution transformer (which might be right outside your house). New suburbs tend to have nice low impedance (high efficiency) transformers (5% cf 10%).

and just how do you measure the RMS short-circuit current? without a very good CT (wide dynamic range) and a digital scope, it gets pretty tricky.

then of course it makes a *big* difference when in the line cycle you apply the short circuit.

peak current is a much more meaningful number when considering fault currents. If the circuit interrupter works, over what time scale do you make the RMS measurement?

which wont tell you much about the short-circuit current at all.

those numbers, BTW, came via the EWRB, which ought to know.

20mm glass fuses are rated at 125A.

how instantly? ms? us? ns? ps? fs? as?

you cant pull Trevor up for making an "instantly switch" claim for a polyswitch, then do the same thing yourself for a circuit breaker - at least not without looking silly :)

case in point: the CB switch mechanism has a finite, non-zero mass. it requires infinitely high acceleration to move from one side to another instantaneously. Physics 101 :)

Cheers Terry

Reply to
Terry Given

**I just tested a few. They do not 'switch'. They do, indeed, limit current, depending on the temperature of the device. Naturally, since the fault current causes heating, the internal resistance rises, which, in turn, causes more heating and more temperature rises. Once the rated current is reached, it shuts off power rapidly enough to protect devices which can tolerate a modest overload (ie: Loudspeaker drivers). 
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

"Terry Given"

** No it is not.

** Madness - additional effects can only* reduce* the amp flow under short circuit.
** No - "manic" is the term for your illness - Terry.
** The term is "manic" - Terry - go look it up.
** Then you are totally stuffed.

** You're a desperate lunatic.

........... Phil

Reply to
Phil Allison

OK, how big does an IGBT have to be before it stops beng electronics?

I've used 100mA SCRs, and 2,000A SCRs. Is the former electronics, if the latter is not?

mighty peculiar definition.

Really? I note you snipped out rotating loads on the same network. Oops, every power systems engineer in the world is wrong.

better than being pathetic enough to require a god.

Reply to
Terry Given

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