On Jun 2, 8:23*am, bud-- wrote:
On Jun 1, 1:30*pm, Winston wrote:





bud-- wrote:

(...)

Nice post a couple levels up.

Thanks!

Meters like the Flukes are "Category" rated - 1, 2, 3. When working on
high capacity services, like 1000A 480V, you really want a cat 1
meter. You won't wind up wearing it as in your previous post. The
problem is not just the meter. Arc in meter causes vaporized meter
leads that turns into an arc between busbars - very dangerous.

I learned about 'flashover' only very recently.
Once you know about it, it's obvious.

Called arc-flash in US. The hazard isn't always obvious. One of the
smartest electricians I have run across wanted to measure the motor
current in a food plant. The motor starters were in a motor control
center, which is a frame with many motor starter modules - busbar feed
in back. He defeated the door interlock, opened the module door and
put a clamp-on ammeter on one of the contactor motor wires - all
absolutely routine. No one knows what happened - the guess was there
was a loose screw. In any case there was an arc-flash. He had bad
burns, some from vaporized copper condensing on his skin. He spent a
lot of time in the hospital with multiple plastic surgeries. But he
survived. You can be killed from burns, concussion from the explosion,
shrapnel.

These days to be OSHA compliant and make the same measurement you
might have to wear an arc-flash suit. OSHA interest in arc-flash is
relatively recent. Some related equipment issues are now in the US-
NEC.





In addition to the nominal voltage you have the hazard of transients
that could start an arc which will then sustain at the nominal
voltage. In the US, OSHA may also take strong exception to using a
meter without the right cat rating.

One of the hazards in high capacity services is available fault
current, which can be 200,000A. Fuses have a rating for available
fault current. The fix is to use "current limiting" fuses. For high
currents they have a clearing time of under 1/4 cycle. You handle a
200,000A available location by thefuseclearing before the current
increases to anything near that value. The earlier Flukefuseis
certainly current limiting. Thefuseopens before the meter leads turn
to plasma.

So *that's* the '40,000 A' number in the 11Afuseratings.
I always wondered about those stunningly huge numbers.

A commonly available Bussmann FRN 20A fuse is "current limiting" and
rated for 200,000A available fault current. But much bigger.

--
bud--

Two somewhat related incidences:

To pass a very stringent British Telecom specification that equipment
would survive adjacent equipment blowing a fuse, I desigened a
'transient generator' using 4 12Vdc 500A vehicle batteries in series
with a 50 ft 0000 Awg coil (to provide a bit of inductance), a starter
solenoid, and a fuse. The idea is the fuse shorts the 48 V, then
blows, and you get a 300+volt surge that wants to put more than 300A
someplace. If your adjacent equipment uses a 'shorting' protection
method, God save us all! The surge will strip off 1/4 inch wide traces
on a PCB. Anyway, I tested all kinds of fuses and discovered the fuse
designers very intelligently designed the fuse to 'gently' blow, just
die, go away. It turned out the BEST fuse for this test equipment
was a small 3AG 1A FB, went off like a flash bulb, and created the 'in-
spec' waveforms.

While visiting an Alcoa plant (in Iowa?), I saw a large casting of
aluminum, approx 18 feet long, 14 to 18 inches tall and 10 inches
wide, of a very strange shape. I was told that that was ordered by a
utility company. It is a FUSE and they had just blown one on one of
their high tension distribution lines and this was the replacement!
Now you want shrapnel and melted metal. Blowing that kind of fuse
must be awesome to behold.

Regards,