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--