3 Feb
2011
3 Feb
'11
8:17 p.m.
On Thursday, February 03, 2011, Tony Duell wrote:
Yes. Things would have heated up quickly and/or blown fuses if it was
long-duration.
Yes, it's high.
I'd agree. The problem is convincing people to spend the time tracking
down an intermittent fault somewhere within more than a dozen possible
pieces of equipment. Sometimes it's easier to just wait for it to go
BANG!, and pick up the pieces. There's nothing irreplacable or unique
that it could destroy, and the equipment is generally designed to
contain such a problem to avoid hurting a person who's nearby when it
happens.
Yes, I do understand the difference. My point is that a fault that's
200A on a supply that can probably supply a fault current nearing 100kA,
isn't that big compared to what's possible, (eg, not a "dead short").
And for whatever it's worth, unless it's a special purpose breaker, a
circuit breaker doesn't trip as soon as the current crosses its rated
current, where "soon" can be as long as minutes depending on the breaker
if you're only overloading by 10% or so. I was amazed at how long
(10-15 minutes) a particular 20A breaker sustained a 25A load before it
tripped. Then again, the thermal response of a breaker tends to match
the effects of wires (that it's protecting) to keep them from
overheating.
Pat
--
Purdue University Research Computing --- http://www.rcac.purdue.edu/
The Computer Refuge --- http://computer-refuge.org
On Wednesday,
February 02, 2011, Tony Duell wrote:
Yes, I mean 200A.
Ouch. I how most of that is either for a short duration (switch-on
surges) on in quatrature to the supply so that no real power is being
consumed. The
ground fault breaker (RCD by you) on the 480V supply the
work's main datacenter (which is rated at 1MVA, or 1200A
currently - an upgrade to 5MW is coming this year) was
tripping on us when it was set to 200-300A,
Dop you mean that? 200A (not mA) of earth leakage wouldappear to
be dangerously high in any installation. When
you're dealing with high-ish voltage (277V to ground), and
lots of power being used (about 1000 amps per phase), leakages can
add up
Even so, it's high. It's around 1/15 of the total current, which, pro
rata would be a leakage of 1A on a medium-sizedd PDP11, say. If my
PDP11 had that sort of leak, I would be repairing it. Period. quickly...
And, it's most likely a spike, not a continuous leakage
current, eg, a short-lived arc to ground from some worn out
insulating parts.
That is precisely the sort of thing I would repair and not just turn
up the breakers to get round it! Worn-out isulation (whatever that
may mean) is not something I am going to trust. In any case a
200A short-term fault isn't all that huge. Ratings
for typical US house circuit breakers are around 10,000A
interrupting capability.
That is surely the maximum fault current they will safely break. Not
the current they carry before they trip. Over here most decent
domestic MCBs will break 16kA safely -- that is the peak current
that might flow if there's a dead short across the mains. But said
breakers will trip on a current of 32A, say.