4 Feb
2011
4 Feb
'11
11:25 a.m.
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. 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.
Perhaps you can explain to me the point of an earth-leakage breaker under
thsoe conditions... It's nvercurent device (that is, a derive that cuts
th emains if the current in the power-carying conductors exceeds a
certain value). The normal reason for having them over here is to protect
people in the event of an insulation breakdown, the unbalanced current
flowing throughj the person to ground will trip the breaker.
I doubt that 200A would flow though a person touvhing a live conductor
(if it would,then there'd be no point in the RCD in most isntalations,
the nromal current-operated breakers would trip). And if it fif, that
person would not notice it. Ever.
So I am seriosu. What fault is the 200A earth-leakage breaker protecting
against?
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.
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.
The same is true of fuses, of course. A 20A fuse does not melt the
instant the currnet rises over 20A.
The currnet agianst time (I-t) characteristics of fuses and circuit
breakers are specified i nthe data sheets (and at least over here there
are British Standards giving hte ones ocmmonly used). It's often
important to allow a higher-than-normal inrush current. I am sure we've
all used anti-surge fuses, for example.
However, I didn;t think that earth-leakage brekaers (RCDs, etc) had much
of a time delay as standard. The idea being that there really shouldn't
be an earth leakage current, not even fopr a faraction of a second.
-tony