5 Feb
2011
5 Feb
'11
7:23 a.m.
"Do it right" is subjective. You may easily
spend hundreds of man hours
True enough...
However, I don't think that turning up circuit breakers (whether
over-currnet ones or RCDs) to get over intermittant breakdown of old
insulation is ever 'doing it right'.
Similarly, it's hard to think of a case where increasing the currnt
rating of a protective device for no good reason is 'doing it right'. If
the device is trippign becauseo f a genuine current surge that yuu know
about and can show is safe, then fine. But otherwise, IMHO you need to
find why the deive is tripping, not just turn it up and hope the problem
goes away.
hunting down a problem that won't show up until
there's a major fault,
which costs maybe a few man-hours to fix, and is simple to find once it
has gone "boom".
Given the sort of energies you are dealing with, I am not at all
convicves it will go 'boom' safely.
So I am
seriosu. What fault is the 200A earth-leakage breaker
protecting against?
Basically, an equipment fault which causes current to flow to ground,
but which isn't enough to trip a breaker or blow a fuse.
It's actually required by the US National Electrical Code to have such a
ground-fault interrupter on any power distribution system that exceeds
800A. I believe that part of the reasoning is that you can
have a "dead short"
that's through a thin enough conductor (or strand of multi-stranded
conductor), that you won't draw enough current to blow a fuse (possibly
hundreds of amps), but which you don't want to trip from 90deg out of
phase loads from capacitors or inductors that are acting as power
filters.
[...]
Say you have a 18AWG or so "strand" of a
thick stranded copper wire, on
a circuit that's fused for 400A or more. The strand (shorted to ground)
will quickly heat up to dangerous levels, and probably won't cause a
high enough current draw to blow the fuse.
Hang on a second. Your RCD trips at 200A or so. What if you have an earth
fault that sould only carry 100A. It will not trip either breker, but it
certian;y could dispate enoguh power to do a lot of damage.
Yes, but breakers tend to emulate "slow-blo"
fuses more than "fast-blo"
ones.
Depends a lot on the breakers and how they work.
Electromagnetically-operated circuit breakers tend to be fast-tripping,
thermal ones slower/ Both types are made (or were made last time I looked
at them).
-tony