In article , G&M
writes
"Rick Hughes" wrote in message
...
"tony sayer" wrote in message
I'd much prefer everything to be on a 30 ma RCD after all a 100 ma ones
not going to be much fun to try to trip through your body
Agree on life protection, and the 30mA RCD wil provide protection to the
AC
outlets, Radial and Ring Final circuits.
The Best Practise oft talked about is not to have lighting on a 30mA trip
duwe to nusisance tripping, many on this group for example have advocated
no
protection for light circuits.
I stuggle with this. When a light blows here it takes out the circuit trip
but has never to my knowledge tripped our 30mA RCD. Is there something
system dependent here ?
There are some misconceptions about what an RCD actually does, well
modern ones.
An RCD (Residual current device) is designed to trip when there is a
current UNBALANCE between what is coming IN and what is going OUT on the
circuit it is protecting.
Now under normal operating conditions it matters not how much current is
flowing through the device, provided that its within its ratings like 64
amps for example.
Now as long as the current in, through say the live terminal, is matched
by the current out through the neutral terminal, then all is fine. Now
if for some reason there is more going in than coming out above the
nominal tripping current, say 30 ma ,actually slightly less, then the
device will trip.
Supposing that you inadvertently touch the live wire somewhere in the
house, assuming that that particular circuit is protected by the RCD,
then there will be an element of current flowing through your body to
earth. Now if that current that you are "conducting" is greater than the
current trip level, then off goes the trip because it has detected less
current flowing OUT than the current coming IN hence an UNBALANCE!.
It doesn't even matter if you touch earth or not or even if your house
is on an earth or not, or even if there is no earth on your system, it
doesn't matter as far as the trip is concerned. The reason is that the
neutral line is earthed at the substation transformer, be that in a
substation proper, or on a pole mounted transformer. Due to this if say
you touch a live wire then you shunt some of the current via the trip
and the earth mass back to the substation and if that value of current
is greater than the trip level off will go the trip!.
Its down to a current unbalance through the trip, simple as that. If for
instance a cooker element develops some leakage between the element
casing then some current will flow to earth and if that current is
greater than the tripping unbalance current then off the trip will go.
Sometimes you may experience this as the element will only do this when
it gets hot and then the insulation will breakdown and the trip current
will be exceeded!.
Another "*******" problem comes if there is a short betwixt the neutral
line and the earth line in the wiring somewhere. Normally if most every
thing is switched off then no tripping, but as soon as some appliances
are switched on then off the trip will go!. The reason is that under
light or no load conditions the trip will be sensing an unbalance state
so the current flowing in will match the current out, because some of
the current will flow back along the neutral lead through the trip and
some direct to earth through the earth wire. Now as soon as some load is
put on the system and the current flowing through the combined earth and
neutral return starts to increase then the trip will sense that there is
more coming in than going out via the "authorised" route via the RCD, so
a unbalance exists and as soon as this exceeds the trip current off
again.
In my opinion all the wiring in your gaff should be on a 30ma trip
because there might be the time when you are working on something and
forget to turn it off, or pull the wrong fuse or something similar I
wouldn't fancy trying to trip a 100 ma trip with my body either thanks!.
So as long as you see the trip as a device that monitors the current in
and the current out are in balance, then all is fine if however there is
a difference between these two values greater than the trip level, then
the device will operate.
Hope this helps....
--
Tony Sayer