"Lurch" wrote in message
...
On Sat, 15 Jan 2005 00:10:15 GMT, "BigWallop"
strung together this:
snipped
*ignore all this and carry on following the advice from elsewhere in
the thread. *
BW, you can have absolutely nothing pulling a load through an RCD and
a NEUTRAL (not negative, that's DC), to earth fault *can* trip it.
Sorry.
Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth. If the appliance is open circuit, all
switches open, then the phase should not be connected to neutral through any
load, and so this can't cause an imbalance in the phase to neutral loop
which the RCD is there to detect. So, for arguments sake, we'll say that
neutral and earth are in the same potential state. The phase is at higher
potential than both neutral and earth. Phase and neutral are "not"
connected through any load (open circuit). A straight short occurs between
a neutral and an earth conductor. What happens at the RCD?
Therefore, the fault must lie between phase and earth for the RCD to trip
the supply, because neutral is already at, or so close to, earth potential
that the RCD will not detect any difference on this side of the supply.
This would only happen if there was a closed circuit between phase and
neutral, and a neutral conductor passed a fault current to earth at such a
low impedance, which should be highly unlikely because neutral is already,
or very nearly, at earth potential.
How I was told and read how it all worked was like this. The conductors and
loads in a circuit act as resistance to the passage of electrical current.
Placing two resistances in series only increases the total resistance in the
circuit. But, if two resistors are placed in parallel to each other, the
total resistance is actually reduced by a tangible amount in the whole
circuit because the electrical current has two paths to flow through on that
part of the circuit rather than just one. This reduction in total
resistance in the circuit by placing a parallel resistor in it is what
creates the imbalance for the RCD to detect. The RCD should only have one
conductor with current going out and one conductor with current coming in.
By making two conductors send current out, and still have only one conductor
sending current in, the circuit becomes unbalanced.
That's why I always thought an RCD was placed in line to prevent shock or
damage from an imbalance on the phase to earth side of the supply, not on
the neutral to earth side of the supply as well. An RCD should also give no
detection of a fault across phase and neutral because such a fault would
also be detected as a balanced load across them. Reading the IEE and RCD
manufacturers literature would also point me to believe this is true.
Yes you can have a slight increase in potential through heavy loading on
other parts of the same neutral path, but these should never be enough to
create a dangerous situation for the RCD to detect and trip open because
earth should also be induced by these same small increases in potential that
are effecting the neutral paths. So what you are saying is that the neutral
or earth of the OP's supply are not operating properly to remove the
unlikely event of a low resistance current flow between neutral and earth,
while the neutral conductor is being induced by some other loadings on the
same part of the circuit. This would cause the neutral to have a much
higher potential than earth, and so would cause an imbalance for the RCD to
detect. So do you think the OP should also have the supply and wiring
tested to be sure that the safety devices fitted are going to properly
protect from the potential harm of an electric shock?
(BTW, I use the word "negative" for circuit descriptions nearly every day
and I was wrong to use it here as we're talking AC not DC, but old habits
die hard. My apologies)
http://www.safety.ed.ac.uk/policy/part3/52.html
http://www.tlc-direct.co.uk/Technica...CD%20Works.htm
http://www.reo.co.uk/files/kbase/How...CD%20works.pdf
http://www.collinseducation.com/reso...20Breakers.pdf
http://www.tlc-direct.co.uk/Book/5.9.2.htm