ARW expressed precisely :
That is correct, so a sufficient PD between the N & E to produce 16mA is enough to trip an RCD. Assuming a 1 Ohm - path 0.016mA x 1 Ohm = ???volta
HOW MANY VOLTS NEEDED TO CAUSE A TYPICAL 30mA RCD TO TRIP Adam ???
Well let's amuse that we are starting with a voltage of zero as all the MCBs are switched off.
Effectively zero but that's not the whole picture.
+----- RL -----+ +------------ Live (open circuit) | | | Substation RCD Sens | | | (A)---- RN -----+ +-----+----- Neutral | ¦ | NE Fault | ¦ | +----- RE ------------ Protective Earth | | RNG REG | | Ground GroundPoint (A) will float above ground due to RNG. In the presence of the NE fault a circuit is formed and current will flow, how much current depends on the N E loop impedance and the source impedance of the voltage at (A).
With a 1 ohm loop impedance you only need 30 mV to get a 30 mA current. With 230 V available you can have a source impedance up to nearly 8 k ohms.
Quite.
Only if the N E fault is down stream of the double pole MCBs. If it's between the RCD main switch and the double pole MCBs you can still get a trip.
Can you illuminate us with the voltage on the neutral pin on the switched side? And that it may be different to neutral supply side? The MCBs may switch live, but not all switch neutral? That simply means the live side current through the RCD is zero.
This is most unlike you, either that or Rodney is posting on your behalf.
I had this experience when I traced and 'moved' an old perished rubber T+E. The cable the insulation was such I had left a Neutral-Earth short.
Even with the MCB off the RCD would trip after a few seconds to a minute. I had to disconnect the Neutral and left the MCB in the off position for the RCD to behave.
No, because of basic physics such as ohms laws. But I can however do some penis puppetry.
I'm genuinely surprised at you. If you don't accept Neutral and Earth can be shorted on the switched side, and that they could be at different potentials then I give up trying to explain any further.
Others here and on other forums do understand. First hit.
There are many others.
That was not my point and it was clearly explained in my first post where I said ALL MCBs were turned off.
Not sure what the point was.
Can you confirm that some MCBs are single pole?
If you do, can you accept that a current can pass on the neutral side and not the live side causing an imbalanced current in the RCD, hence tripping?
Fredxx laid this down on his screen :
We are wasting our time, trying to explain to him.
I am extremely surprised that Andy is not aware of this (if it is indeed Andy, not a puppet), it is so basic.
Lets hope he never finds himself in a situation where a site has an RCD installed at the incomer, where here drops all the MCB's out for perhaps a lighting DB, with lots of computers running on the site - then he shorts N to E on his lighting circuit.
I agree it is (should be) only a problem where an MCB is a single pole type, but single pole ones are the far more common.
Fredxx laid this down on his screen :
RCD's being tripped because of an N to E short, when a circuit is assumed to be isolated by an MCB - is something which puzzles many amateurs, when messing with electrics. I am astounded that Andy claims not to know or understand this, as he claims to be a professional.
I think Andy understands(*) it when not all MCBs are off. The N E fault allows some of the load current to by pass the RCDs sensor N side creating an imbalance and trip. Which is correct enough.
He's then assuming that with all the MCB's off there can be no load current and there for nothing to bypass the RCD and trip it. But is missing the fact that the incoming neutral can have enough volts on it to produce enough current to trip the main switch RCD when the N E loop forms a circuit via a N E fault.
The early ones were voltage operated. 40V appearing on the earth would trip them. I installed lots as an apprentice. However instances arose with simultaneous nearby faults when they didn't work. So they were abandoned in favour of the current ones, which back them had lots of nuisance trips.
So long as both poles are isolated. The point is that current can flow on a neutral/earth path if only one pole is isolated. As with these combined mcb/rcd single pole devices.
Whilst you might not be forced to improve the electrics you can certainly class them as unsatisfactory even if they were installed to the relevant regs
In this case in the centre of Sheffield I am expecting the supply to be TN.
Had a NE fault on a newbuild the other day that stopped the test button working on a dual RCD CU. The test button did not work on the RCD with the NE short.
hi-impedence earth? then the test current would flow back through the neutral and the rcd wouldn't trip.
Not sure yet. I'll get some more info. And I need to draw a diagram of what could have happened
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