Well worth setting the alarm for Sunday morning

The only thing I was still open to was any voltage on the N from the supply side of the switch (not the load) as that would still be a function of the resistance of the neutral in the supply (wouldn't it)? Not if the RCD / switch was open of course but if someone was trying to close it?

Or maybe it does discriminate between the input and output (of the RCD) re it's ability to trip due to an imbalance (maybe it would have to)?

Cheers, T i m

Reply to
T i m
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I went into this about a year back when I had problems with RCD tripping in a large single phase heating project. Apparently tripping can occur when spikes are present on the incoming supply. Somehow interference pulses are sufficient to induce an imbalance ant trip the RCD. This is very rare and of course the RCD does have to be on.

In my case it was due to wiring errors and I was misled by one RCD that didn't trip because it was bought abroad at short notice. A nicely packaged device with a part number the manufacturer didn't recognise and would be better described as a "switch" :-)

AB

Reply to
Archibald Tarquin Blenkinsopp

Unless Fido's piddled in the consumer unit and got the busbar.

Dead Fido lying next to the CU in a puddle would be a giveaway though.

Owain

Reply to
spuorgelgoog

Yep.

Yep, in the L side.

It really depends on the type of supply and how good (or not) any N - E bonding is.

With L open, shorting N - E here would not trip an RCD. We are on a two wire supply. The MET is bonded to the supply N and the other required bonds (incoming services etc). The N is also connected to ground at the pole. It would be very difficult to get any voltage between N and E.

Change that to a three wire supply with some resistance between the N and E when measured post RCD. Resistance means that there can be a voltage between the N and E. Voltage means a current can flow if the circuit is completed by shorting N and E post RCD. As there is no L current this N E current is an imbalance and could trip the RCD.

OK bit contrived and relies on either or both of the N - Ground or E

- Ground conections nor being very good.

Reply to
Dave Liquorice

That does assume DP MCBs, though. Lots still around that are SP.

Reply to
Dave Plowman (News)

ARW has brought this to us :

Your point being?

Reply to
Harry Bloomfield

Dave Liquorice expressed precisely :

Correct.

It most certainly will cause a trip.

Yes..

You cannot rely on it not tripping even on a two wire supply - that is especially true, if it is a large premises and other supplies at the premises are still operating/ not isolated.

Exactly. N to E voltage here varies between 15 and 30v and a N to E short I can guarantee will trip RCD's.

Reply to
Harry Bloomfield

ARW brought next idea :

You sound very confident of that, despite my explaining that there can be voltage on the N and enough to cause an RCD to trip.

Reply to
Harry Bloomfield

I only go because of Remembrance Sunday and the Scouts.

Reply to
ARW

If N and E are shorted what sort of PD are you expecting to get between them?

Reply to
ARW

If you have a NE short what is the PD between N and E?

Reply to
ARW

ARW expressed precisely :

Zero or close to zero, depending on the path resistance.

However, prior to the N to E short....

Reply to
Harry Bloomfield

What did you charge?

Bill

Reply to
Bill Wright

At the short, none, but if there was a pd before the short the current created by the short would trip the R Current D?

The only question in my mind is would a pd seen between N and E on the supply side of the RCD and a E - N short after it, stop someone resetting the main RCD?

Cheers, T i m

Reply to
T i m

There is a RCD not connected to earth?

Reply to
ARW

I'm sorry, but I am with HB here.

If there is a Neutral-Earth potential difference before the main switch is thrown, and there is a Neutral-Earth short after the switch, there will be a current flowing in the neutral leg of the RCD.

IF TN-C then there is no Neutral-Earth potential difference and so there can be no imbalance with all the MCBs turned off.

As another poster has said, this type of RCD trip can only happen with single pole MCBs.

Reply to
Fredxx

ARW presented the following explanation :

Yes, why would it need an earth?

They work quite simply on a current imbalance, which is what the C in RCD stands for current. If the RCD sees a difference in current flow between it L & N connections, it trips and safely disconnects the supply.

At this point I will point out that you do fully understand, are fully aware why an N to E trip will cause the RCD to trip - but unable to accept you made an error, that you are attempting to still score a point.

Quite easy to prove, drop all of the MCB's out and short N to E on any of the sub-circuits. No more to be said, I think.

Reply to
Harry Bloomfield

T i m explained :

It would, it would (N to E voltage dependent) immediately trip. You would not be able to hold the RCD in the ON position, they are designed to not allow that.

Reply to
Harry Bloomfield

Multiply zero by any number and you get zero.

A 30mA RCD needs a minimum of 16mA to cause a trip.

Reply to
ARW

Go and sit in the dunces corner with him.

Reply to
ARW

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