Voltage operated ELCB - what's wrong with them?

[ianmacd]

Sounds clear and makes sense when I get a minute I will try it at home.

Guess his power never came back on so he couldn't post more .

 

[ianmacd]

Absolute nonsense.

Unfortunatly not. If the RCD is supplying MCB's that are all open then there will be no current flowing though the RCD to get diverted down the CPC to create the imbalance. If some of them are on then this would not be the case and the RCD will most likely trip when shorting N&E on any circuit on that RCD.

 

[ianmacd]

If you had a line to earth fault on the system it may or may not trip the RCD. The only way the RCD would trip is if there is 30mA difference between the live and neutral currents. Your additional L-E fault with the N-E fault in place effectively means its a L-N fault. However - the probability is that the MCB would trip - as the N-E fault effectively makes this a DIY TNCS system. Problem is - if the fault was on the lighting circuit the lighting cpc could be taking a substantial fault current.

Thats correct assuming the fault is to the CPC and not to another route to earth (like the floor). I would hope, however that since it is in effect an L-N fault now, the MCB will trip and should do so before the cpc in the lighting circuit burns out. I would be more worried about it on a cooker or shower radial.

 

[ADS]

Absolute nonsense.

Really, Lurch

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I had a change of heart

 

[ADS]

Bang on for all MCBs off, now if you have just isolated the circuit you're working on....

Which I believe I said here:

If, on the other hand, other circuits (on the same neutral bar as our isolated circuit) are energised and under load, then current could flow through the isolated circuit's neutral and down to earth.

This would create an imbalance in our RCD and it would trip - but this would only happen under these conditions.

 

[ADS]

If you had a line to earth fault on the system it may or may not trip the RCD. The only way the RCD would trip is if there is 30mA difference between the live and neutral currents. Your additional L-E fault with the N-E fault in place effectively means its a L-N fault. However - the probability is that the MCB would trip - as the N-E fault effectively makes this a DIY TNCS system. Problem is - if the fault was on the lighting circuit the lighting cpc could be taking a substantial fault current.

I see what you're getting at now, Dave, and you're right if, as Ian said, it's a circuit fault. If it's through a person to earth it would operate pretty much as normal - except for the offset current in the neutral fault.

Obviously, being a double fault on the system there's not much we can do to combat it, as the 'Regs' and our means of protection only deal with single fault conditions - or so I was told on the 17th course.

 

[Lurch]

The main point in that part of the post was that ALL MCBs are OFF. Thus, no current can flow in any part of the installation. So, since there can be no current flow, no matter how many N-E faults one creates within the installation there can be no current imbalance to trip the RCD.

No, there doesn;t need to be current flowing in a circuit to trip an RCD, there needs to be an imbalance between the 2 live conductors. If all circuits are off but there is a potential difference between N and E then shorting N & E could cause the RCD to trip if enough current is drawn due to this potential difference.

An RCD doesn;t measure that the return matches the outgoing, it looks for an imbalance, either positive or negative, so even with all circuits off if you supply a voltage to one of the outgoing terminals derived from somewhere other than the other outgoing terminal the RCD will trip.

Perhaps people should stop making wild statements stated as facts when they don;t know what they're on about. Heard that before somewhere....

 

[ianmacd]

Indeed if you IR though an RCD it can/will trip.

I would hope, however, that there would be no PD across N & E.

 

[ADS]

If all circuits are off but there is a potential difference between N and E then shorting N & E could cause the RCD to trip if enough current is drawn due to this potential difference.Perhaps people should stop making wild statements stated as facts when they don;t know what they're on about. Heard that before somewhere....should have listened and payed attention then

So where is this potential going to come from, then - all the circuits are off and the neutral is referenced to earth (0v) - no current is flowing in the neutral so tell me how the neutral is getting to a different potential to earth.

If what you were saying is true we could never work on circuits simply isolated at the MCBs - we'd always be in danger of that neutral with a 'voltage' on it.

And as for bonding our exposed conductive parts to this neutral (TN-C-S), I'm never touching anything again

By the way, Lurch, this:

An RCD doesn;t measure that the return matches the outgoing, it looks for an imbalance, either positive or negative, so even with all circuits off if you supply a voltage to one of the outgoing terminals derived from somewhere other than the other outgoing terminal the RCD will trip.

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......is a contradiction within itself.

The RCD is still measuring that what is going out is equal to what is returned via the neutral - in this case what is going out is 0 amps - the imbalance is the current in the neutral conductor compared with this

 

[Lurch]

The RCD is still measuring that what is going out is equal to what is returned via the neutral - in this case what is going out is 0 amps - the imbalance is the current in the neutral conductor compared with this

See this is where you're going wrong, you don;t fully understand how an RCD works.

It doesn;t measure that what goes out comes back in, it looks for an imbalance between the 2 outgoing terminals. If there is nothing leaving, but something returning it will still trip, an RCD can still trip with no load. I've been through this before, look at how the test switch is wired on an RCD, this is exactly how the test switch works, and it works either on or off load.

When you do actually understand how an RCD works, then you can come back and complain about people incorrectly stating facts.

 

[retiredsparky]

It's all to do with the touch voltage exceeding the maximum stated in the 15th, 16th and 17th edition regs.