RCBOs with TT

[Paul_C]

The problem of isolating a faulty circuit still remains with a single-pole MCB, however. A fault on the neutral can cause the main RCD to trip, depending upon the currents involved from the various loads in use at the time, but one cannot isolate that fault to restore power to the rest of the installation just by turning off the MCB for the faulty circuit, any more than one can isolate it to prevent the main RCD from tripping with a single-pole RCBO.

A D.P. RCBO coupled with a time-delayed main RCD can prevent more than the faulty circuit being isolated automatically when the N-E fault occurs. But beyond that convenience, the reasons for wanting a D.P. rather than an S.P. RCBO are just as valid as arguments for using a D.P. MCB instead of an S.P. type.

 

[JohnW2]

The problem of isolating a faulty circuit still remains with a single-pole MCB, however. A fault on the neutral can cause the main RCD to trip, depending upon the currents involved from the various loads in use at the time, but one cannot isolate that fault to restore power to the rest of the installation just by turning off the MCB for the faulty circuit, any more than one can isolate it to prevent the main RCD from tripping with a single-pole RCBO.

That's obviously true, but I think that colintd's point was that, with a split-RCD setup, the fault would not take out the entire installation, whereas with an up-front Type S RCD, that is what would happen.

Kind Regards, John.

 

[bernardgreen]

The RCBOs I have just purchased are double pole.

I got that wrong. They are single pole. That was a bad day. I didn't buy them as double pole but for some reason I thought they were when I got them home.

 

[JohnW2]

The RCBOs I have just purchased are double pole.

I got that wrong. They are single pole. That was a bad day. I didn't buy them as double pole but for some reason I thought they were when I got them home.

We all have days like that! However, I'm a bit confused given that what you wrote before was ...

Which makes me feel that double pole RCD and RCBOs should be used as cutting only the live would leave the circuit at the potential on the neutral which could be hazardous if ( when) there is a significant potential on the neutral. The RCBOs I have just purchased are double pole.

... which led me to assume that you had deliberately bought DP ones. Mind you, as I implied in what I wrote at the time, I'm sure your wallet will be pleased that you didn't!!

Kind Regards, John.

 

[bernardgreen]

After some thought my question is " Is there any point in disconnecting the neutral at the RCBO to "protect" the circuit in a PME installation ? "

The CPC cannot be disconnected and that is electrically the same as the neutral so if the neutral is bouncing ( no longer at true ground potential ) then the CPC will be just as hazardous as the neutral.

There is obviously some concern about bouncing neutrals as I have to ensure that the CPC ( the earth wire ) of my cottage's electrics cannot come into contact with the CPC of the adjacent shop which is a TT installation. The official reason is that in " a fault situation " heavy currents could flow from my CPC to the CPC of the shop and create a fire hazard. The only way that can happen is when the CPCs are at different potentials. As the water systems are connected I have to put in a length of plastic pipe and mark it as " electrical safety separation, do not replace with metallic pipe "

The other slight problem is a redundant lead water pipe that come into my kitchen up through the soild concrete floor. Do I need to bond that to the MET ? If I do am I not connecting my CPC to an earth rod and creating the same hazard as would be there if my CPC was connected to the CPC of the adjacent shop and it's TT earth rod.

Common sense suggests I cut the lead pipe at floor level and cover it. But on the the other hand common sense suggests the lead pipe could be one more addition to the multiple protective earths that ( apparently ) PME systems depend on to "protect" the neutral and derived CPCs.
( PME = protective multiple earth ).

One more case where "fit all" regulations do not fully fit the actual situation.

 

[JohnW2]

After some thought my question is " Is there any point in disconnecting the neutral at the RCBO to "protect" the circuit in a PME installation ? " The CPC cannot be disconnected and that is electrically the same as the neutral so if the neutral is bouncing ( no longer at true ground potential ) then the CPC will be just as hazardous as the neutral.

Indeed, and I think I made essentially that point earlier. Of course, the potential of neutral (relative to true earth) doesn't really matter if you have succesfully created an equipotential zone (i.e.no true earth accessible within the property).

There is obviously some concern about bouncing neutrals as I have to ensure that the CPC ( the earth wire ) of my cottage's electrics cannot come into contact with the CPC of the adjacent shop which is a TT installation.

Yes - we've discussed that before. It certainly is 'untidy' having a MPE and TT installation in such proximity.

The official reason is that in " a fault situation " heavy currents could flow from my CPC to the CPC of the shop and create a fire hazard.

I'm not sure they have thought that through properly. The shop's TT electrode is unlikely to have an impedance less than 50Ω - so the 'heaviest' current between CPCs under worst conditions is going to be less than 5A - and there's no conductor you're going to have in your installation which would become a fire risk at 5A.

The other slight problem is a redundant lead water pipe that come into my kitchen up through the soild concrete floor. Do I need to bond that to the MET ? If I do am I not connecting my CPC to an earth rod and creating the same hazard as would be there if my CPC was connected to the CPC of the adjacent shop and it's TT earth rod.

The same situation, yes - but, as above I don't see how that can be a fire (or any other sort of) hazard.

After Common sense suggests I cut the lead pipe at floor level and cover it. But on the the other hand common sense suggests the lead pipe could be one more addition to the multiple protective earths that ( apparently ) PME systems depend on to "protect" the neutral and derived CPCs.

Concrete floors can get damp, so (since I really don't see it as introducing any hazard) I think I would want it bonded - and, as you say, that would add to the 'PME'.

Kind Regards, John.

 

[bernardgreen]

Concrete floors can get damp, so (since I really don't see it as introducing any hazard) I think I would want it bonded - and, as you say, that would add to the 'PME.

Damp floor in contact with true ground and PME "earth" in the house.

How do I bond a concrete floor. ?

I am off to buy some oil lamps and a gas cooker.

 

[JohnW2]

Concrete floors can get damp, so (since I really don't see it as introducing any hazard) I think I would want it bonded - and, as you say, that would add to the 'PME.

Damp floor in contact with true ground and PME "earth" in the house.
How do I bond a concrete floor. ?

Bonding the floor in general is clearly impractical for you (would probably have had to be done before the concrete set )

I was thinking more about damp concrete acting as a conductor between the cut-off gas pipe (probably very well earthed) and the floor surface - which I would have thought was far more viable a path than through the mass of concrete in general - and then, of course, that bonded gas pipe within the concrete will do something towards 'bonding' the conctere floor in general.

Kind Regards, John.