TT Systems

[AdieB]

Hi quick question I have to change a consumer unit out. It is a TT supply system. All breakers will be 61009. I have spoken to my part P tech department and they say that I still need a 100A 100mA S type main switch up front. Another guy says NO as the breakers are all 30mA controls. The first guy said it was required incase there was a neutral fault. But as the neutral also connects into the 61009 will that not provide protection?

AdieB

 

[Leesparkykent]

There isn't a requirement as far as know to install an S-type upfront although wouldn't be a bad thing either if you did...the rcbos would have to be double pole or single pole switched neutral.

 

[AdieB]

There isn't a requirement as far as know to install an S-type upfront although wouldn't be a bad thing either if you did...the rcbos would have to be double pole or single pole switched neutral.

I was going to use a Lewdon board with RCBO - */30/SP which is single pole but if you look at the diagram it shows that the RCBO coil is looking for out of balance between phase and neutral which would result in a trip. Would this not provide adequate protection?
AdieB

 

[davesparks]

I was going to use a Lewdon board with RCBO - */30/SP which is single pole but if you look at the diagram it shows that the RCBO coil is looking for out of balance between phase and neutral which would result in a trip. Would this not provide adequate protection?
AdieB

You shouldn't need the diagram to know that it compares line and neutral!
But no that does not provide protection against neutral faults as it will not clear a neutral fault, only detect it.

Think about what will happen if a neutral to earth fault occurs, the rcbo will detect the flow of diverted current from neutral to earth via the fault and then disconnect the line leaving the diverted neutral current flowing through the fault making it nice and hot ready to start a fire

 

[Dave OCD]

And for that reason a dual RCD board would be better, but surely Dave when the line is disconnected by the individual RCBO, fault current from neutral to earth in that circuit will also cease ?

 

[Lucien Nunes]

No, because N-E fault current could be from any or all circuits not just the faulty one. The neutrals are all commoned in the busbar and current can just as easily flow out to the fault from the DB as back from the load on the faulty circuit. Hence the need for DP switching to disconnect 'both ends' of the faulty neutral conductor.

 

[davesparks]

And for that reason a dual RCD board would be better, but surely Dave when the line is disconnected by the individual RCBO, fault current from neutral to earth in that circuit will also cease ?

The neutral current from that circuit will cease yes, but it will continue to be diverted from the rest of the installation, hence the overheating of the N and E conductors (if it was just the one circuit's neutral current then this wouldn't be an issue)

 

[AdieB]

Is there a regulation that says all TT systems with 61009s fitted on all circuits has to have a S type 100A 100mA DP RCD up front. I can not find one. Out of two Napit Tech guys say there is not a requirement. I have also just received a copy of a previous EICR of a job that requires retesting where again it is a TT system with a 100Amp isolator controlling two DP 80A 30mA ( split board) which then controls 60898 mcb's so is this ok as the RCD's are DP? Or should the main isolator be 100mA DP S type as the 30mA will trip before the 100mA

 

[Piratepete]

Hi AdieB
During my recent NICIEC assessment I was advised to fit a 100mA S type RCD up front on TT systems, because the CUs are now steel and have the additional risk of an internal SC between the flexible conductors that supply the RCDs and earth. It actually states this in the yellow On Site Guide on page 34 below the twin RCD diagram. Two other solutions for TT systems are shown on page 33 - the second showing RCBOs on to a solid busbar not requiring the S type RCD up front.

Cheers
Pete

 

[AdieB]

Thanks for the reply I will check my yellow book which is in van. The two boards I am speaking about are both plastic / polycarbon type did your guy say anything about this type of board?