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?

 

[AdieB]

Just reviewed book and if I am reading correctly if you install an AMD 3 metal board on a TT system then you have to install a 100mA 100A rcd external to consumer unit not as a Main switch.

 

[Simonslimline]

Just reviewed book and if I am reading correctly if you install an AMD 3 metal board on a TT system then you have to install a 100mA 100A rcd external to consumer unit not as a Main switch.

What reg number is this!!

 

[Leesparkykent]

Just reviewed book and if I am reading correctly if you install an AMD 3 metal board on a TT system then you have to install a 100mA 100A rcd external to consumer unit not as a Main switch.

I would phone your CPS technical line and see if they class an RCD as similar switch gear and if they regard it as such then it would have to be housed in a non combustible enclosure so IMO would be pointless installing it externally and may as well swap the main switch for a 100mA s type RCD. I've seen it argued a few times that an RCD may not be classed as similar switch gear as offers no over current protection....IMO I think this is wishful thinking and would have to be housed/enveloped in a non combustible enclosure. I think the general consensus is to take the tails through a purposely made nylon tail gland, adequately support the tails from the board to the meter to minimise movement and replace the main switch for a 100mA S-type to provide earth fault protection to the flexible links that feed the RCD's on a dual RCD board.

 

[Deleted member 26818]

There is no requirement in BS7671 to provide RCD protection specifically for TT installations.
It may be necessary to use an RCD to provide earth fault protection due to high earth fault loop impedance.
Traditionally a 100mA 'S' type RCD was installed up front.
Where the 100mA 'S' type RCD was installed in a metal CU, an insulating gland would be used where the tails entered the CU.
The 100mA 'S' type RCD would allow discrimination for any downstream 30mA RCDs.

As far as I am aware, no RCDs will detect a loss of supply neutral.
However RCDs/RCBOs with a functional earth will operate in the event of a loss of supply neutral.

Where all circuits are provided with individual 30mA RCD protection, there is no reason to install an up front 100mA 'S' type RCD.
Where a metal split type CU is used, then an upfront 100mA 'S' type RCD will provide protection to the internal wiring of the CU.

Double insulation provides earth fault protection to the tails upstream from any RCD.

 

[Piratepete]

Just reviewed book and if I am reading correctly if you install an AMD 3 metal board on a TT system then you have to install a 100mA 100A rcd external to consumer unit not as a Main switch.

Answering your first question - If they're plastic boards a Main Switch and two 30mA RCDs is fine in a TT situation. I assume these are for a non- domestic application?!

On this question, for a metal board in a TT situation, the S type RCD can replace the Main Switch see Figure 3.6.3(ii) on page 33 of the yellow On Site Guide.

Pete

 

[Midwest]

Reg . 411.5 TT Systems, and specifically 411.5.2 gives option to use either an RCD or OCPD as earth fault protection, and key phrase 'the former (RCD) being preferred'. If using OCPD, 411.5.4 provides formulae to calculate that conditions are met. Up front S Type RCD may be required to protect distribution circuit supplying CU.

 

[AdieB]

I have not got my yellow Regs book with me I have the NICEIC site guide page 101. If installed in a TT system there could be a fault between the two incoming tails and the metal casing. Resulting in all exposed conductive parts connected to the main earthing terminal t become live. Due to high external earth fault loop impedance of a typical TT system The service fuse may not operate. Therefore I read it as the RCD must be positioned external to the consumer unit

 

[Simonslimline]

I have not got my yellow Regs book with me I have the NICEIC site guide page 101. If installed in a TT system there could be a fault between the two incoming tails and the metal casing. Resulting in all exposed conductive parts connected to the main earthing terminal t become live. Due to high external earth fault loop impedance of a typical TT system The service fuse may not operate. Therefore I read it as the RCD must be positioned external to the consumer unit

We work to BS7671 though don't we!

 

[Deleted member 9648]

I have not got my yellow Regs book with me I have the NICEIC site guide page 101. If installed in a TT system there could be a fault between the two incoming tails and the metal casing. Resulting in all exposed conductive parts connected to the main earthing terminal t become live. Due to high external earth fault loop impedance of a typical TT system The service fuse may not operate. Therefore I read it as the RCD must be positioned external to the consumer unit

As has already been pointed out,it is likely that the Am3 requirements will necessitate installing the upfront RCD in a metal enclosure as other means of complying are not really practical, which defeats the object.
The answer is simply to take precautions to prevent any possibility of a fault between incoming tails and the CU metal case. Easily achieved, and has anyone ever seen a fault to a metal cased CU from the tails? I haven't....and I've seen plenty forced against sharp metal K/O's with no grommet!
Properly done the risk of a fault to a metal CU from the tails is just about zero.

 

[Deleted member 26818]

I wonder if the concern, is not that the conductor insulation will be damaged by the metal enclosure, but more that with these modern terminations, the conductors will fall out and then come into contact with the metal enclosure?

 

[Midwest]

Prior to A3, the recommendation was an insulated CU on a TT supply, because there was a risk, however small of a metal CU becoming live, because of the conditions previously mentioned.

Now with A3, a non-combustible CU must be used for all supplies on a domestic property, ferrous metal being the suggested material. Therefore the 'grown-ups' have had to come up with a recommendation for using a metal CU with a TT supply. The yellow OSG has made those recommendations, and using workmanship procedures & suitable products.

Hence 100ma S Type RCD as main switch, to prevent single insulated internal cables (manufactures) between said S type & dual RCD's, making live accidental contact with metal enclosure, or the use of RCBO's (no S Type RCD) with the chance of internal busbars (connecting the live side of RCBO) 'minimal'.

The use of 6181y cable as tails, with suitable gland, negate the use of stand alone S Type RCD, outside of CU.