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

 

[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.