TT Systems

[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?