replace 100mA RCD

[s1m0n]

Hi,

I have a job lined up to update a consumer unit,
It is a TT system, there is a 100A 100ma RCD unit between Meter and the consumer unit, the customer wants this removed.

If I fit a Slip board consumer unit, 100A mains switch and 2 X 80A 3oma RCD
so all circuits are coved by 30ma RCD, is there a need to keep the 100a 100ma RCD unit?

Thanks

 

[Eggster71]

You could replace the main switch with 100mA incomer? The only reason I can see you'd really need this though would be to prevent anyone using a non RCD way by re-arranging the board, that way you cover all the bases. You'd need to make sure you install an insulated board though as a metal one would require protection incase the tails came into contact with the enclosure.

 

[FUSEServices]

100mA RCD is there to cover the TT system. If Ze goes over 21ohm, prevents the earthing conductor rising above 50V

 

[s1m0n]

Thanks for the feed back so far,

I'm aware of the 100mA RCD keeping touch voltage to 50v, if all circuits are covered by 30mA RCD
(2 in this case to seperate lighting circuits etc)
then this would do away with the need for the 100mA RCD, would it not?

Thanks Simon

 

[Guest123]

Yes absolutely.

Unless as was stated before the CU is of a metalclad construction.

 

[s1m0n]

I am going for an insulated CU,

Cheers

 

[Sintra]

Hi,

I have a job lined up to update a consumer unit,
It is a TT system, there is a 100A 100ma RCD unit between Meter and the consumer unit, the customer wants this removed.

If I fit a Slip board consumer unit, 100A mains switch and 2 X 80A 3oma RCD
so all circuits are coved by 30ma RCD, is there a need to keep the 100a 100ma RCD unit?

Thanks

If you have a smoke alarm circuit.

Fire detection circuits must be supplied independently of other circuits and not protected by an Rcd protecting multiple circuits. 560.7.1

Except for TT systems, the circuit supplying a smoke alarm should not be protected by an RCD. For TT systems the smoke alarm must be supplied from the fixed equipment section of the ccu, not through a 30mA RCD.

 

[malcolmchisholm]

On a TT system, I've been asked to replace a Wylex fuse board with a DB equipped with RCBOs on every circuit. This is to prevent any circuit fault disabling any other circuit.

One circuit feeds a sub board, which will also be fully equipped with RCBOs, so the sub board will be fed by a MCB.

Can I do away with the master RCD, which is 100mA?

Regards,

Malcolm Chisholm

 

[KFH]

I can't quote a regulation but I did a similar on last week and put a 100mA S type RCD in prior to the first CU to protect the cable to the sub board and protect anyone who tried to dig it up (buried SWA, not sure how deep). If it is not time delayed the master 100mA can trip with any of the RCBOs.

 

[Guest123]

One circuit feeds a sub board, which will also be fully equipped with RCBOs, so the sub board will be fed by a MCB.

Can I do away with the master RCD, which is 100mA?

Regards,

Malcolm Chisholm

No, as the EFLI will be too high for the the MCB alone to offer earth fault protection. Either supply the distibution citcuit via an RCBO or exchange the 100mA RCD for a time delayed 'S' type one.

 

[malcolmsanford]

Would you be using DP RCBO's as regulation 537.2.1.1 is often interpreted as meaning that both LIVES must be isolated, but as this reg does not highlight fault conditions there as always been controversy as wether this means ADS or just isolation via a DP main switch.

I have to say that for a TT system, and the use of RCBO's I would have to be thinking of DP types for safety. I agree the regs do stop short of insisting on DP protection, but I have always wanted that level of protection either by a 100mA up front RCD on pre 17th standards, or now the use of 2 RCDs as you will get in a split board or High integrity board.

 

[Chr!s]

Would you be using DP RCBO's as regulation 537.2.1.1 is often interpreted as meaning that both LIVES must be isolated, but as this reg does not highlight fault conditions there as always been controversy as wether this means ADS or just isolation via a DP main switch.

I have to say that for a TT system, and the use of RCBO's I would have to be thinking of DP types for safety. I agree the regs do stop short of insisting on DP protection, but I have always wanted that level of protection either by a 100mA up front RCD on pre 17th standards, or now the use of 2 RCDs as you will get in a split board or High integrity board.

As far as i can see disconnection for ADS (TT/TN) is isolation of the line conductor onlyin general. The only time isolation of both live conductors is when an RCD is being used against risk of fire Section 532.

TT/ requires live conductors to of a circuit to be isolated, now REG 5372.1.1 allows provision for a group of circuits bases on risk assessment. Many TT fall foul on this in my opinion.Also Reg 314 comes into play.

Where RCDS are installed in series, the down stream RCDS should ideally be double pole, single pole devices will leave N -E faults for the upstream device to possibly operate. Again it comes down compliance with Section 536 and risk assessment. Reg 531.2.9 See note.

Regards Chris

 

[malcolmsanford]

^^^^^^ described much better than I Chr!s but think we are both singing off the same sheet.

Also though Chr!s if you use the RCD as a main switch for the use of ordinary persons as in a domestic CU, then that also must be double pole. (TN/TT)

 

[Chr!s]

^^^^^^ described much better than I Chr!s but think we are both singing off the same sheet.

Also though Chr!s if you use the RCD as a main switch for the use of ordinary persons as in a domestic CU, then that also must be double pole. (TN/TT)

Agreed

 

[Chr!s]

Also one final point RCD'S do not limit touch voltage, only, ​only duration.