Tt And 100Ma Rcd

[Electroglow]

have not ever done a TT Cu change before. Now my Q is..... if i remember correctly a 100ma RCD is required front end before the RCD consumer unit? Please could some one explain why this is a requirement as all ccts are on rcbos or will be. The CU is about 1 meter from the meter. all Bonding is up to date Ect. Thanks 

 

[OnOff]

Metal cu then is it?

 

[steptoe]

Are the RCBOs single or DP ?

 

[OnOff]

I didn't read the OP properly! But ref the RCBO's  - if single pole i.e. they only switch the line in the event of a fault rather than the neutral also which IS a line conductor. 

Now no doubt starts the periodic debate on this...............

 

[steptoe]

No no no,

The neutral is a LIVE conductor, not a line conductor

 

[batty]

Problem with having TD RCD before CU if you have a neutral to earth fault the main RCD may still trip that is if you are using single pole RCBO's. If it is a standard domestic TT installation with insulated CU I would not put a TD RCD in.

 

[steptoe]

so how would you get your isolation under fault conditions then if using SP RCBOs ?

 

[batty]

so how would you get your isolation under fault conditions then if using SP RCBOs ?

I have always found that with neutral to earth faults when using single pole RCBO's the only circuit affected is the circuit with the neutral to earth fault. That is using a main switch board. If there is any RCD before the board though that will be affected. Obviously phase to earth faults will be cleared by the RCBO anyhow.

 

[steptoe]

you still need to disconnect all live conductors according to the regs, otherwise how do you isolate?

N-E can 'daisychain' under certain circumstances too.

 

[batty]

Getting late Steps will have a think about this tomorrow.

 

[binky]

This debate was had before, in answer to the post, you don't need 100mA RCD at front end if protecting all circuits with double pole RCBOs that disconnect both Live and Neutral. There is some debate though involving metal clad CUs, which if I remember correctly, it was decided still needed RCD protection in their own right, in case of shorts to the metal case.

 

[steptoe]

You can even do away with a front end RCD for MC cu if other measures are taken,

But for most domestic its not viable either financially or otherwise.

 

[M107]

Are the RCBOs single or DP ?

Noooooooooooooooooooooooooooooooooooooo not again :slap

I'm currently doing a job at the moment TT so plonked in a TD 100mA on the tails, put in new (secondhand headbang customer supplied metal clad MK cu), the customer then has the meter moved & the meter monkeys give the thumbs up ABC we can pme.

Have left the TD in place on the tails customer wants submain out to shipping containers & other stuff round the yard....... Should get some photos of this job if I remember.