TT system, RCD or RCBO

[Bobby34]

At the risk of asking a silly question could someone advise me on the following:

Client has a TT system with only half the circuits covered by an RCD, they want a light adding on a light circuit thats currently not covered by the rcd.

Now with half circuits not going to meet the disconnection times if I work on just the lighting circuit would I still have to cover all the circuits that haven't got an RCD with a new RCD or am I allowed to cover just the lighting circuit with an RCBO because that's the circuit i'l be working on?

Because of the results when testing I'm thinking i'm going to have to cover all the circuits but I'm just wanting to check the RCBO route incase the IR is low (old wiring) and nuisance tripping could occur (I know the clients won't be wanting to pay extra if I have to start going digging).

Thanks in advance

 

[Deleted member 9648]

You'll really want a DP RCBO to disconnect both live conductors on a TT.

 

[buzzlightyear]

Yep found them thanks mate

6 064 16 LEGRAND 6a RCBO 30ma 10ka B TYPE

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2

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£25.95

 

[Deleted member 9648]

A N-E fault on the lighting circuit you have altered will possibly take out the RCD serving the rest of the installation. The only way to prevent that is a device which disconnects the N.

 

[Bobby34]

A N-E fault on the lighting circuit you have altered will probably take out the RCD serving the rest of the installation. The only way to prevent that is a device which disconnects the N.

Ah yes I didn't think about the DP, if I remember theres a reg aswell that says TT systems need a DP..... they don't like making it too easy for the sparks do they haha!

 

[Welchyboy1]

Only if there is an rcd upstream of your does it need to be double pole

Which by the sound of your post there is not

However legrand do double pole rcbos

 

[westward10]

There is no requirement for a protective device on a TT system to be double pole just that it disconnects in the event of a fault.

 

[Deleted member 26818]

There needs to be Double Pole isolation, which is provided by the Main switch.
Is there no up front 100mA RCD in this installation?

 

[Deleted member 9648]

My interpretation is that part of the installation is on an RCD and part isn't, it's possible that a N-E fault on the RCBO circuit could result in the other RCD operating for an unrelated fault because a parallel path would remain in place even if the RCBO tripped. A DP RCBO would prevent that. I do not think it is a requirement, but if possible I would fit a DP device if I was altering the circuit protection as part of the work carried out.

 

[Welchyboy1]

My interpretation is that part of the installation is on an RCD and part isn't, it's possible that a N-E fault on a single pole RCBO circuit could result in the other RCD operating for that fault because a parallel path would remain in place even if the RCBO tripped. A DP RCBO would prevent that. I do not think it is a requirement, but if possible I would fit a DP device if I was altering the circuit protection as part of the work carried out.

In my head it sounds like a tenby split load 16th ed board with first half Circuits direct from main switch and second half 30 mA rcd protected, thats the issue i usually find

It shouldnt affect the rcd downstream with an rcbo in place on the unprotected side but if there was another rcd (e.g. 100mA s type) between the origin and the new single pole rcbo, the N-E fault on the rcbo circuit would likely effect this device even when rcbo has tripped open and also cause the rcd to trip if sufficient load was applied through it

As such a double pole rcbo would be required to completely isolate said fault

Thats my understanding for what its worth

If an N-E fault would effect downstream rcd devices then i should think we would be in trouble combining RCDs onto PME supplies?