100mA RCD on PME system

[kingeri]

I am going to a job this afternoon, new shower circuit. I noticed yesterday that the Wylex CU has a 100mA S-type RCD as the main switch and all the circuits are on 30mA RCBOs. PME system as far as I could tell (certainly not TT). Didn't give it much thought then, but was thinking about it last night in bed (sad, I know). Initially I thought there is no problem with this arrangement, a bit unnecessary perhaps. But I was thinking, as the RCBOs are single pole, a sustained N-E fault on any of the circuits would trip the upfront RCD and take out the whole installation. Other than that, I can't see an issue with it, unless I'm missing something. Do you folks think this in itself is reason enough to recommend replacing it with a vanilla main switch, or would you just leave it in situ? And have I missed anything?

 

[kingeri]

Anyone? I was hoping for something before I set off!

I obviously haven't done a Ze yet, which may explain the presence of the 100mA RCD I suppose (if it's very high).

 

[Richard Burns]

I would not worry about it, if it was TT then it would be the same risk of total power out on a minor NE fault and you would only be able to solve that by DP RCBOs.
Any other fault will be discriminated.

 

[kingeri]

Many thanks Richard.

 

[HandySparks]

I'ts a balance between safety and convenience. The 100mA type S is acting as a backup to the RCBOS. I'd be inclined to leave it in. Would be worth asking the householder whether it's ever tripped.

 

[bigspark17]

Is it a TT thats been converted to PME.. i wouldnt worry about it @kingeri

 

[kingeri]

Cheers guys. I can't see it as having been TT in the past, but I suppose you never know!

I will leave it in, it is providing dual redundancy in a way!

 

[sparkdog]

Vanilla main switch? I prefer the strawberry ones myself.