Adding 30mA RCD protection.

[marcusone]

What are the options for providing 30mA RCD protection for all socket outlets on an existing TT system that has only a 100mA RCD covering all circuits. Not a split load board. Many thanks.

 

[JohnD]

RCBO is ideal. What make and age is your CU?

 

[marcusone]

Not that old, about ten years I would say. I was thinking RCBO but was wondering if that is a problem with the existing RCD not being time delayed.

Cheers

 

[JohnD]

If you have two RCDs on the same circuit and neither is time delayed then they are likely to both trip at the same time.

However I have found the small ones in RCBOs and sockets are a bit quicker than the big ones under test (about 10 mS IIRC). I had assumed it was because they have less inertia and it isn't mentioned in the specs. I haven't tried a test with two RCDs in series except with a main one time delayed. You are probably going to have to splash out on a S type. They aren't cheap.

 

[Steve]

Wont a 100mA and a 30mA discriminate?

 

[JohnD]

Not with a fault of >100mA. They will both detect it and start to trip.

Both will start to go as soon as the cycle reaches their trip current. There is a slim chance that if one trips in say 10mS and the other was going to have take say 25mS, the second one might not trip. Never tried it and wouldn't rely on it. That's why you use the delayed ones; they won't start to trip for a period long enough for the standard one to have finished and cut the current.

 

[securespark]

Yeah, the idea of a TD RCD is to stall it to give the other a chance to trip. A fault covered by the 30mA RCD should trip only that RCD, not any other devices upstream of it.

 

[marcusone]

Thanks for feedback. Looks like I'll be recommending a new split board with the necessaries.

 

[JohnD]

RCBOs are better.

 

[Steve]

RCBOs are better.

YES, WE KNOW.

 

[plugwash]

RCBOs are better.

but be carefull using RCBOs on a TT, the single module ones don't tend to isolate the neutral when they trip leaving a significant possibility of a NE fault tripping the main 100ma RCD