Ye olde RCD question

[Aberporth]

Hi there
I have a scenario

A house with a TT earth has everything running via 30ma RCD switch out in a bedroom cupboard (presumably for convenience when it trips.

Whilst not very trippy it is prone avery few weeks to find a reason to trip which is a nuisance as one circuit feeds a fish pond which needs aeration.

Would it be sensible to replace the existing RCD with a 100ma one and protect those ccts vulnerable (outside, shower etc) with individual 30ma ones?

Is there any reason this is very silly?

thanks

Aber

 

[davy_owen_88]

Fitting a standard 100mA RCD will probably not make any difference, what you need is a time delayed (S-type) RCD. You should protect every circuit with the 100mA TD RCD and then fit 30mA protection for those circuits that require it - sockets likely to supply outdoor equipment, showers and certain fittings within a bathroom.

 

[plugwash]

That sounds reasonable just a few things to be aware of

1: make sure the 100ma RCD is a time delay type to ensure that the 30ma ones trip first.
2: When RCD protecting the individual circuits avoid using single module RCBOs are they ususually don't isolate the neutral when they trip.
3: Try and find out if a PME upgrade is availible, often they are cheaper than the time delay RCDs

 

[JohnD]

Would it be sensible to replace the existing RCD with a 100ma one and protect those ccts vulnerable (outside, shower etc) with individual 30ma ones?

Yes. you can use RCBOs which are great. A second best solution would be to have a split-load CU. you should use a 100mA Time delayed RCD on your main incomer. if it is not time delayed it will often trip at the same time as your 30mA ones (unless you can arrange only ever to have very small leakages). Better still ask your electricity supplier to provide an earth, then you won't need an RCD on the incomer. it might cost you £60 to £100 but will be money well spent. time delayed RCDs are rather expensive.

bah too slow

 

[plugwash]

Yes. you can use RCBOs which are great.

Sure they are handy in some cases because of the space saving over seperate MCBs and RCDs but the single module ones very often don't isolate the neutral meaning they won't clear a N-E fault (and since they won't clear it it may go on to trip the main RCD)

 

[Aberporth]

Thanks chaps
That is really useful. I am assuming the exisiting 30ma RCd is not time delayed. How do you tell if it is? Does it have something written on it to distinguish it?
thanks

 

[JohnD]

It might say "S" or "S-Type" on it. For some reason "S" stands for "Time" or you could look for a part no which may be recognised.

However time delayed ones are AFAIK invariably 100mA or greater not 30mA

 

[tim west]

Yes. you can use RCBOs which are great.

Sure they are handy in some cases because of the space saving over seperate MCBs and RCDs but the single module ones very often don't isolate the neutral meaning they won't clear a N-E fault (and since they won't clear it it may go on to trip the main RCD)

That's a bit cruel and heartless, you could have said they are the best thing since sliced bread or the like, now you've hurt JohnD's feelings

 

[JohnD]

RCBO on every circuit ==> no need for a "main" RCD

 

[Spark123]

It might say "S" or "S-Type" on it. For some reason "S" stands for "Time" or you could look for a part no which may be recognised.

However time delayed ones are AFAIK invariably 100mA or greater not 30mA

iirc the "S" stands for "Selective".

 

[securespark]

Gold star for numerical spark!!