TT earth help

[Notaspark]

Does a supply with a TT earth need to have a 100ma RCD protecting all circuits?

 

[ericmark]

No
It needs a method of automatic disconnection and in most cases a RCD will be required to allow this to work but it could be 500ma or 30ma there is no set figure and today often the 30ma are used for personnel protection are on all circuits so it does not require a second one.

Edited to correct error pointed out by JohnD

 

[JohnD]

50ma

 

[Zambezi]

Does a supply with a TT earth need to have a 100ma RCD protecting all circuits?

I think it is a bit more involved than that. What type of CU you have, the run from the meter to the CU, does it have other 30mA RCDs or RCBOs protecting the individual circuits.
Short answer is yes and long answer is no.

 

[ericmark]

50ma

OK I know what I wrote why repeat without a comment?

There are now three sizes of RCD in common use.

30ma These are used to protect personnel.
300ma These are used to protect for fire.
500ma Used in CONSTRUCTION AND DEMOLITION SITE INSTALLATIONS for sockets over 32A see 704.411.3.2.1

In order to get discrimination between RCD's both the S type (delayed) and other sizes will be used.

In the house using a 100ma was traditional but there is no reason why this should remain so.

Edited to correct mistake pointed out by JohnD

 

[JohnD]

down here we use 30mA RCDs for personal protection.

what make are your 50mA?

 

[Notaspark]

CU is split load with 30mA RCD protecting rings only. I assume I need to have this rectified?

 

[JACKC]

As you have a split board with rcd protesting the sockets you can fit 30mA rcbo's to the unprotected circuits, or you can if you have room fit another rcd to protect the non rcd side. Personally I would fit the rcbo's despite the expense as it wolud only trip the faulty circuit and not all of them connected to that rcd.

 

[ricicle]

Are you sure the board is not split with 100mA TD main and then 30mA to protect those circuits requiring so. Is this an insulated unit ? Are you sure the supply is TT ?

 

[Notaspark]

Incoming power is two tails coming through party wall with no earth. CU is connected to a main earth terminal with a short length of 16mm. From MET to gas and water with 10mm. Connection (10mm) from MET to earth rod (in pit) outside of property. Only 1 30ma RCD protecting half of split load unit. It was called TT last time I looked in a book .

Edited 'cos my fingers are too fat!

 

[oharaf]

Are you on about the main isolator switch? If so I thought that this needed to be 100mA.

Then for each circuit the actual makeup could have a number of variables

i.e. dual RCD covering 2 split loads say 80mA and 63mA

or all RCBO's

or pre 17th a straight split load board of 30mA on one half and mcb's toher half

Yep you got TT by the way

 

[aptsys]

Are you on about the main isolator switch? If so I thought that this needed to be 100mA.

Then for each circuit the actual makeup could have a number of variables

i.e. dual RCD covering 2 split loads say 80mA and 63mA

???

 

[JohnD]

A not mA I think?

 

[oharaf]

A not mA I think?

Sorry John got me confused here.

If he has a TT system he should not have a 60947-3 Isolator should he? Am I not correct in thinking it should be a 100mA RCD?

 

[JohnD]

I was thinking of

i.e. dual RCD covering 2 split loads say 80mA and 63mA

I hope you meant, an 80A 30mA RCD and a 63A 30mA RCD.

Yes, I would formerly have said a 100mA "S" RCD on the incomer, but these days with an insulated CU and all circuits on 30mA RCDs or RCBOs, no need