Low ZS on a TT system

[Von Vardo]

Hi guys, I've got a TT system with a Ze of 21 ohms. Two ring final circuits are on the same rcd and are both giving a Zs of 1.7 ohms - does this indicate a neutral - earth fault?

 

[PC Electrics]

Maybe. But may also be getting parallel paths through the bonding to a neighbouring property whose install is a TN.

 

[SPARTYKUS]

Hello

It might be indicating that your parallel path back through a water Main Equipotential Bond or Gas is connected to another property with TNCS, bringing your reading down massively. From that figure I am a betting man on it.

Do you catch my drift, so to speak?

Could be gas or oil also. Or something else!

SPARTYKUS.

 

[SPARTYKUS]

Ah! PC Electrics beat me to it!

 

[telectrix]

try disconnecting the main bonding and see what you get then.

 

[Von Vardo]

Thanks for the answers. There's no bonding to this board - it's a sub db in a garage fed from a 100 amp isolator. Anything else it could be? I haven't had time to do
many tests but would like a heads up before I go back tomorrow!

 

[SPARTYKUS]

Are you sure there's no bonding at the main board being passed on so to speak?

 

[telectrix]

have you earthed SWA armour at both ends?

 

[Von Vardo]

One core of swa is connected to MET, the other end connected to earth bar in db. No bonding in db. I have 3 rfc, 2 lighting circuits, 1 circuit to an outbuilding and 1 circuit feeding central heating. The garage is attached to the house and has living space above. The water is bonded to the main db in the house.

 

[poheZ]

there maybe no bonding but if there is a boiler and the cable supplying the boiler has a cpc then that will make contact with the pipe work.

here is an idea why not do some dead testing, it's dead easy and a dead cert on finding a problem, dead. (got one more in)

 

[Von Vardo]

The problem is that even if there is a parallel path through the water system the whole property is on tt and there's only one neighbour within a mile so I can't see it being a shared tn

 

[telectrix]

are you saying that the house is TT or just the garage is TT?

 

[poheZ]

that has nothing to do with it, any earthing system will have a parallel paths to earth. Just do a dead test, cos i bet ya aint done that yet!

 

[Von Vardo]

The house has 2 boards and another in the garage. All are tt. I'll do the whole range of tests tomorrow and post the results!