New circuit results on TT system

[sparky b]

Installed a 16a radial on a 16th edition split load Hager c/u.

S type RCD main switch and one side 30ma RCD, all circuits protected by 30ma RCD except 2x 6a lighting circuits.

These are the results I got:

PFC L/N 1.60ka L/E 8a

ZE/RA 239 ohms

R1+R2 0.20 ohms

ZS by calculation 239.20 ohms

RCD 1x 34.0ms 5X 16.0ms

My concern is (unless I've done something wrong somewhere) that a ZE/RA over 200 ohms is considered unstable and what should be done to rectify. I'm aware the high ZS on the new circuit is covered by the 30ma RCD but what about the 2x lighting circuits that are not, are they covered by the S type RCD or should i put them on the 30ma or should I just leave them as they were already like that....

Sorry for the long winded post, not much experience on TT systems, just wanna make sure all is ok (or not).

B.

 

[kme]

The Ra is too high to be considered stable - check the connection to the rod; if OK then another rod is suggested. You haven`t "calculated Zs" of the circuit, as Ra was taken with bonding conductors disconnected; whereas Zs is taken WITH them connected - Actual Zs will be lower 99.9% of the time.

I`d probably note that the lighting doesn`t comply with current regs; though they aren`t retrospectively applied.

KME

 

[sparky b]

Thanks KME, little black star clicked for ya!

 

[steptoe]

is the S rated RCD a 100mA one?

if it is a higher one, say 300 or 500 then it deffo needs to be changed to a 100mA, so your lighting circuits ARE RCD protected, just not at 30Ma which is simply a code 4, and no further action is necessary really.

240 ohms isnt majorly bad, but you should try another rod possibly and try get it as low as possible , <100 ideally,

what sort of ground are you going into?

 

[sparky b]

is the S rated RCD a 100mA one?if it is a higher one, say 300 or 500 then it deffo needs to be changed to a 100mA, so your lighting circuits ARE RCD protected, just not at 30Ma which is simply a code 4, and no further action is necessary really.

240 ohms isnt majorly bad, but you should try another rod possibly and try get it as low as possible , <100 ideally,

what sort of ground are you going into?

Yes, it's 100mA rated, so that's all good then.

The rod is in a slabbed area, probably just mud underneath but very close to drains....

 

[steptoe]

MUD SHOULD GIVE YOU A GOOD READING,

another rod may well be just what the Doc ordered,

personally I would recommend to the customer they should consider this for a future issue.

 

[sparky b]

MUD SHOULD GIVE YOU A GOOD READING,another rod may well be just what the Doc ordered,

personally I would recommend to the customer they should consider this for a future issue.

Thank you, will do. a little clicky star thing for you too.....