No bonding with RCD

[Penfield]

Apparently if the pipe work in a property achieves certain resistance there is no need for bonding if a RCD is used. What is the figure that needs to be achieved? I have been looking through the regs this morning but haven't found it.

 

[DanBrown]

Apparently if the pipe work in a property achieves certain resistance there is no need for bonding if a RCD is used. What is the figure that needs to be achieved? I have been looking through the regs this morning but haven't found it.

If im correct in saying, 7666 Ohms is the maximum resistance allowed. I did: V/I = R 230/0.030 (30mA) = 7666 Ohms.
So long as the resistance is below this reading, the RCD should trip according to plan. Thats why we use them on TT systems a lot, because of the high resistance readings that could occur, and the 7666 ohms is lets face it - pretty high....

Other comments would also be appreciated so i know if im on the right lines.

Hope this helps anway.
Dan...

 

[Chappers]

I thought the level given in the regs was 1667 ohms? Can't check as it's beneath a pile of stuff at the moment and I'm not brave enough to start disturbing it.

If I recall rightly, a voltage of maximum 50V with a current of 30mA is considered to be below shock levels, and so this should be the maximum value permitted when an RCD is used. R=V/I would then be: R=50/0.03, so R=1667 if rounded up from 1666.666-recurring. If the resistance was higher, a higher voltage would be required and as such the 50V limit is exceeded and values are above those deemed enough to cause a shock.

 

[Guest123]

Correct Chappers, 50V is the "safe touch voltage" hence the calc R=50/0.03=1667ohms. which is maximum permissable Ra or (Ze) for a "TT" system although 200ohms is generally regarded as the maximum acceptable before you need to do something about it.

Cheers.

 

[DanBrown]

Chappers, Good thinking... Totally forgot about the 50v touch voltage.
You are quite right. 50v/0.030a = 1.667 ohms.

 

[Penfield]

Thanks, forgive me if this is a daft question but all the replies so far seem to be concerning a TT installation. Would the same formula apply to TNS etc? I just want to make sure I fully understand.

Paul

 

[Guest123]

Hey mate.

That calc is for the maximum permissible Ze with the "safe touch voltage" in order to hit your disconnection times for 30mA RCD's

With respect to TN-C-S it's max Ze of - 0.35ohms
TN-S it's max Ze of - 0.80ohms

Cheers.