Maz zs

[Deleted member 9648]

And so the argument continues

A discussion mate.....a discussion

 

[Smugley]

I include a line on the Schedule of Test Results for each RCD, followed by each MCB that is fed from it.

In the Max Zs column you could put 1667 for the RCD along with other details relevant to the RCD (Operating Current and trip times), and then enter the relevant Max Zs for each MCB.

EDIT: Oops - should have read the question properly!!

 

[IQ Electrical]

That's acceptable but the original question was referring to an RCBO device.

 

[Graeme Harrold]

Fully agree for TT entering 1667 for max Zs, but for TN I list the MCB value and work to it as I have encountered too many sticky RCD'd as the clients NEVER do the quarterly check. By entering the MCB value, it focuses you on getting the lowest value for the MCB type.

Each to their own, its just the way I prefer to do it.

 

[Deleted member 9648]

Fully agree for TT entering 1667 for max Zs, but for TN I list the MCB value and work to it as I have encountered too many sticky RCD'd as the clients NEVER do the quarterly check. By entering the MCB value, it focuses you on getting the lowest value for the MCB type.

Each to their own, its just the way I prefer to do it.

I think there is a mis-understanding here.....on a new circuit on a TN system I would expect the measured Zs to be well within the max value for the MCB,or other overcurrent device and would ask questions if it wasnt..(Poor design?Poor teminations?).....But on the cert I would record the 1667 ohms value as a max where an RCD protects the circuit-
A. Because technically that is the maximum allowed value...and
B. Because it saves a few seconds looking up the protective device value, and when you have a wad of certs to do anything that makes it less of a chore is to be welcomed.

I fully agree that the Zs on a TN system should be within the limits of the overcurrent device,the max value written on the cert is just academic.
That said most of the Zs values which slightly exceed the max for the protective device are on existing circuits which have been extended and altered.IMO where the excess is a fraction of an ohm over,and other tests are satisfactory,the RCD can be used to meet the required disconnection time and the slightly high Zs ignored.