spinlondon,
The inside cover of the NICEIC sheets show the 80% values as an aid to the inspecting electrician.
The NICEIC certificate requires the actual maximum value to be entered.
Recording the maximum values is not a problem, and is often a help. It also helps to justify 'failing' any test results that exceed them to the client.
I don't see why this itself could be regarded as a rip-off, but I agree, there are too many inspection reports carried out that are incorrect or incomplete, mostly because the inspecting electrician does not really have enough knowledge or experience to do it correctly.
Yes, one would take a Zs measurement with the circuit switched on, but not necessarily under full load conditions where the cable conductors could reach an operating temperature of 60 degrees. This would increase the actual Zs from the reading taken with the cables at ambient temperature, hence the 80% values.
So the reasoning behind this is this:
A water heater circuit (not RCD protected) may just pass a Zs test at just under the maximum value. This would appear to be OK. however, in normal use the current that the water heater pulls will cause the phase conductor temperature to rise, increasing it's resistance. This increase may then take the Zs of the circuit over the max permitted value. Particularly if the heating effect of both the P&N conductors also cause the ECC temperature and resistance to rise. Additionally, the heating effect of any other grouped cables may also contribute to this increase in Zs. In the event of an earth fault the required disconnection time may not now be achieved. So a test that originally passed now fails.
If the original Zs test is below the 80% value then it is considered that the heating effect of the cable in normal use will not be an issue. If 80% of the maximum is exceeded than additional investigation may be required.
