Geoff asked me to start a new thread on this issue -
I cover mains impedance/resistance measurement on my training course but to explain why it is measured: -
NHS staff don't normally need to do this measurement - which is to ascertain the phase to phase impedance for high power radiology generators. It is measured at the room isolator terminals.
The equipment supplier normally does this check so don't panic.
However, it is important for the electrical safety group to ensure that during commissioning, the supplier is asked for the technical details of the power requirements. This includes maximum kVA and maximum mains impedance.
With these values, the designers have the responsibility of making sure the supply cables are big enough to keep the resistance below the maximum impedance value.
If the cables are too small, the radiographers won't get the full power out of of the unit. Then there will be trouble.
Happy for others to comment but I think for this purpose, the ESG should ensure they get the right information out of the supplier so that the designers get the cable CSA right.
If the mains impedance measurement finds it's too high, then the contractor needs to correct their mistake or the designer has a big bill on their plate.
