Testing with a UK Single Pole RCBO in Circuit


I am about to buy a Megger MFT1553 multi tester as I am getting one of my staff fully qualified to 17th Edition and need a decent test unit. Megger is after all the original brand of insulation testers (like Hoover to vac cleaners)
I am concerned to read the small print on the Crabtree single pole UK RCBO brochure that states "CAUTION These devices are electronic units and should be disconnected from the supply during insulation and earth fault loop impedance testing."
I have used an MK RCB but its spec sheet does not give this warning, although MK tech support advised me to remove them and temporarily replace with MCBs prior to doing loop or insulation testing and replace the MCBO to do a full RCD test. He said it is not in their instructions as electricians should know about this.
This warning is NOT applicable to the European double pole RCBOs or conventional RCDs as there is no working earth connection.
There is no reference to an RCBO in the Megger instruction manual. I phoned Megger and they were very helpful and said that the small white earth wire is so thin it could not possible carry an earth fault current but was probably used as a screen to the electronics inside as no manufacturer is giving away any secrets about its real function. They assured me that you could use their MFT to test everything with an RCBO in circuit as it is only a small pulse that is used for a non-trip loop test. They said it would be wrong to modify the circuit for testing purposes as you should test everything exactly "as is" which sounds correct.
I have read on the web that the cream functional earth wire ensures a missing neutral or an N-E fault will trip the RCBO but again I am unconvinced about this. We are using RCBOs in 1 bed flats as a full split CU is overkill and trips too many circuits at once.
Some web reports say an RCBOs can give false loop test results which seems rather unlikely as you are measuring a very low loop impedance impedance.
I am concerned about the truth of these warnings and why are some manufactures putting it in writing while others are not.
Any ideas - and what exactly does that cream functional earth wire do?
Graham
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