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11 Point test for Safe isolation on 3-phase Electrics in the UK

Sergio Fernandez

Safe isolation on 3-phase systems in the UK currently requires a 10-point test plan

A quick internet search bring up the following 10 point test plan.
L1 – L2
L2 – L3
L1 – L3
L1 – Neutral
L2 – Neutral
L3 – Neutral
L1 – Earth
L2 – Earth
L3 – Earth
Neutral – Earth

Personally I think the industry should write it down as follows

Earth to L1
Earth to L2
Earth to L3
Earth to Neutral
Neutral to L1
Neutral to L2
Neutral to L3

L1 – L2
L2 – L3
L1 – L3
It shows clearly the first 7 sets of test FROM LEAST dangerous connector/probe first


Steps to Safe Isolation

    Permission: Secure authorization to isolate the circuit.
    Identify: Locate the correct isolation point.
    Switch Off: Turn off the 3-phase supply.
    Lock Off: Secure the isolator with a padlock and keep the key.
    Warn: Place "Danger" notices and "Do not switch on" labels.
    Prove (Before): Test your voltage indicator on a known live source/proving unit.
    Test: Perform the 10-point test above.
    Prove (After): Re-test the indicator on the proving unit. 

The Lock Off: Secure the isolator with a padlock and keep the key.  Should EMPHASISE preferred lockoff is upstream from DB/CU



Important Notes:

    Always use a GS38-compliant voltage tester; voltage sticks are not suitable for proving dead


Now for the 11th test part
Use a suitable clamp meter on the tails to check for leakage current initially set to Amps then mA after the isolation step/lockoff step.  This is vital to help detect PEN faults/Diverted Neutrals




As always please be polite and respectful in this purely academic debate.





Come on everybody let’s help inspire the future

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  • It shows clearly the first 7 sets of test FROM LEAST dangerous connector/probe first

    If you were going down that route then wouldn't you have Earth to Neutral first? Although I have to question the logic behind that - they're only "least dangerous" if things are as they should be - if things are awry 400V could be almost anywhere - so you really should be treating all of the tests with the same respect. I suppose if all the connections were OK but the isolation hadn't been carried out then you'd could discover that with s 230V rather than 400V test and stop there - but I guess if there was a problem you'd likely continue with the rest of the tests to get as much evidence as to the cause (it tells you more if you know all three lines are still live vs just one of them), so the advantage seems slight.

    If I were to add another test, it might to be to terra firma,..

        - Andy..

  • in reply to AJJewsbury

    If I were to add another test, it might to be to terra firma,..

    Inclined to agree - confirming N and E are the same is not as good as confirming they are also near ground ;-)  Of course you cannot always - the location may be on a roof or in a closed basement.

    Sometimes phase rotation matters.

    They are only in that order for speed - at no time do you move more than one probe, so it all takes about 20 seconds and the voltages are all grouped, so 3 off 400V test, 3 off 230V test and one off zero volt test. reversing the order as proposed keeps the same advantage.

    Mike.

    PS in some parts of the world  where domestic 3 phase is more common it is usual to have  either 3 single phase sockets or 1 three phase  outlet built into the CU so a simple plug in tester can be used with no meter probes or screws to be loosened reducing the risk of a probe slip and arc hazard.

  • in reply to AJJewsbury

    And, we are testing to avoid danger, so test the most potentially dangerous first?? 

    Safety philosophies/approaches are tricky !

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  • in reply to AJJewsbury

    And, we are testing to avoid danger, so test the most potentially dangerous first?? 

    Safety philosophies/approaches are tricky !

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