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Tackling AFDD Tripping

sparkiemike

How are people tacking AFDD tripping?

In the past if I had a call out for a tripping RCD/RCBO or MCB there are well established procedures and tools to track down the fault.

These range from the visual inspection, insulation resistance testing, earth leakage measurement, RCD ramp testing and RCD time testing. It would not take too long to track down the fault whether it was faulty appliance, water ingress, damaged cable or even a duff protective device. The repair might have taken a bit longer but at least you knew where the fault was.

I had a call out this weekend for a AFDD that was tripping on a ring circuit. The new consumer unit (with 3- AFDDs, RCBOS and surge protection) has been in service the end of November and no issues reported. The customer did his own diagnosing and suspected the fridge/freezer as the circuit stopped tripping when he removed the appliance from the circuit. However, when he plugged the fridge/freezer in to another ring circuit with AFDD protection via a extension lead on a reel it did not trip. At this point I did not know what type of fault it was as the customer did not make a note of LED status on the AFDD.

The fridge has no damage and continuity and insulation resting testing all OK

Ring circuit was intact and insulation testing OK (greater than 500Mohm). The instrument readings were exactly the same as they were at the end of November. Plugged it back in and no tripping. I also ran a 1.8kW load on the same socket for a few minutes to see if I could get it to trip -  all OK.

Ten minutes after leaving the circuit tripped, I returned and from the flashing light sequence on the AFDD it was definitely an ARC fault. Reset the breaker and is was tripping regularly every few minutes.

I plugged in the fridge into another circuit, but this time with my much shorter extension lead. Then proceeded to inspect all sockets and checking tightness of terminals - no issues. Although there are some terminations not accessible for inspection.

Then I noticed the other circuit tripped (with fridge connected via extension lead) - so the conclusion that it is definitely the fridge. As the fridge/freezer is still under warrantee I advised the customer to contact the manufacturer. He plugged it into the original circuit in the hope to keep it running. It did not and I told him again to not run the fridge.

Later that night I get a message that now the other circuit is tripping every time they use the microwave -  not the circuit with the fridge and apparently fridge not plugged in.

At the moment I am not 100% sure it is the fridge and can't rule out a faulty AFDD or has the faulty fridge caused the  microprocessor in the AFDD to go faulty.

As a last resort I told them to switch off all circuits and main-switch and then switch-on one by one. Thinking that these devices have microprocessors maybe they need a restart every now and again - bit like my router at home.

Any suggestions on diagnosing ARC faults?

  • 0

    A bit of monkey engineering to try - swap out the AFDD and replace with a conventional RCBO and see if that makes a difference. If it does, then leave as is and bin the AFDD. It is immature technology and my guess is that not all of the bugs have been ironed out of the software.

    I would be intrigued to know which brand has been used..........

  • 0
    sparkiemike said:
    maybe they need a restart every now and again

    That should not be the case with a deeply embedded micro such as this; if it was needed, the code would not be fit for purpose - especially as it is within a safety-related device.

    As an aside, any software within a device like this should have gone through an auditable verification process to make sure that it will operate correctly when needed. Note, however, that any "failsafe behaviour" would likely be to trip, so it could be that an internal fault (of the software) is being detected or it's working "correctly" but is subject to false-positives!

  • 0 in reply to keylevel

    "...........any software within a device like this should have gone through an auditable verification process to make sure that it will operate correctly when needed......"

    You mean in the same way that Microsoft does with Windows? Scream

  • 0

    Looks like we are all going to have to start going round with something similar to an oscilloscope that captures waveforms when they look like they may trigger an AFDD. then have overlays showing what each manufacturer believes their device will consider acceptable and unacceptable.

  • 0 in reply to Alan B

    Far easier, cheaper and quicker to just revert back to RCBO by swapping out, then discarding the offending chunk of useless kit in the nearest WEE depot.

  • 0 in reply to whjohnson

    Agree, so far I have avoided installing them as I have a choice in most domestic installs. For places like nursing homes I wonder if it can be argued at the moment that false tripping causes more risk than the fire risk they are preventing.

  • 0

    Just wondering.

    Are AFDD's double pole or do they have some element of a shared neutral and could a fault on the neutral of one circuit trigger the AFDD on another circuit. Also would a fault up stream of the AFDD cause the AFDD to trigger.

  • 0 in reply to Alan B

    Indeed. Good point well made. I too have steadfastly refused to fit what I perceive to be the hard sell of nothing but a fancy gadget of no real world use.

    I could foresee these kind of trip events happening when the things were first muted and have never been convinced that it would be a good idea to have fragile silicon chips situated right next to a high current arc chute inside such a small form factor - recipe for disaster.

    The less electronics we have inside consumer units the better.

  • 0 in reply to whjohnson

    Contactum

    PDF

  • 0 in reply to Alan B

    1P+N are switched, so double pole

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