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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?

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  • 0

    My understanding (which may well be incomplete or wrong) is that the standard's tests for AFDDs work on the assumption that once there is a break in a cable which causes intermittent arcing, the arcing chars some of the insulation over time, and the carbon makes future arcing easier. So the tests are with carbon rods. A lot of the youtube tests try to create arcs with small copper to copper gaps, which tend not to last more than a second or two, and may be seen by the AFDD as a motor-type spark and not trigger a disconnect.

    So when a youtuber shows an AFDD failing to disconnect, it may be that the AFDD is actually working correctly. Or it may be being completely useless. Or the standard isn't up to scratch. We can't tell from the video.

  • 0 in reply to wallywombat

    Indeed - actually the type test for AFDDs is quite odd.  A section of PVC twin cable without CPC is pre-damaged with high voltage, to create a local decomposition.

    The standard is too expensive for causual readers, but luckily you can read a pretty good description of the process  on page 16 here.

    You are quite right passing the test does not guarantee real world operation  - copper copper arcs tend to either burn clear or weld shut and stop arcing, carbon electrodes or brass can usually be persuaded to maintain an arc for a while.

      Cables with a CPC like we use in the UK are likely to trip an RCD once the PVC carbonises to the point of conducting, if arcing  or not, and LSF type cables do not respond in the same way as the PVC, and tend to produce rather less conductive char.

    The doc above is a Swedish chap doing a rather more lab based check on AFDDs with other scenarios, that  may or may not be more realistic.
    He too find situations where the varuious makers offerings are not the same.

    As I said above, the devices and the standard, appear very immature, and the absence of testgear and diagnostics to allow any kind of fault tracking is a serious obstacle.

    Mike.

  • 0 in reply to mapj1

    I have reached out to Kevin at Chauvin Arnoux UK to see if they are working on an AFDD tester or are working on bringing anything to market to test AFDDs with the prospective of an installer/tester. 

  • 0 in reply to mapj1

    Indeed, and another point made in the video but glossed over here, is who is going to pay for the time spent by an installer going round all of the sites where he has installed AFDDs to do a firmware update every time one is issued?

    There are some Klien AFDD testers around in the USA but they don't test to the standards here.

  • 0 in reply to whjohnson

    Are you thinking of these testers ?

    The Klein Patent of 2008 gives away how they do it. I suspect something similar could be done for the 220V equivalent.

    It only really does one sort of arc waveform, but it would at least trigger that part of the AFDD.

    There is a Siemens patent that achieves the same ends with a differnt circuit and generates a slightly different waveform though that style is not out in the wild in any actual test gear to my knowledge.

    Note that apart from the wait 20secs between retests, for the Klein uint, there are no special energy related precautions. The Siemens patent mentions a small fan to cool the resistor bank- which could probably be ommitted with a similar time limitation

    It may become more common once the 20 years is up and patents have expired.


    Mike.

Reply
  • 0 in reply to whjohnson

    Are you thinking of these testers ?

    The Klein Patent of 2008 gives away how they do it. I suspect something similar could be done for the 220V equivalent.

    It only really does one sort of arc waveform, but it would at least trigger that part of the AFDD.

    There is a Siemens patent that achieves the same ends with a differnt circuit and generates a slightly different waveform though that style is not out in the wild in any actual test gear to my knowledge.

    Note that apart from the wait 20secs between retests, for the Klein uint, there are no special energy related precautions. The Siemens patent mentions a small fan to cool the resistor bank- which could probably be ommitted with a similar time limitation

    It may become more common once the 20 years is up and patents have expired.


    Mike.

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