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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 in reply to Sergio Fernandez

    Certainly worrying that an AFDD can be "dead" out of the box, and the issue not be detectable on-site.

    I didn't quite follow the Grenfell fridge comment though - the last time I took a fridge apart (which was a long time ago admittedly) the one thing it didn't have was an isolating transformer - all the major components (compressor motor, light, edge heater, thermostats) were all directly connected as I recall. Maybe "smart" ones these days are all driven by electronics? (or maybe the electronics just drives a relay that controls the motor?)

    Maybe an AFDD wouldn't have seen the fault anyway (an overheating compressor motor I would imaging be closer to resistive heating than arcing until it was too late anyway), but I'd still like to understand the thinking behind the comment.

            - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy

    whjohnson said:
    Also, for those who think that an appliance arc fault would be picked up, for example the grenfell fridge - a AFDD would not have detected it.

    I think that comment was by

    It would be good to hear his thought process on this. 

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    Certainly worrying that an AFDD can be "dead" out of the box, and the issue not be detectable on-site.

    That would be a question for the R&D Team at Hagar to investigate.  Maybe they are unaware that the AFDD POST (Power On Self Test) does not detect certain issues and then fails safe.  It would be interesting to get some imput from Hagar on this.

  • 0 in reply to Sergio Fernandez

    What aspect of a fridge catching fire do we think an AFDD would detect that say an MCB or RCD would not ?
    Certianly if there was not much else switched on even a classic arc signature would be ignored - AFDDs have to be made to ignore arc-like waveforms of less than a few amps to avoid tripping on normal operation of brushed motors.


    By the look of the forensic evidence of dr-j-duncan-glovers-expert-report  in interview he seems to reckon that a poor crimp inside a hot point Fridge freezer caused the insulation to overheat and then burn.

    "A probable cause of the fire is a poor crimp connection for... an overheated wire connector within the compressor relay compartment for the fridge-freezer (Hotpoint Model FF175BP) from Flat 16."

    .....

     "The overheating connector in my opinion was the first event that started burning the insulation on the wires that led to a short circuit."

    ..... 

    "The overheating of the crimp starts the fire. It overheats, it glows, it ignites."

     (transcript of that verbal evidence here ) Pages 60 to 85 are most relevant.

    Equally in his actual  written report he also mentiones

    " Conversely, the following observations do not support a fire origin within BPS/I,
    the fridge freezer from Flat 16.
    (1) No arc damage was observed within BPS11.
    (2) The 13A fuse for the fridge freezer power supply cord is intactinot blown.
    (3) Whirlpool has reported that there have been no recalls and no legal
    proceedings/claims against Whirlpool, Trulesit, Or its representatives involving
    a. Hotpoint FF175BP or FF175B G."

    But given the charred mess remaining, it would be a brave soul who concluded much at all without some sort of caveats. Note that the no 7 'kitchen' breaker had tripped at least by the fire had taken hold, while other circuits did not, and the owner turning off the main switch as he escaped in many ways helped to preserve the scene as nothing else was tripped later due to cable burning through.

    Mike.

  • 0 in reply to mapj1
    mapj1 said:
    led to a short circuit

    Excuse my ignorance in this but.  Was the short L to N or L to E or is the power in

    Hotpoint Model FF175BP)

    Direct Current. 

    Are AC and DC arc the same same signature?  My understanding is that DC can jump arc or spark further distances than AC

  • 0 in reply to mapj1
    mapj1 said:
    (2) The 13A fuse for the fridge freezer power supply cord is intactinot blown.

    13 amp seems a bit high for a fridge

  • 0 in reply to Sergio Fernandez

    I am afraid that I  do not know about anything relating to Grenfall for sure any more over and above that in the evidence publically available, that I have linked to above - if you have more please share.

    I think my fridge has a 13A fuse - the inrush is quite fierce - 80-100 amps for the first cycle or so, I suspect others are similar, and a 5A one may die after a few starts.

    M.

    A DC arc does not self extinguish at the zero crossing like AC but the ca[ability to draw an arc length is comparable if enough current is available to feed the ionisation of the plasma. In practice DC swithches need larger gaps and a much faster rate of separation for the same voltage. The fridge freezer will be mains though.

  • 0 in reply to mapj1

    Not sure I'd even be as confident as he was looking at that to be honest.

    But he did do some CT scans of crimps and found serious voids.

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