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Arcing noise along length of cable

DaveBP
Hi all, Can anyone help explain this phenomenon? 


In a singles in conduit install, one of my team noticed that in a switch room, when energising the MCCB to a 63A socket, an arcing noise could be heard in the trunking above her head. It sounded quite localised. She immediately switched it off, and we made arrangements to use a different 63A outlet for the equipment being powered from this circuit. 


On disconnecting the equipment, (a Final Distribution Unit in 7909 speak) it was clear that in the plug, either the L1 pin had been overloaded or had been arcing as a result of a loose termination, which is sad, as the unit had been recently maintained. The socket had suffered damage to it's L1 pin too, and the internal wiring (L1) between isolator and socket had suffered thermal damage. 


The installed wiring checks out fine on continuity and IR, and we haven't yet been able to find any damage in the trunking where the arcing sound was heard. - no smell, but to be honest it's hard to visual as the trunking is absolutely packed, and larger circuits have clearly been put in first, at the back. Socket outlet will be replaced.


Question is; is there any reason the arcing would present itself audibly, some 60m away from where there was clearly a fault, or could there be a second fault where the arcing was heard? The load on the FDU was around 16A a phase, and would have been mostly SMPS in LED luminaires, (so quite reactive?) but these luminaires don't light illuminate on power-up, hence using the MCCB as a means of switching-on. 


This all happened before lock-down, but thankfully we're heading back in to work, so it will be one of the first jobs to sort out. I'm thinking we might be able to get one of those inspection cameras with a flexible neck into the mass of cables, and hopefully trace a good length of the larger cores to rule out damage where the arcing was heard. 


Thanks, 


Dave 






  • 0
    I rather doubt that the arcing would have been heard outside the trunking 60 m away - I would expect the cables to damp any sound quite effectively. If it had been loud enough, one might think that it would have been audible at the socket.


    That said, why not sound a buzzer by the socket and see if you can hear it in the switch room?
  • 0
    I'm trying to imagine what any arcing could have been that would not result damage and therefore in low IR readings somewhere, especially in a full trunking. 

    My inclination would be to test, including continuity, then assume that the direction was deceiving and that she heard the plug / socket arcing.
  • 0
    OMS:

    You've not got any circuits in there from different supply sources by any chance - eg, some stuff operating from say a UPS that wouldn't necessarily have it's output synchronised with other systems ?


    Regards


    OMS


    Hi OMS, 


    I'll have to check once I'm back on site, but there may be some new FPs in the trunking that are fed from static invertor, - new emergency lighting install. If not in the trunking then in the vicinity on basket. 


  • 0
    Voice pipes could be used over distances as long as 300 feet (90 m).

    Wikipedia- Speaking tube.

  • 0
    Wouldn't that level of movement imply quite a substantial fault current such that the OCPD should have tripped, and hence there shouldn't have been many vibrations to hear?

    I suspect you only need quite a small amount of energy to produce something audible if the conditions happen to be right (or wrong) - simple speakers can operate on a few dozen mW or less. I believe there have even been cases of a dentist being able to hear a radio station due to a loose filling (acting like an old crystal radio set) - the energy available for that (just from the radio waves) must have been tiny. Even with a crude setup of a couple of parallel wires carrying a few tens of amps and trunking acting as a sounding board - especially if the setup happened to have a resonant frequency similar to that of the noise, it feels possible to me.


       - Andy.
  • 0
    Sparkingchip:

    Voice pipes could be used over distances as long as 300 feet (90 m).

    Wikipedia- Speaking tube.

     


    Interesting as a resident of the borough to read of the "Gosport Tube", but I would still say that if you stuff the tube full of cables, it will not transmit sound. Nonetheless, a chum of mine who is a professor in Nottingham, once made the very valid point that if you do not know the answer, you have to do an experiment; which is exactly why I suggested the buzzer.


    Please don't over-think this one.


  • 0
    A rather brief demo of wires attracting and repelling due to current flow - possible that very sudden changes in current produce a kicking in the trunking that has a similar  interrupted buzzing sound as the arc.

    And in another link here  The theory..




    121212



     

  • 0
    I'm definitely with Andy Jewsbury here... the fault has to be substantial to physically throw cables around, but to simply make an audible noise, especially as arcing tends to be high frequency.. steel trunking tends to sound tinny (pun not intended), inasmuch it accentuates higher frequency noises.


    I can imagine that a significant series arc could cause current fluctuations enough to cause a crackling or arcing noise a long way away from the actual fault.
  • 0
    MHRestorations:

    I'm definitely with Andy Jewsbury here... the fault has to be substantial to physically throw cables around, but to simply make an audible noise, especially as arcing tends to be high frequency.. steel trunking tends to sound tinny (pun not intended), inasmuch it accentuates higher frequency noises.


    I can imagine that a significant series arc could cause current fluctuations enough to cause a crackling or arcing noise a long way away from the actual fault.




    Morning,

     

    Where's the high frequency coming from?  A 230V arc might be caused twice every cycle, i.e. once per half cycle, and that's about it (the arc current itself will follow the sine wave, where there is current flowing).  So the crackling or buzzing should have a 50Hz sound to it.  


    Also, a series arc will limit the current drawn as it is inline with the load and as far as impedance goes, it is additive.  I.e. if the cable doesn't move under load, I don't see why it would be moving (due to magnetic effects) under series arc conditions. 


    A parallel arc would be different and could cause higher currents (low impedance from phase/phase, phase/earth) but that would imply cable damage, which has not been shown up by the IR testing. 


  • 0
    Dutch of the Elm:

     Where's the high frequency coming from?  A 230V arc might be caused twice every cycle, i.e. once per half cycle, and that's about it (the arc current itself will follow the sine wave, where there is current flowing).  So the crackling or buzzing should have a 50Hz sound to it. 


    No, I don't think that is correct. The frequency is much higher than a mains hum, which is two and a bit octaves below middle C. It is more like little shots firing off so you hear the frequency generated by each arc.


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