statter:

Sound can travel long distances so the noise could have travelled but if the trunking is full this would be less likely. 

Why not put a thermal camera on the area in question?  That would highlight hot spots. 

Alternatively you could check conductor integrity by measuring volt drops on each conductor under load on the circuit and comparing to calculated values.  That coupled with decent insulation resistance test results should highlight major issues.

Good idea re- the volt drop test, - trunking is really full, so was surprised if the sound would travel so far, I was expecting that the cable may have been damaged from new cables being added, but there's nothing new in that area. 

Dutch of the Elm:

Hi Dave,


Has the continuity of the trunking been checked, and is it bonded to Earth?


I'm assuming it is metal trunking..


Seems like there was a significant overload on L1 but I'm not sure that this is related.  What type cable comprises the circuit?

Trunking system is well constructed and all well linked with earth straps to DBs, it is indeed steel containment (300x300), all single 6491x conductors, TPNE. 25mm2 live conductors, 16mm2 CPC. Protection is 63A IEC157 MCCB no earth leakage protection at source. 

Dutch of the Elm:

Have you checked IR readings L's-trunking, and  N-trunking?  I'd assume that normal IR testing would show up any inter-cable faults but it might be worth to check each line to trunking.


If trying to find an arcing fault, it might be best to use a test voltage of 500V or more, as opposed to 250V. Subject to disconnection of the load etc.

IR was with CPC connected to system earth (and so including trunking), 500v, so it would seem that the cable run itself is clear (all off the scale at >299Mohm)

AJJewsbury:

During faults singles can be thrown violently apart due to the large currents and the resulting magnetic fields interacting. I wonder if it might be possible for the harsh switching of the current by the arcing fault to set up some small sympathetic vibrations between the singles within the trunking along the circuit length - which might then become audible at some point where the packing allowed a little movement and the cable happened to be close enough to the trunking wall for the sound not to be absorbed by the other cables.

   - Andy.

Hi Andy, 

Yes, maybe the location of the noise was the first place outside of the DB where there was a bit of slack and the cables could vibrate about a bit. I suppose this could have been repeated at other points along its route, but it would have been heard closest to where the fault was being switched from. Luckily none of the loads suffered any damage. 


 

AJJewsbury:

During faults singles can be thrown violently apart due to the large currents and the resulting magnetic fields interacting. I wonder if it might be possible for the harsh switching of the current by the arcing fault to set up some small sympathetic vibrations between the singles within the trunking along the circuit length - which might then become audible at some point where the packing allowed a little movement and the cable happened to be close enough to the trunking wall for the sound not to be absorbed by the other cables.

   - Andy.

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?


It seems IR tests are showing the circuit as clear.


Perhaps once Dave BP is back on site some current measurements could be taken (clamp meter or better, an oscilloscope) with that suspect circuit switched-on but at first under no load, then subject to load.  That might at least show up if there is some odd cable fault.  Maybe have a thermal camera at the ready too.