LV 400V AC armoured cable glands are geeting overheat(Temp. measured more than 60 deg C)

May need more detail to be certain, but my first suspicion would be substantial currents circulating via the cable armouring and causing heating. Is there a neutral ? or is this a three phase, 3 wire system ?

Are large harmonic currents being produced and circulating via the cable armouring.


Single core SWA cables are prohibited in the UK, and whilst this prohibition may not apply in your locality, the laws of physics are still the same, and substantial currents can be induced in magnetic armouring of single phase cables.


In a power plant, do not discount stray currents from some other part of the system, being diverted via LV cable armour.


The line currents in the paralleled cables look very unbalanced and may be symptomatic of some other design or installation flaw.

Is the current flowing in and out under each gland compensated ?  I am not too sure from your description if each gland surrounds 3 phases,  and there are 2 glands in paralllel, or if there are 3 glands, each carrying two cores but of the same phase each.

For best results you need flow and return currents within any one hole to be near zero sum,  ie. all 3 phases - if not then two effects occur -

The first is magnetic heating of any magnetic metal that the cable passes through - typically problems arise with cables entering thick walled steel boxes.  A simple saw cut connecting the holes for flow and return current fixes this. If need be that slot can be back filled with epoxy or braze metal, anything  interrupts the magnetic path forming a loop around an un-cancelled current in the steel

Nor is it clear what the armour of the cables is made of - steel armour suffer the same problem.

The second problem is one of induced currents - if an un-cancelled current flows in cable inside a metal sheath, and it does not need to be magnetic, this introduces a voltage along the braid/ armour . Put two such armoured cables carrying different currents, and connect the armours at both ends, and a one turn air cored transformer is made, where the primary is the wanted current, and the armour forms a shorted secondary - the voltage is very low, but so is the resistance around the loop, especially with aluminium armour, The trick is either to have insulated bushes under the load end glands on all but one cable, so the armour is earthed at the origin but the shorted loop is broken. (or an insulated plate for the glands to bolt to, and then a separate earth connection - often the glanding boxes have a removable panel to make this easier.)


Or one can break the armour in the middle, and cross link the armours from different phases, so that the induced voltages more or less cancel or at least the total voltage on each armour is a sum of so many metres of phase 1 and so many metres of  phase 2 and so many metres of phase 3.

I agree with the responses above.  Some other ideas to investigate...


Phases do look very unbalanced for motor load.  Is there a poor connection on one phase somewhere or a motor with a phase down somewhere?  Neither of these will help with the armour currents though.

Are the two transformers feeding Bus A and Bus B on the same tap setting.  If not and if they are paralleled somewhere this would give rise to large circulating currents.

Have you got the same problem (perhaps to a lesser degree) on other MCCs.  Have a look.  If not are the armour earthing and glanding arrangements the same or different?


Hope this helps.

These are single-core cables, yes?  Are the glands brass (non-magnetic), or are they steel, or cheap fakes steel coloured to look like brass?  A steel gland on a single-core cable will get hot.  Check them with a magnet.  Is the gland plate steel, or is it aluminium?  If the gland plate is steel it needs to have a slot cut between the holes for the glands so that there is not an unbroken circuit in magnetic material around a single conductor or a single phase.  Where the cables are armoured I assume they are AWA?

This appears to need linking to your discussion about main protective bonding 

https://communities.theiet.org/discussions/viewtopic/1037/24004?post_id=117756#p117756

This discussion is really beyond the scope of my experience, so don’t consider me an expert.


This appears to me to be the relevant part of the regulations.


BS7671:2018


521.5 AC circuits electromagnetic effects 


521.5.1 Ferromagnetic enclosures electromagnetic effects 


The conductors of an AC circuit installed in a ferromagnetic enclosure shall be arranged so that all the line conductors and the neutral conductor, if any, and the appropriate protective conductor are contained within the same enclosure.


Where such conductors enter a ferrous enclosure, they shall be arranged such that the conductors are only collectively surrounded by ferromagnetic material.


These requirements do not preclude the use of an additional protective conductor in parallel with the steel wire armouring of a cable where such is required to comply with the requirements of the appropriate regulations in Chapters 41 and 51. It is permitted for such an additional protective conductor to enter the ferrous enclosure individually.


521.5.2 Single-core cables armoured with steel wire or steel tape shall not be used for an AC circuit.

NOTE: The wire or steel tape armour of a single-core cable is regarded as a ferromagnetic enclosure. For single-core armoured cables, the use of aluminium armour may be considered.

This discussion is really beyond the scope of my experience, so don’t consider me an expert.


I would consider protective conductor to the  sub main distribution board as the main earth conductor and the earth terminal in the sub main distribution board as the main earth terminal and run the main protective bonding conductors from this MET to the extraneous parts.


I would then run additional protective conductors, if required, to the distribution boards in parallel with their circuit cables as per regulation 521.5.1 which I have copied out above.


Hopefully that makes sense.


 Andy

Hello Team Mr. Broadgage, Mr. Mike, Mr. Statter, Mr. John Russell & Mr. Sparkingch ip.

First of all I apologized for late reply & would like to thanks & appreciate the prompt response concerning to our discussion.   

As per the preceeding reply's i came to noticed that some of the points regarding our LV system is not clear so let me elobrate the system bit in detailed.

Pl see the attached following below files,

1. LV System SLD concerned breakers position are marked. 

2. Q1 Breaker cable entry & gland earthing condition. Q1 & Q2 cable & gland arrangement is identical. 

3. Q7 Breaker cable entry & gland earthing condition. Q7 & Q10 cable & gland arrangement is identical.

I think if the proper gland earthing will make then this issue will be sorted out. Kindly also share the correct way of gland earthing. 

It would help if it was known if this system has been up and running for some time with no obvious issue, until now, or if it has only recently been put into service.

Hello Alcomax,

This system was running since 2011 & outer insulation failed because the cable insulation was exposed to gland heat since long time. this is what we are thinking. 

kindly share your comments.