Single Core Steel Armoured Cable

[AusPowEng]

I have an industrial client who has suffered a cable failure whereby the screen bond on a single core cable (6.6kV Cu/XLPE) at the end of a short run had a ‘thermal excursion’ burning through the termination kit and cable insulation. Eventually, the protection activated with a phase-earth fault.

Amazingly the single core was manufactured with steel wire armour instead of Al or Cu armour. The screen is also Cu. For the purposes of this discussion, the cables are at least 240sqmm.

The armour was earthed (grounded) at both ends via to the armour clamping glands used on the switchgear cubicle entires. The gland plate is non-ferrous. The screens were also earthed at both ends of the cable.

The cables were in service for 12 months before the failure. What would happen if a 1C cable has steel-wire armour? I would have thought the armour heating (due to eddy currents associated with the current induced in the armour) would have caused cable failure almost immediately after being placed into service.

Initial evidence indicates a manufacturing fault. In my industry, sending someone off to find such a cable is a good joke to play on graduate engineers. Right now however, I am open to worldwide experience, even if to eliminate the obvious. Has anyone else ever seen steel-wire armour on a single core MV cable?

 

[waross]

It sounds like submarine cable.The one that I worked on is armoured by steel rods about 1/4" in diameter. There is poor conductivity from rod to rod, and so there is not much path for circulating currents. The cable that I saw was used to carry one phase of a 13,200 V wye circuit about 1000 feet under sea water.
respectfully

 

[waross]

Sorry, I used very poor wording. The cable is about 1000 feet long, the water is not 1000 feet deep.
respectfully

 

[AusPowEng]

Waross, Thanks for this quick reply. In this instance, this is a standard surface installation with 3 x 1C per phase from a gas turbine to an incomer circuit breaker panel.

On the issue of the path for circulating currents, I suppose that for an armour to be effective as mech protection, the individual wires must be adjacent. There is just tangental contact at best.

 

[sslobodan]

It is common thing in Russia (you asked for international experience) In their old systems but now days they also use Al or CU cables.
In general somebody sell them a cable they had in stock and could not sell to anyone. I cant imagine why would anyone go from Coper wired cable except for low cost. Problem with that cable was that he was over loaded so temp. factor of cable heating was sliding on permissible. He could hold up current that he had if he was placed correctly 0_0_0
0 - cable
_ - blank space diametar => from diametar of cable.

they probably did 000 (the laying method problem I have on field so I must calculate for it) or in bether case
0
00 (triangle form)
and when the summer came ambient temperature raised and so the isolation started to wear down. It is a common problem and that is why you need to calculate permissible current in worst case scenario:
40 degree (Celsius) with ambient temperature 000 configuration for a maximum curent load to be at least = to a sum you need. it varies from cable suppler to suppler but in my data for these are facts:
ambient teperature in C | % of nominal load
25 | 100
30 | 92
35 | 85
40 | 75
45 | 65
50 | 53
55 | 38


So if you have 240mm2 cable with 480 A max load
In are where temp. can exceed 45 C his max load can be 312 A
That is for 000 config. and for 0_0_0 config. his current would be 560A (25 C) 364 (45 C)
Now on high voltage cables difference of those (52 A for wrong heat calc. or 80A for laying method difference) led to a burning down that cable, although he must have been calculated to allow max current to pass (that is why he needed 1 year to burn down). It makes no difference in material Fe CU or AL only the size gets bigger (ever watched 600mm2 AL cable is being layed on -35 C ?)
If anyone interested I can post A entire calculation for laying cables with correction factors It took me 4 years to collect them and they wary from manufacturer to manufacturer but nowadays they allow bigger currents for cables. It would take some time to translate it on English but its the must for anyone in this field

 

[RRaghunath]

I guess the steel armour with both ends earthed provides a good shield against flux leakages.
The currents in the armour would be substantial but the cable sizing calculation would have taken the extra heating in to account.
This kind of arrangement is necessary where large currents are involved and there is a possibility of leakage flux linking the structural steel or other steel closer to the cable route.