Circulating current in armoured single core cables
Circulating current in armoured single core cables
(OP)
In one of my Project we have used 630 sqmm/XLPE/AWA/Cu cables for Chillers installed in roof from MDB's in ground floor.For each Chiller of 1600Amp 9 runs of 630 sqmm cable is laid (3 runs for each phase),each phase cables are grouped together and cleat ed with trefoil cable cleats in trench and riser. Each length is 170 mtrs and its both ends are gland ed with aluminium cable glands in aluminium gland plate and for each group one 1Cx300 PVC Y/G cable is pulled.
But last week local authority put one comment "all single core cables to be non armoured" to avoid any current circulating in the armour.
Cable pulling and termination is completed so we cannot change any cables now, so can any one can please advice how to solve this issue technically.
But last week local authority put one comment "all single core cables to be non armoured" to avoid any current circulating in the armour.
Cable pulling and termination is completed so we cannot change any cables now, so can any one can please advice how to solve this issue technically.






RE: Circulating current in armoured single core cables
The easiest way to resolve the circulating current issue would be to change one of your gland plates to paxolin, or some other insulating material, so that the armours are earthed at one end only.
Regards
Marmite
RE: Circulating current in armoured single core cables
The cables are run in 3 trefoil groups as 1.RRR 2.YYY 3.BBB and both ends it is aluminium gland plates.
RE: Circulating current in armoured single core cables
You can get isolating glands from the likes of Hawke and Raxton which are designed to isolate the armour from the gland body which might be easier than replacing the glandplate. An insulating glandplate will need to be fairly substantial to withstand the electromagnetic forces during a fault: glass-filled epoxy or glass-filled polyester would be more suitable than paxolin, but would likely be at least 16mm thick.
You really need to get the conductors into the correct formation as described by Marmite.
RE: Circulating current in armoured single core cables
The present connection scheme will have higher circuit reactance than YRB grouping with greater circuit impedance and conductor voltage drops. The worst case voltage drop may be during motor starting when you can least afford it.
Under Canadian code it may be acceptable to derate the cables if you allow sheath or armour currents to flow. That may mean adding a foutrth group of cables (and changing all connections so that all trefoil groups are RYB). You will then be paying each month for the losses occasioned by the sheath or armour currents.
Bill
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"Why not the best?"
Jimmy Carter
RE: Circulating current in armoured single core cables
But, in my opinion, still the induced voltage in the armor remains elevated. See [for instance] IEEE 525- IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths. Appendix D. Calculation of Induced Voltages.
For one or 2 parallel three-phase systems the induced voltage may be calculated following this standard. For more parallel three-phase systems the induced voltage has to be calculated following general induction rule:
Ep=Sum[Epq=ω*(Iqa+jIqr)*ln(Spq/dp)]|q=1 to n,q<>p| where:
p is the considered parallel single-core cable in which is this induced voltage calculated and q is the inductor cable.
I don’t think the armor could be in contact with the gland plate since the cable is PVC jacketed. Just in case of jacket damage a contact may be present.
RE: Circulating current in armoured single core cables
Armoured glands most definitely would connect to the armour. That's the whole idea of them. I guess a stuffing gland sized to pass the cable outer sheath could be used and the armour just left cut short inside the board, but that's a really rough and ready way of terminating the cable and I wouldn't allow it on my site.
RE: Circulating current in armoured single core cables
If you but each group together i.e. RYB1,RYB2,RYB3,...etc, the sheath induced voltage and accordingly circulating current, will be reduced greatly.
Isolating gland will help eliminate the circulating current however
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If you open one end, the induced voltage should be checked with normal and fault current situations to be safe for personnel. , a figure of 65 V is usually quoted for normal operation and 5 kV for faulty cases.
RE: Circulating current in armoured single core cables
RE: Circulating current in armoured single core cables
There's still a lot of the old stuff in service in the UK, and probably will be long after I retire too.