Bonding of Single Condutor Armoured Cables

[hanksmith]

I have a 4160V, 3-phase system with 3 single conductor armoured cables installed to feed a unit sub. They are installed at 4160V and pull about 200A maximum. I have been told that the armour is not bonded at either end.

If this is the case wouldn't the armour have an induced voltage very close to 4160V?

I have my doubts that this was checked out properly and I think that the cable armour is bonded at both ends to reduce this induced voltage, bonding at both end has a different problem as this would allow the flow of current in the armour.

Thanks

 

[jghrist]

The voltage induced in armor that is isolated from ground depends on the relative capacitance between the conductor and the armor and between the armor and ground. Assuming that the cable shield is grounded, and is under the armor, then I'd estimate very little electrostatically induced voltage.

The voltage induced by the current in a single point bonded armor is different from the electrostatically induced voltage; it depends on the current in the conductor, not the voltage.

 

[hanksmith]

The Client did not mention any shield for this cable, they made no mention of anything being bonded to ground, they made it sound like it is an armoured cable with nothing bonded to ground.

If this is the case would the armour not have an induced voltage, possibly quite high?

The the historical current in the cable is about 200A but it is sized to be able to fully load two 1.5MVA transformers (4160/600V)

 

[rbulsara]

I would think the armour should be bonded to ground at both ends, just like a enclosing metal conduit would be.

 

[rbulsara]

hit submit too fast..forgot individually armoured cables... but still they can be treated same as shields, which are bonded to ground at both ends

 

[hdp]

A number of years ago, the utility I worked for was revising standards for paper insulated, lead covered cable risers. The lead engineer wanted to begin to use single-point bonding to increase the ampacity of risers (Southern climate). I received an urgent call from the field...why does this pole smoke when we close these switches? This was a dip, under an interstate highway. One crew thought that the single point bond had been done in the manholes by a splicing crew. As it turned out, no bonds were installed and the lead covering (with PVC jacket) was totally floated. The riser terminations were mounted on a metal 3-phase terminator and arrester bracket. This was a 25kv class 3C-500 MCM cable operating at 12.47kv. As others have already pointed out, the voltage on the lead (or armour) depends on the nature of the capacitive voltage divider that the floating covering creates. In this particular case, the voltage was high enough to track through the wood pole to the pole ground and heat associated metal parts, igniting the pole at a through bolt.

 

[hdp]

Oops, the cables were actually single conductors, not a 3C cable.