shield current capacity
shield current capacity
(OP)
if a power cable has a rated short circuit current capacity of 31.5 KA for 3 second does it neccessarily mean that the shield of the cable should have the same rating? if not then what would determine the shield current capacity?
thanks.
thanks.






RE: shield current capacity
If the system is solidly grounded, the sheild should be capable of caring a bolted ground fault for the time it takes the relaying to clear the fault. The same applies for resistively grounded systems, only now the maximum ground fault current is much reduced.
Try these links to Okonite & Southwire:
http://www.okonite.com/engineering/shielding-currents.html
http://www.okonite.com/engineering/shielding.html
http://www.southwire.com/tech/library/pc/pwrcab04.htm
If I haven't answered the question try and rephrase, and it may sink in.
RE: shield current capacity
RE: shield current capacity
thanks for your response .I read those articles too.what I want to say is that I have ordered a cable with 31.5 KA short circuit capacity.I had assumed that there would be a 3 phase fault somewhere outside cable so the cable would have to resist the current.but now if I assume that there would be an earth fault in one of the cables(single core ) and also assume that cable shield is bonded at both ends to earth then fault current would be shared in two paths ,one in the shield and the other in the earth .the earth fault current would definitely depends on the current capacity of the shield. my situation is that tenderers have offered different shields say:tape ,wire ,tape and wire and with different capacities ,31.5 KA(shared in shield and armor ),7 KA,and even 630 A.I don't know what to do .do I have to force them to give me a larger shield cross section and with special design ?or 630 A is just enough for me assuming that total earth fault current would be large enough for the relays to operate .it's quite possible that I totaly misunderstood the concept .so please help me.
thank you
RE: shield current capacity
Bung
RE: shield current capacity
If you ground at both ends this could still conceivably be the case if the fault is close to the supply end on the cable the shield back to the supply would carry most of the fault. If a cable fault is not the most serious case (ie: line differential relaying) and a through fault could last much longer, then you need to analyze the total copper going back to the source of ground fault current. (Current divider) (ie: shields, sheaths, ground cables) I personally would not include the earth path because of the low resistance of copper.
You may want to read some other threads on shield grounding.
RE: shield current capacity
except some formula in the okonite web site but in that formula you have to know the shield current then evaluate the cross section while I don't know what is the fault shield current.with regard to circulating current
actually we intend to perform cross bonding system on our system so I don't think that there would be a problem although again I couldn't find a single formula regarding
circulating shield current calculation and the amount that it would be reduced by this method in any document ,text book or manufactureres web site.any help on this topic?