Table 310.104(E) Cable Insulation

[turtlepokerman]

Can anyone give me an explanation of how duration of line to ground fault is tied to the insulation level (100%,133%, 173%) of the cable? See notes 1,2,3 in NEC 2011/2014 Table 310.104(E). I'm curious how these times were determined and if fault current has anything to do with this duration.

Reason I am asking is if I have a 5kV low-resistance grounded system with a relay that only has instantaneous (ANSI 50G element) where the INST pickup current is set to 20A, do I technically need a timer that opens the circuit after 1 hour if my cable (133% level of insulation) is operating at <20A of SLG fault current.

Any help is appreciated!

 

[davidbeach]

It’s not the current, it’s the voltage. On a system that isn’t effectively grounded a ground fault on one phase causes a voltage shift on the unfaulted phases. The line to ground voltage on those phases can approach the line to line voltage. That elevated voltage is what the increased insulation is for.

 

[cuky2000]

Check the following:

https://www.eng-tips.com/viewthread.cfm?qid=293206

 

[7anoter4]

For 100% and 173% recommendations are clear why: it depends on the potential to ground of the remaining healthy phases. What is difficult to understand it is why 1 hour it is the maximum allowed for 133% insulation. If the system is not grounded the voltage rise on the healthy phase will be sqrt(3)*Vo that means as in 173% case. The cable insulation could withstand then 30% more than rated but not more than1 h?

 

[turtlepokerman]

7anoter4,

Yes you have the same question as I do. Why does the 1hr. time limit come in for 133% insulation? Also how much current/potential is allowed on the shield for over 1 hr on a 133% insulated cable. If it is zero (Current/Voltage) then I would be forced to add some type of protection to guarantee the fault is cleared in the amount of time, because I don't think I can say maintenance can clear a fault in under 1 hour.

Thanks everyone for the discussion. I'm just trying to figure out what I need to do to be compliant with 310.104(E).