Incident Energy Calc of Faulted MV Cable in Manhole

[jtinge]

It has been suggested by some of my colleagues that IEEE 1584 equations can be used to calculate the incident energy resulting from a faulted MV cable in a manhole. Am I correct in my understanding that the equations in IEEE 1584 are only applicable to a fault between conductors having an air gap? If this is correct, I don't see how the equations are applicable to finding the incident energy of a faulted medium voltage cable that won't have any air gap between phases or to the grounded shield. The only way I can trick the software to making this calculation is to insert a bus with an air gap at the manhole location, but this does not realistically model a faulted cable or splice and I don't have any confidence in the results. Does anyone have any experience or guidance with this situation?

 

[waross]

If there is no air gap at a fault location there may not be an arc. This would be a bolted fault and the energy would be dissipated in the resistance of the cable and the source.
An arc implies an air gap, however small.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree with waross but I think you could take the distance between bare conductors-that means about 2 insulation thickness-as possible air gap in a phase-to-phase short-circuit case...

 

[waross]

And on that I agree with 7anoter4. The arc may "fringe" so you may want to also do a calculation based on about 150% of two insulation thicknesses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The IEEE 1584 calculations do not apply because they are based on test results and the tests done do not in any way resemble a cable fault in a manhole. A MV cable fault would be Ø-grd, not Ø-Ø because of the cable shield.

Using ARCPRO, which was used for the NESC arc hazard tables (>1000V) might be an alternative, at least these are based on single phase faults. These are based on faults in air, however, so they are not strictly applicable either.

Do you have to label the equipment in manholes for arc flashes resulting from cable faults? If you are interacting with the cable or equipment in a way that might cause an arc, you are doing something like switching or removing a separable loadbreak connector. In these situations, the arc would be in air, not through the insulation. What kind of interacting with the cable or equipment would you be doing that would cause an insulation failure in the cable? I guess that moving the cables could initiate a fault if it the insulation was damaged and sitting there ready to blow.

 

[dpc]

Well, if there were no arcing faults in vaults, there would never be fires in vaults, and that is obviously not the case.

I agree with jghrist that the IEEE 1584 equations don't really apply. But there is clearly some risk.

Even a fault caused by direct contact between conductors is not necessarily a bolted fault because the conductor material will quickly vaporized and arcing can ensue as the distance between the conductors increase.

Although a fault is likely to start as a line-to-ground fault in a cable, it may be something else before the fault is clear.

David Castor

http://www.cvoes.com

 

[jtinge]

I appreciate the responses. At our site we don't permit anything other than cables and splices in our manholes. While we have had cables faults, the damage has been limited to small blowouts at the fault site. Protective relaying usually clears the fault within a few cycles. We usually have to thump the cable to find the fault location. Not having transformers or other oil filled equipment has probably limited our risk of manhole fires. I agree there is still risk in manhole work with energized cables; however, I am at a loss as how to assess that risk. As previously discussed, the current state of the art arc flash hazard analysis methods don't seem to be applicable to internally faulted medium voltage cables. That being said, how does NFPA 70E assess the risk of medium voltage cable examination in a manhole as a HRC 4? This doesn't seem to align with PPE recommendations in the NESC.

 

[jghrist]

The NFPA May 2003 Report on Proposals (ROP), Annex J, has some additional information on the Hazard/Risk Category Selection Table.

Under "Examine insulated cable, in manhole or other confined space", the "Arc Energy Hazard and Risk Assessment" is "Wide range of energy and confined space, risk moderate".

The HRC is 4, with a note "A job safety analysis is required to justify that this task be done with the equipment energized."

The "Substantiation for Selection of Hazard/Risk Category" is "Risk moderate --> HC 4 or 5, Insulating gloves required due to the nature of the task."

 

[dpc]

There are numerous differences in the approach to arc-flash safety between NFPA 70E and NESC. They will probably eventually converge towards something like a consensus, but it will take a while.

Unless the facility is under the control of an electric utility, you are better off trying to use NFPA 70E.



David Castor

http://www.cvoes.com