Underground Cable Fault & Heating

[willj]

Hi All,

Would like to know if anyone could offer their observations of the heating involved with a underground cable fault, either 240 or 480V.

I am having an issue where I would like to understand how much heating could possibly occur.

This would be for a supply service on the secondary side of a distribution trans, before the service entrance.
This would be a high impedence fault that would not trip any protection, so the fault could exist for an extended period of time.

So, how hot could the ground get around the cable fault and at the surface. Buried 1 ft or 3 ft below grade?

I would appreciate any links to reports, pictures, or first hand experience.

I suspect the fault "zone" of earth would get quite hot.
Would it melt objects around the fault? Hot enough to start a ground fire?

Thanks in advance for any comments.

Will...

 

[itsmoked]

Kinda hard to say with no suggested current...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jghrist]

Assuming 75°C cable and assuming that the protection would trip before the cable was damaged, then the upper limit would be 75°C. This is the conductor temperature. The soil at the cable would be less because of temperature drop through the insulation. I would expect that the soil temperature would get pretty close to ambient not very far from the cable.

 

[willj]

To provide more info - what I was trying to get to:

Given a fault condition exists on the cable - L-G for example.

High resistance fault - current is below the upstream fuse capacity on the primary side of the transformer.
And as the fault is before the downstream breaker the breaker is no longer in effect. Nothing really to protect the cable at this point.

The current most likely is well above the rating of the cable.

Hopes that clears up what I am looking for experience on.

 

[itsmoked]

It doesn't clear up anything.

The current most likely is well above the rating of the cable

How can this even be?!? The breaker protecting this cable will not allow it.

Are we talking 1A at 240V? 240W

Or 200A at 480V? 96,000W

Can you see there might be a heating difference between these two cases?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[willj]

Here is a rough sketch.


Primary TX Secondary Service Panel
---------- ||-------------- ---------|
Fault

Here are some numbers:

Example
- Primary current: 25 amps
- Secondary current: 250 amps

Secondary Volts: 2 cond, 240V, + neutral

Below the 50 Amp primary utility fuse, and before the distribution panel breaker.

As you can see this fault would go until it clears it self or is detected and the service disconnected.

-----------------

I am not trying to dig into the numbers too much.
I am more looking for if people have observerd an underground cable fault before and what were the effects.

Thanks

 

[ausphil]

we have observed thousands of underground cable faults, and some low voltage cables can be in service with quite high fault currents for long periods of time.

we run 400A, 600A and 800A fuses on LV distributors, and to have a phase to earth fault in the order of tens to a hundred amps can certainly occur. To the fuse, it just looks like load current and will happily sit there until the heat generated eventually changes the form of the fault (maybe creates a full open circuit), or creates a lower resistance path which in turn draws more current, which creates a runaway situation until finally the current becomes sufficient to blow the fuses.

to answer your orginal question, we have seen link boxes (underground boxes to allow easy switching and interconnection of distributors remote from the substation) which are pitch filled, where the temperature has been so high that the pitch has melted, and the bitumen around the box is soft. So yes, faults in LV cables can sit around for a long time generating heat until some condition changes to make it a more substantial fault. This is especially the case if you have paralelled the section of distributor out of 2 substations (for some maintenance work or other failure etc and haven't restored the system correctly), because you have 2 supply paths and 2 supply fuses that allow even higher current to flow intot he fault and cause heating.

also, when we dig on fault locations, you can get some assurance that you have dug on the fault location even before you see the cable due to the heat that is held in the soil. It may get to, say 30-40 degrees (celsius) above the cable, but when you get to the cable, maybe 60-70 degrees.

"hot enough to start a ground fire", I would say probably not, unless you have something that is flammable that takes little temperature rise to start giving off vapours for a fire to start.

ausphil

 

[racobb]

Yes there can be the unusual instance where a high impedance fault allows just enough current to severely damage the cable over time, but may be low enough not to blow a primary fuse that is 150 plus percent of the transformer rating. These kinds of events are rare in that they do not escalate to enough current to either blow or melt the primary fuse out. Not much can be done and I would not loose to much time being concerned with the issue. I have seen overhead triplex cable with one hot conductor welded to the neutral and still operate, but that is definitely the exception. Didn't repair it energized though! Same could happen to an undergound conductor.

Have seen overhead 15 and 25 kV conductors dance around on the ground and turn sand/silica into glass and melt large holes in asphalt. Pretty scary!

Can it happen, yes, often no. They are definitely the exception.

Hope that helps.

Alan

 

[Marmite]

My observations are similar to Ausphil's. The UK system is similar. I've seen paper insulated lead sheathed cables where the lead has completely disappeared over a considerable length due to a persistent earth fault. One fault location technique is to look for dry patches after rain. Sometimes the tarmac lifts with the force of the fault. Often the ground is steaming when you dig out the hole for the fault repair. There have also been instances of melted plastic gas pipes in the vicinity of faulty cables. There is potential for a lot of heat but probably not that likely with 2 single cores and 240V, especially if they are aluminium, which tends to burn back into the sheath and create an open circuit.
Regards
Marmite

 

[willj]

Thanks all for your input.

It does give me insight that these faults although uncommon do happen. And that it can get quite hot given certain circumstances.

I would still appreciate any more feedback from others that have experienced high ground heating with this type of fault.

Thanks,
Will...