Feeder Failure Point

[noonan]

We were having a discussion the other day about where feeders (MV, underground) usually fail. One engineer said his experience was that they usually fail in the manhole. His reasoning was that a feeder inside a duct is more protected due to the relatively dead-air space. In the manhole, the feeders see more physical abuse and movement during energization.

The opposing point of view said his experience showed the feeders usually fail in the duct. He didn't have any theory as to why, just his many, many years working with these systems.

Both of these engineers are very experienced and well respected, each with over 25 years in power distribution.

Thanks for any thoughts.

 

[bob]

I would think that a cable that fails in duct was damaged during installation.
Cable that fails in a man hole is usually at a splice that was installed improperly.

 

[mdshunk]

Blowouts at terminations and splices.

 

[ultramegabob]

terminations are weak points, wire rarely fails in the center unless it has been comprimised somehow.....

 

[mdshunk]

And splices are normally located in... drumroll.... manholes.

 

[noonan]

Thanks!

Sounds like most things, the interface is the weak point.

 

[jghrist]

By far the largest percentage of the length of the cable is in the duct not in the manhole. Failures that would be randomly distributed along the length (like manufacturing defect) would mostly fail in the duct because that's where most of the cable is.

 

[rcwilson]

From my experience in refineries: Most cable failures occurred in ducts. Could have been due to higher operating temperatures in the ducts or cable fabrication and installation problems.

Failues during commissioning were 95% at the stress cones, splices, or terminations. During construction the failures were in manholes and were physical damge not electrical failures.

 

[noonan]

By far the largest percentage of the length of the cable is in the duct not in the manhole. Failures that would be randomly distributed along the length (like manufacturing defect) would mostly fail in the duct because that's where most of the cable is.

OK, I agree that based on the fact that most of the cable run is in duct, that's where the greatest chance for failure would be. But, the environment inside the duct may or may not be the same as in the manhole.

If the run is just passing through the manhole (no splices or taps), then I would think the failure would probably occur in the duct.

If there are taps in the manhole, do you still think the failure point will probably be the duct?

 

[weressl]

We were having a discussion the other day about where feeders (MV, underground) usually fail. One engineer said his experience was that they usually fail in the manhole. His reasoning was that a feeder inside a duct is more protected due to the relatively dead-air space. In the manhole, the feeders see more physical abuse and movement during energization.

The opposing point of view said his experience showed the feeders usually fail in the duct. He didn't have any theory as to why, just his many, many years working with these systems.

Both of these engineers are very experienced and well respected, each with over 25 years in power distribution.

Thanks for any thoughts.

Based on statistical data as compiled in the Gold Book (IEEE-493) Splices and terminations represent a higher failure rate than straight, uninterrupted portions of the run.

Underground cables - both directly buried, in conduit and ductbanks - are subject to earth shifting either due to natural settling or heavier than designed 'overhead' loads and vibrations. I direct burial the cable can stretch, in conduit and ductbank the conduit can break and shear the cable.

Proper design of depth, reinforcement of ductbank, load(mechanical) consideration and burial construction can minimize these potentials.

 

[zog]

We were having a discussion the other day about where feeders (MV, underground) usually fail. One engineer said his experience was that they usually fail in the manhole. His reasoning was that a feeder inside a duct is more protected due to the relatively dead-air space. In the manhole, the feeders see more physical abuse and movement during energization.

The opposing point of view said his experience showed the feeders usually fail in the duct. He didn't have any theory as to why, just his many, many years working with these systems.

Both of these engineers are very experienced and well respected, each with over 25 years in power distribution.

Thanks for any thoughts.

PILC, XLPE, TR-XLPE, or EPR? Shielded or unsheilded? Open air or direct burial? They all have different most comon modes of failure that is most common.

But generally speaking both of them are correct, and this is why.

The most common failure is in the manholes at terminations and splices becuse with todays simple splice and term kits everyone out there thinks they know how to do MV cable terms and splices, seems so easy, but failures from poor workmanship are more common now than before when only certified and experienced splicers did this type of work.

However a properly constructed AND TESTED (Via tan Delta or VLF) splice is a better system than the cable itself so if the work is done correctly, statisticlly the rest of the cable is most likely to fail therefore the duct that contains the majority of the cable is most likely to be the failure point.