Old Cables
Old Cables
(OP)
Currently working on a job that has cable installed that I plan on using, the problem with the cables is that they have been installed for over 40 years now.
The cables are in use so I know they are working, I am just adding additional load to them. If the cables fail they will shut down a good part of the plant.
Are there any de-rating factors to use to adjust for the age of the cable?
Thanks
The cables are in use so I know they are working, I am just adding additional load to them. If the cables fail they will shut down a good part of the plant.
Are there any de-rating factors to use to adjust for the age of the cable?
Thanks






RE: Old Cables
RE: Old Cables
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Old Cables
All 1 conductor cable.
some are armoured.
some are un-armourded, un-sheilded.
The un-armoured cables are in concrete duct banks, the armoured are in dedicated cable tray
RE: Old Cables
RE: Old Cables
We did this and our client's unscheduled electrical outages dropped to a small fraction of what it was previous to the test/replace effort.
I know (from experience) that standing in a meeting and telling management that you want to replace cables that are still working is sometimes a hard sell, but you're talking about the future reliability. It's far easier to schedule replacement at your convenience that run around screaming in the dark when they fail at some critical juncture in the future.
Document all conversations for your records. That way, if they turn you down on replacement, you'll have an "I told you so" file for the untimely failures.
Good luck!
old field guy
RE: Old Cables
I think a shut down is possible, from the research I have done the PD test is not great for cables unless they are about 90% failed, it's great for testing the terminations though.
I have now been told that a tied up power factor test is best for testing cables but I have never heard of this test before, I did a google and it came up with lots of hits, if anybody has some decent literature on the topic it would be greatly appreciated.
Thanks
RE: Old Cables
As a contingency I would have replacement cable located and available.
Your raceway may be bad as well. If you have embedded raceway it may be cracked and/or collasped. The most common problem is raceways get silted up over the years the cable can cemented in place. It can be impossible to pull out.
Line up your ducks so you can install new cables in a hurry. Tha may include new raceways.
I have done cable that were not that old and we put in new cables and swapped connections to make the transitions. If shutdown time is valuable new cables and racways can be very cheap.
RE: Old Cables
Similar, but slightly different equipment.
"I think a shut down is possible, from the research I have done the PD test is not great for cables unless they are about 90% failed, it's great for testing the terminations though."
PD testing has come a long way in just the last year or so, there are different methods and equipment (And databases) out there, some are much better than others, the only problem areas is when there are 2 different cable types spliced together. These guys have a great system and Don is probally one of the leading PD gurus in the industry. http://www.nooutageelectricaltesting.com/
"I have now been told that a tied up power factor test is best for testing cables but I have never heard of this test before, I did a google and it came up with lots of hits, if anybody has some decent literature on the topic it would be greatly appreciated."
Cable testing methods has been discussed here many times before, and there were some really smart cable guys in the discussion, search for those threads. Dont listen to what you have "heard", do the research, read the standards.
ANSI/NETA
IEEE 400
IEEE 576
ICEA
All 4 of these are recognized MV cable testing standards, none of them discuss Pf, they all are very similar.
One other good reference is the EPRI cable testing procedures that were updated last year, could be the most recent in depth study.
RE: Old Cables
1. VLF 3Uo 60 min. withstand test, 2 .tan delta or other integral method test (like RVM,IRC...), 3. PD off-line test. Generally each of the method has some + and -, you always must combine them. For exmpl. if the cable insulation contains a lot of water - there are no air/gas voids - no or less PD´s - PD test seems to be OK but cable can faild under VLF test.Although I am a big PD testing fan, in this case I guess it will not tell you a lot (40 years old 5 kV cable construction - a lot of PD´s along the whole cable lenght possible or even PD location impossible because of the multidischarge activity and multi signal reflection, a high water/humidity cable insulation content possible in case of PILC, WT in case of XLPE/PE insulation). You should think about replacement in the near future as well. Testing can prolong the time and sort the cables.
RE: Old Cables
RE: Old Cables
The time in use its enought to take the decision.
RE: Old Cables
I haven't posted to this site in a while but, with all the confusion I hear in the above discussion, it appears that we need to open up this topic again. We have had this discussion several times on this forum but perhaps many of you don't remember our previous discussions. I recommend that you do a search on this site for 'cable testing' if you would like to review our past discussions.
I will assume the subject cable systems are built with extruded insulation. Are they?
Yes, a 40 year old cable is 'old' but, if the financial impact of an outage is low and the cable system's reliability assured with a thorough diagnostic test, there are very few good reasons to replace it.
1) Moderate loading is not likely to be an issue
The constant loading has very little to do with the insulation reliability of the shielded power cables. If we are talking about the reliability of mechanical connection you might have a point. Extruded cable systems rated 5kV and higher almost never fail by conduction or thermal issues. Extruded cable systems almost always fail through an erosion process associated with partial discharge (PD).
2) PD and TD are not effective with unshielded cables –in fact most electrical tests are not effective on unshielded cable!
hanksmith says that some of the cables are unshielded. Unless a cable has a shield to define a ground plane, you are only testing the points of the cable that do. Otherwise you are testing the air around the insulation, which is not important!
3)On-line PD finds <3% of defects
Why are some of you recommending an on-line PD test for the shielded cables when the circuit can be de-energized? If reliability is critical, the only test that can assure reliability is to repeat the manufacturer's off-line PD test.
4) TD cannot assure reliability
TD detects aging mechanisms in cable insulation. The problem is that you can not tell where the "losses" are in the cable system. Most TD tests in field are not performed with guard circuits at the terminations. If you do not use guard circuits, your measurement is useless. Most field TD equipment sold in North America do not have guard circuit capabilities! Losses measured over the surface of an old termination or at joints can be 2 to 3 times higher than losses in cable insulation. Even if a TD test is not grossly affected by a termination or joint, what are you going to do with a measurement that tells you that your old cable is, well.... old! This sounds like an expensive calendar to me. If you are trying to compare the relative losses on 1000s of utility cable to prove to management that your old cables are old the TD approach may make some sense. If you are working with critical cable systems, and TD test is a very poor choice to assure reliability.
I know one company that replaced 30+ out of 40+ terminations because of "bad" TD readings. I asked how they knew which terminations to replace. They didn't know, so they just replaced the terminations at both ends of each phase with a 'bad' TD reading! I asked they used a guard circuit. They asked me, "What's that". My friends, unless you use guard circuits, your Power Factor, TD, Dielectric Spectroscopy, Relaxation Voltage,..... (all general condition assessment tests) results will have serious errors. Sadly, the company who did the TD testing, is also in the cable repair business. The more 'bad' TD readings they measured, the more repair business they received! My advice... Before you decide what test to use and what to do with the results, consult an expert and the IEEE standards.
5) For the record, PD tests do not measure leakage. Leakage has very little to do with extruded cable system reliably.
6) A "Tip up" test refers to the shape of the voltage vs. loss curve. To the best of my knowledge there is no such thing as a "tied up" curve. If there are losses in system under test, the voltage/loss curve will tip up in a non linear fashion as the voltage increases.
7) Careful with VLF destructive withstand – you may do more damage than good
If you can afford an outage, and you simply want to blow out some weak points on a shielded power cable a very low frequency (VLF) AC destructive withstand is a reasonable choice. Remember a destructive HIPOT cannot assure reliability.
8) I agree with Zogzog that you need to read the standards. However, the guys associated with the link provided need to read the cable testing standards too. While they provide many great electrical services for other electrical components, to the best of my knowledge they only provide, on-line PD testing for cable systems. If the manufacturers only did on-line PD testing on all cable components sold over the last 40 years, all of our cable systems would be in big trouble!
Cheers,
-Ben
Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants
RE: Old Cables
Many thanks in advance!
RE: Old Cables
RE: Old Cables
VTPOWER
RE: Old Cables
Dodgergrl,
I assume that by MV 90 you are talking about 5kV class cables systems insulated with EPR insulation without an outer metallic shield. You have a serious challenge on your hands. Unfortunately there are no highly effective, predictive diagnostic tests on the market for unshielded cable (without an outer metallic shield). Without a shield, the voltage stress distribution is not even during voltage application. Thus any voltage applied is only going substantially to stress the insulation which is in contact with a ground plane. Without a ground plane, you are stressing the air around the cable not the cable insulation!! The best you can do is to apply low levels of DC voltage (less than operating voltage) for less than 5 minutes to determine it there is any substantial tip-up in a micro-ammeter during the voltage ramp. Make sure you ground the cables for a few minutes after applying the DC voltage.
There are some tests that are in the R&D phase that may help in the future but, for now you are stuck with the DC tip-up test for unshielded underground cables. This is one of the many reasons that the industry is moving away from unshielded cable in medium voltage applications.
Cheers,
Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants