Spare Cables - energize or de-energize? 2

[wbd]

Here is the situation:
3 transformers each feeding a switchboard
Each switchboard has a breaker that connects to a cable to another switchboard as a cross tie for backup
Each cross tie cable has a breaker at each switchboard
All cross tie breakers are open.

My question that I have been struggling with is it better to leave the cross tie cable de-energized or energized?

My thoughts are that if it is energized and the cable faults, you would know about it before it is needed. The other thought is that it may be more detrimental to the cable to leave it energized with no load on it.

Thoughts/opinions/experiences?

Thank you all in advance.

 

[cranky108]

How would it be more detrimental? What are the probability of failures, and what would be the mechnisum of failures?

You want to know you have a viable backup, so either you must reduce the risk of failure. That said figure what are the failure modes, and how do you reduce them?

With transmission lines, or power transformers we do that by using reduced voltage to energize them. For oil filled equipment we monitor the oil quality. What other factors can you control?

 

[davidbeach]

A breaker at one end of each should always be closed leaving the cables energized. If they fail while not truly needed you'll know that in time to do something about it before you really, truly, need the cable. If underground and subject to water in the conduit then it's even more useful to keep them energized.

 

[magoo2]

If the cable is likely to be surrounded by moisture, then I agree with david and keep the spare energized. If it is indoors in a dry environment, then I would probably keep it energized just to know that it is still working.

 

[racookpe1978]

From above, it seems clear that underground or potentially any cable likely yo be "wet" should be isolable by breakers and kept energized.

Let's assume not underground, and not oil-surrounded. Could it then be isolable with breakers, but those breakers only shut for testing once a year? Or wold that"cycling" of the isolation breakers and insulation actually be worse than continuously keeping it hot?

 

[davidbeach]

Thermal stability is always better than thermal cycling. But a short (length wise) cable kept energized won't be thermally much different from the same cable deenergized. If you don't have complete physical security, an energized cable provides a certain level of protection against theft, and a likely detection of that theft should it be attempted. The annual test only tells you that it was good the last time it was tested; some point it may no longer pass the test and you'll have no idea how much of the year you went without a suitable tie cable. The mechanical impacts of energization also mean that moment of energization has a slightly higher risk of failure than some moment in which the state of energization did not change.

 

[Marmite]

As long as the cable is live it is being monitored by the protection. If you leave it dead for any length of time the suitability for re-energisation is no longer a given, and the risk of it not being available when you need it increases. When you need it in anger it could have been rendered unserviceable in the intervening period by a number of methods, such as moisture ingress, third party damage, theft, vermin etc. Someone could even cannibalise the "spare" circuit breaker for parts.
The voltage isn't mentioned but the safety rules in some companies would require any medium or high voltage cable left dead for more than a day or so to be tested before it is made live.

Regards
Marmite

 

[wbd]

Thank you all the replies. BTW voltage is 480V.