Energized cables, no -load on 3

[wbd]

Hello,
I'm trying to refresh my memory, Years ago, I had heard that it is not good for the life of cables to keep them energized with no load on them. Specifically, I'm thinking of a loop system where the loop is open at one switch and the cable is energized from the other side to the bottom of the open switch.
I don't remember the exact reasoning but seem to remember something along the lines of the fields and cables not being able to kept dry from the heat of current passing through.
Does anybody have any comments on this?
Thank You

 

[MikeHalloran]

Unterminated conductors may become Normanized.

...


My friend Norman was the project manager for a flight simulator, just entering its installation phase. One fine day Norman entered a room at the customer's facility, noticed an unterminated cable lying on the floor, said "What's this?", and before anyone could answer, picked it up.

Did I mention that the cable was as thick as your arm?

Did I mention that the conductors had been stripped?

Did I mention that the cable was live?

The resulting short lit up the room, then darkened it, taking out a 1200A service and shutting down part of a military base for a while.

Norman was, ahem, cautioned, about picking up stuff without permission after that.

;-)



Mike Halloran
Pembroke Pines, FL, USA

 

[RalphChristie]

Do not know the answer to your question, but where I live you would rather keep the cable alive, due to copper theft. That is a major problem in our part of the world.

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[cuky2000]

The only negative effect that I can think at this moment is the overvoltage caused by Ferranti effect. When that happens, the only current flow is the capacitive charging current. This causes higher voltages at the open end of the line.

In rare conditions, this overvoltage may be above 10% of nominal system voltage, with possibility to exceed the cable insulation rating or creating overstresses to power system components.

 

[BJC]

If the cable is de-energized and fails you will not know it util it's too late. If the energized cable fails ( from a rat gnawing through it or a backhoe cutting it ) you will have time to repair it.

 

[davidbeach]

If the cable is de-energized and fails you will not know it util it's too late. If the energized cable fails ( from a rat gnawing through it or a backhoe cutting it ) you will have time to repair it.

Besides that, if it's energized, it'll bite back. I have always recommended to my clients that all underground cable, once initially energized, be kept energized when ever possible.

 

[itsmoked]

It also helps you find the person responsible for cutting it.

 

[benlanz]

What voltage class of cable are we talking about? Low voltage <1kV, MV 5kV- 35kV or...

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[wbd]

It is a 5kv class cable. It feeds underground as part of a loop configuration and the loop is open at this switch with the cable energized to the bottom of this switch.
Thank You

 

[ITSTOAST]

We had a 5kv feed and transformer in a basement as a spare and kept it energized to keep moisture out. Sump pump quit and winding blew out when water reached it. I still think it was a good idea. Mike I like your Norman story, been there done that

 

[Skogsgurra]

I think that we should have a MYTH FAQ. There are so many subjects to put in there. Let's see... This cable thing, that power line heating, the running a CT with open secondary (yes, some truth there), the DC motor without field (yes, I know), the bridge consumed by DC corrosion, the...

I think that I will start a new thread on this.

Gunnar Englund

http://www.gke.org

 

[benlanz]

wbd,

I am sorry it took me so long to reply. I am just returning from the IEEE Insulated Conductors Committee conference.

The issue is not a simple one. If you leave a cable energized (with or without a load) you are likely to have a normal distribution of failures over time (if any). If you deenergize the cable you are likely to have most of the failure (if any) soon after energizing. Explaining these statements from a material stand point would be near dissertation length. I recommend that you keep the circuit energized and document the history of the cable as it ages with the rest of the population. When you need the cable again you will have some idea of what level of reliability to expect.


Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[wbd]

Benjamin,
Thank you for the reply. This puts the issue to bed for me.
Barry

 

[Electic]

Despite any theory, typical utility practice would be to either keep the cable energized or remove it.

A de-energized cable laying in the ground is an unknown quantity and at minimum would require a hi-pot or meggar test prior to energization. If the cable has exposed ends it could absorb moisture.

On the other hand, having unneccessary cables plugged into a distribution system can cause unneccessary outages.

 

[petamm]

one notice: energized but not loaded cable still causes the losses, if this is important for someone,should think about...I agree the switching on the long time de-energized cable-it could be a problem. I am sure you know there are many predictive diagnostic methodes to estimate the cable state-I would always recommend to do diagnostics before switching it on or (of course I mean for long time deenergized cables). If not it is better to leave it energized.