voltage presence on deenergized power cable

[qstorm]

This has got me stumped. We are conducting loop check on shielded power cables that are in present service. There are some discontinuities in the shield of the cable which were bonded only now, long after the cables were put in service. It is for this reason that loop checking on the shield is being conducted. On some of the cables, the loop check confirmed the continuity of the shield (about 0.3 ohms on a digital ohmmeter. On other cables a reading of 1kilohm to 3kilohms registered. The contractor insists that there is "continuity", but we are not about to accept and we are of the opinion that the bonding at some points were not satisfactory.

It was found out that even with the cables deenergized and isolated, there is an AC voltage presence in the order of 2 to 4 volts--something that the contractor claims interferes with the continuity check and is responsible for the high resistance.

What do you think can be responsible for the voltage even if the cable has been repeatedly discharged? Can it really interfere in the continuity check?

The cables run about 900 meters along the carriageway of an elevated railway system and are used for DC traction power supply.

TIA

 

[WhiteyWhitey]

Hi QStorm,

Regarding the Residual Voltage on the cable I would be surprised if there was an AC voltage induced in your cable
by the DC traction. I would be interested to know how this could occur.

Are there any other HV services in the vacinity of the cable route?

Is the Cable earthed at both ends during the test?

What is the system voltage of the cable?

In regards to your strange Sheath resistance test results there could be a few things going on that I can identify, eg a bum sheath bond or perhaps and area where the sheath has been disconnected or damaged,

Are the link boxes still accessable? Can you inspect them for bad bonding?

Are the link boxes fitted with surge arresters?

What sort of test gear are you using?

The more information, photos etc etc you can provide the better these educated fellows can assist. Im interested to know how you go though.

Keep us posted.

Cheers,
Andrew

 

[qstorm]

Some additional data:
-there is an existing 34.5 kV aerial distribution line at some areas above the railway
-cables are laid on concrete cable trenches along the sides of the carriageway
-cable is not earthed during the test; will see if test can be conducted with one end earthed
-the cable is energized at 750VDC
-the links are not accessible, they were sealed with heat- shrink tubing
-the voltage and resistance were taken with Fluke digital multitesters

 

[WhiteyWhitey]

Where did you take your test measurement? Could the low result sugest the sheild to earth?

Do you have an estimate of the value you are expecting or a cable specification?

 

[itsmoked]

Almost a mile of cable will have some sort of AC voltage on it if it is anywhere but out in some desolation wilderness.

Yes this can certainly mess up an ohm reading.

You may need to inject a larger current and measure the voltage.

How exactly are you measuring this continuity? One probe on the shield the other where?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[pwrengrds]

Try to measure the resistance using a ductor (digital resistance ohm meter), along the lines of what Keith suggested. That would put a DC current into the line and measure the voltage drop. This can be done using the other feeders as return lines. You can use a battery to supply voltage from the conductor to the shield at one end and see if it shows up at the other to prove coninuity, though not resistance.

 

[BSEE1980]

It looks like induced voltage. Parallel conductors will have distributed capactance all along their path, inducing voltage from one to the other. You said something about DC: if this is from a rectifier or SCR source, the noise and thus induced voltages can be large.

If the other source cannot be turned off, ground one end and use a ductor. Or a battery and light bulb. Or floating AC source tied to ground and ground the conductors one at a time.

Good luck!
Steve

 

[itsmoked]

BSEE1980; Welcome to Eng-Tips!

Keith Cress
kcress -

http://www.flaminsystems.com