MV Voltage Cable Splice

[umrpwr]

I would appreciate others feedback on the following situation.

A customer of mine has one phase cable in a 5kV duct bank approximately 600' in length with parallel runs that has gone bad and needs to be replaced. The cable is 2250 kCmil Al and approximately 40 years old. Due to age of the cables in the duct back and the fact that that particular size of cable is not readily available the customers engineer has requested that we repair the cable by splicing a smaller cable onto the undamaged cable portion of the cable in the run at a manhole. The new cable would be 2000kCmil and would be approximately 100' in length of the total 600'. The feeder is lightly loaded and not loaded near capacity.

I have recommended against repairing the cable in such a manner and am concerned about what other liabilities and problems may result. Besides being generally bad engineering and construction practices my arguments against do the repair are the following:

Ideally the ownwer would replace all of the cables in the duct at the same time due to the age of the cables. The reasons I told the owner against doing such a repair are:

1.) The age of the other existing cables and remaining life

2.) The new phase cable will have a different impedance and be unbalanced. This problem will become prevalent at heavier loading.

The owner being cost conscious uses the following justification and reasoning for their argument.

1.)The other cables may not be able to be replaced easily in the existing duct bank because of the age.

2.)The feeder is only lightly loaded and any imbalance between phases and cables will not be significant enough to matter.

As a compromise between two the one other option would be to splice new cables of equal length onto all of the phase cables from the manhole. However this is less than ideal and adds another potential point of failure.

I would appreciate any comments on the above situation.

 

[busbar]

Given light loading, could the damaged cable be cut clear and abandoned, changing relay settings or fuses for the modified, lower capacity?

 

[rbulsara]

If the load is within the capacity smallest section of the cable and is protected by the overcurrent device for the short circuit, which I am pretty sure it will be, I do not see a problem with this.

In fact MV cables are protected for short circuit only (comparing cable damagedamage curve vs the TCC of the fuse/ CB). Overload is controlled by design.

By the way, as you already know , 40yrs is beyond the life expectancy of this or any type of cable..they should plan to replace the entire cable or be prepared for such random faioure more frequently...

 

[rbulsara]

Also tapering the cable size as it moves away from the source, or tapping a larger MV feeder with a smaller size cable without seperate overcurrent protection is very common practice in MV distribution systems.

 

[alehman]

I agree with the previous responses. If you can manage without the bad cable, it would be preferable to do so rather than attempting a splice on an old cable, which may not be reliable.

The difference in ampacity between 2250kcm and 2000kcm in free air is about 100 amps. In a duct bank, the difference would be less. Another minor concern is that the load would no longer divide equally between the cables due to the lower impedance of the reduced section. I suspect the imbalance would be negligable.