Separately Run PVC/SWA/PVC Cable for 75KW S/D Starter Motor Create Eddy Current?

[chlee-penang]

Dear Members,

Seeking your view to assess a risk for replacement one length of PVC/SWA/PVC Cable.

Back Ground:

A 90 KW Motor is connected to Star-Delta Starter in MCC at a distance of 200 meters, by 2 runs of 3-core70 mm2 PVC/SWA/PVC cables buried directly underground.

One length having an underground joint covered in concrete recently develops earth fault, one core shorted to ground.

Intend to replace only one length of 3-core faulty cable, laying on surface cable tray that carries other small power and control cables in stead of underground due to majority length of existing cable route already covered by concrete.

Concerns:

Will separate run cable on surface mounted cable creates eddy current since it is not bundled with the other length of cable?

The existing galvanized metal cable tray has metal cover of most of its length.

Thanks,

chlee-penang

 

[Lucien Nunes]

In general, paralleled cables should be arranged to ensure uniform current sharing under all conditions including fault, and it might be difficult to achieve this with your different installation conditions for the two runs. Because of this it would probably be non-compliant with UK regulations other than as an exception. The only real cause for eddy currents under running conditions would be assymmetry in the cable core impedances; this would cause a nett longitudinal current along the tray which could couple into the control circuits as common-mode voltage.

 

[chlee-penang]

In general, paralleled cables should be arranged to ensure uniform current sharing under all conditions including fault, and it might be difficult to achieve this with your different installation conditions for the two runs. Because of this it would probably be non-compliant with UK regulations other than as an exception. The only real cause for eddy currents under running conditions would be assymmetry in the cable core impedances; this would cause a nett longitudinal current along the tray which could couple into the control circuits as common-mode voltage.

Dear Lucien,

Thanks for your expertise advice, seeking your further clarification.


1. Disregarding the two run of cable installed in different route and method, even replacing both cables to be run in same cable tray, the assymmetry in the cable core impedances will still present on each length of cable when starter switches to Delta running condition inducing common mode voltage to other control cables right?

2. Am I right to say that, running one cable on cable tray present common mode voltage, it also creates heat on section of cable tray with cover?

Thanks,

 

[chlee-penang]

Anyone out there could share your view?

Our colleagues feel that there is no harm to replace only one length of cable, on the argument that even replacing both cables, impedance of each run remains the same and the circuit is in series not parallel.