After a short circuit the cable does not always have to be replaced; the decision depends on where the short occurred and how hot the cable got at the time. Qualified electrical staff assess the cable by inspection and measurement; if there are signs of damage, connecting the new motor to the old cable simply carries the problem over to the new motor.

DRG-315S-4 110 kW 1500 rpm cast iron frame DRG three-phase motor – motor cable after a short circuit

Where did the short circuit occur?

  • In the motor winding: If the fault was inside the motor, the cable carried high current until the protective device opened. If this was brief the cable often survives, but a check is essential.
  • In the terminal box: The arc may have burnt the cable ends and lugs; the end section may need to be cut back and renewed.
  • In the cable itself: If crushing, rodent damage, junction boxes that let in water or a sun-aged outer sheath caused the fault, changing the motor without renewing the cable makes no sense.

Signs that show the cable's condition

  1. Blistering, melting, discolouration or a burning smell on the outer sheath
  2. Darkening of lugs and terminals at the ends
  3. Signs of heat in the cable tray or around the gland
  4. A low reading in the insulation test carried out by qualified staff

How the test is done and whether the results are acceptable depend on the applicable regulations and the staff's assessment.

Does the cable cross-section suit the new motor?

If the new motor is not the same power as the old one, or the cable was laid years ago for a different motor, the suitability of the cross-section should be reassessed. On long runs, voltage drop also comes into it. You can see the general approach in the motor cable size calculator; the final decision rests with the electrical design and qualified staff.

What else to check besides the cable

The short-circuit current also passed through the fuse, switch and contactor. Contactor contacts may have welded or melted; do not put the new motor into service until these devices have been checked. Bear in mind too that failures caused by insulation damage or improper connection are not covered by the warranty.

Once cable and panel are ready, you can review models matching the old motor's power in the three-phase motor shop, and for larger ratings look at the industrial motor options.