Arc flash upon single-phase short circuit of 6KV motor leads, followed by three-phase short circuit
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The voltage level is 6KV; the normal operating voltage is around 6400V. The motor features F-class insulation. It is used in a cooling tower fan application. The rated current is 25A, while the actual operating current is 16A. Initially, the three-phase leads of the motor were connected using welded terminals, but later they were changed to single cables that were connected to terminal lugs and then secured with screws. Today, a single-phase ground fault in phase B caused a three-phase arc flash short circuit. I hope experts can help analyze the cause
(1) Loose or poor crimping of the Phase B cable can lead to single-phase arc grounding, a situation that may occur at the moment of startup. (2) Contamination and flashover of the porcelain insulator at the Phase B terminal caused single-phase arc grounding
Regarding this issue with the fan motor of the cooling tower at the 6KV voltage level, where a single-phase ground fault in phase B caused a three-phase arc flash short circuit, my suggestion is that you upload pictures of the fault, especially those showing the interior of the motor. An analysis like this is more well-founded and reliable! Additionally, I can’t agree with the view on the second floor! In particular, the second scenario – \"Contamination of the porcelain insulators at the Phase B terminals, resulting in flashover and single-phase arc grounding\" – is almost impossible; the wiring box of the cooling tower fan motor is sealed, so the possibility of contamination and flashover of the porcelain insulators is low! As for cooling tower fans, the humidity levels at the installation site are relatively high. If the junction box is not properly sealed or if the sealing gaskets have worn out, it is likely that moisture will enter the junction box. Discharge to ground will occur at the weakest insulating points inside the box – namely the terminal posts or the areas closest to the metal conductive grounding elements; it is possible to determine these points by checking which cable grounding wire is closest to phase B. Furthermore, in the case of a single-phase ground fault in phase B, it is necessary to thoroughly inspect the internal wiring of the motor, especially the locations where the windings emerge. If the welds are weak or imperfect, arcing may occur; or if the windings become damp, insulation failure at the weakest points within the three-phase windings can lead to a single-phase ground fault, which may then develop into a three-phase arc flash short circuit. This is a likely scenario! In any case, I ask this fellow sea traveler to carefully inspect the internal windings of the motor; perhaps the cause of the fault can be found quickly!
Is it appropriate to operate at 6400V for a long time?
The second possibility is the occurrence of contamination, and there is also a high risk of moisture getting into the junction box. Therefore, a comprehensive analysis is still required based on the photos of the motor junction box interior and the fault scene.
In the case of single-phase grounding, an instantaneous arc flash can cause inter-phase short-circuit faults; it would be best to send the image so that an analysis can be conducted based on the situation
We have also encountered such problems with our 10kV systems; the possible cause is prolonged operation of the leads, resulting in high starting currents； The contact area at the crimped part is poor ； Climbing discharge due to dirty insulators ； Furthermore, when a motor gets wet and electric heating is turned on, the moisture moves towards this junction box. When overhauling high-voltage motors, this lead wire needs to be inspected; we require that the leads not be crimped, but rather welded or brazed. Since this incident occurred, all the high-voltage motors here have been inspected and brazed.