hi guys
Background:
I currently have a 150m long steel wire armoured PVC/PVC 50mm2 3C+E cable installed which powers a 45kW DOL motor. The motor is powered from a large 3 phase motor control center, which also powers many other DOL motors. Each DOL motor has a fuse, contactor and thermal overload installed. The control circuitry (240VAC) for each DOL motor feeder is powered from the upstream side of the fuses with its own 6A control fuse. The control circuitry for each drive is scattered across each of the 3 phases (ie. drive 1 control circuitry is powered between phase 1 and neutral, drive 2 control circuitry between phase 2 and neutral etc.). Each drive has it's own module within the motor control center.
The 150m cable runs from the motor control center, overhead on cable ladder in an air conditioned switchroom, out through a cable transit in the wall, onto some more cable ladder, underground (buried direct), up onto another cable ladder, into a junction box, out of the junction box, through a cable joint then into the motor. The motor is located in a hazardous area, the junction box glands are potted, the switchroom is in a safe area.
The event:
Last weekend, all 3 fuses blew on the 45kW motor as a result of a short circuit some where. The event also caused all control supply fuses on phase 1 to blow, meaning we lost 1/3rd of the drives powered from the MCC. The contactor for the 45kW drive has also been fused shut as a result of the fault.
The investigation findings:
I disconnected cable between the motor and junction box and the cable between the MCC and junction box. I meggered all cables. On the day of the event, 1 ohm was found between phases on the cable between the MCC and junction box. 1 ohm was also measured between each phase to earth. Cabling in the MCC and from the junction box to the motor was fine (measuring high resistance). The motor was also fine (measuring high resistance).
I measured again today (2 days later), again cables that were fine are still fine. The faulty cable now measures ~50-100 ohms between phases and earth. The faulty cable is 12 years old.
My suspicions:
The only conclusion I can come up with is that somewhere along the cable we've had some water penetration, resulting in a short circuit between phases and a short circuit to the armour (which is tied down to earth at each end of the cable). This explains the resistance changing over time (ie. water drying out) and the obvious short circuit. My only concern is, how does water get into a steel wire armoured PVC/PVC cable that is 10 years old. We run a pretty strict operation in that any accidental tampering with the cable as a result of maintenance/upgrade work would have been reported. The cable has not moved and is securely fixed in place. Feedback is appreciated here. I'm also a little stumped as to why the control fuses blew on the neighbouring drives, but suspect this may be due to fault current being fed back from neighbouring drives up through the MCC phases.
The way forward:
I've heard of cable fault finders being able to do tests to pin point the fault. My concern is that if the water dissappears by the time I get them in, they will not see the fault (ie. no short present). Replacing the cable will cost upwards of $100k. Is there any calculations or tests I can do ASAP to try and nail where this fault is in the cable? I've racked my brain all day and cannot come up with anything.
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