Hello All,
I've been call to a small chapel as the power goes on and off once or twice a week.
Cable is a 50mm 4C fed from a 100A 6kA MCCB 50-60m away. 4pole RCD in Chapel.
Load on each phase is about 20-35A when the A/C kicks in so MCCB and cable OK for that.
IR at 500V Fluke 1653B. Results checked on 2 meters
I pulled the cable out at both ends and did an IR test as we had a bad batch of UAE cable and very poor installation in the ground about 5-6 years ago which has now started to fail .
Results - L1 to L2, L3 N Earth over 500meg - fine
L2, L3 N and E a dead short between them all - 0.00
I had ment to do a continuity test and get a reading but was called away.
Don't worry the cables are capped off and LOTO at the MCCB.
If its a dead short how can the MCCB hold on for 3-5 days at a time before tripping, how can the MCCB ever be switched on for that matter ?
It would be a good idea to get an accurate IR figure but the problem with using a continuity tester is that it will only use a test voltage of a few volts so the figure it gives will be inaccurate and won't correspond to the real life insulation value at 400v. Ideally you need a 500v megger tester than can measure down to 10 kilo ohms range like a Simpson 505 for example but you're in the realms of expensive equipment (> 1000 quid).
If it wasn't tripping the MCCB you could actually reconnect the cable to the supply but not connect to the load and apply power, then put a clamp meter around each wire to see the actual leakage currents. From this you'd get a better idea of your IR. If the currents are very small then you could progress to an earth leakage clamp to get a more accurate reading but I'd suggest a risk assessment before doing these kinds of live tests and appropriate PPE.
Bottom line is that even without further testing your megger tests are sufficient evidence of a cable fault so the other option is just replace it or yank it out, find the damage and fix it.
Again using the 1653B a resistance of 827 ohms was read between L2 and L3.
I threw a small amp clamp around the conductors and left the building off but no current flow was shown !!! Still got the same IR result though, so I don't know.
Yes the cable will be pulled in the next few days.
(400)²/827=193. 193W just from one of the affected pairs of cores. If that's a single fault rather than distributed leakage it'll be getting warm!
Has someone possibly made an illicit tap in the cable and you're reading their connected load? That could explain a lower AC current than the DC resistance would suggest.
.........Again using the 1653B a resistance of 827 ohms was read between L2 and L3.
I threw a small amp clamp around the conductors and left the building off but no current flow was shown !!! Still got the same IR result though.......
You've got a conflict of test results if you're measuring an IR of 827ohms on a scale that's using a test voltage of a few volts and there's no current flow when it's powered up. With a real resistance that low any normal clamp meter would have happily given you a current reading.
I'm with Lucien, you need to investigate further but start thinking what could give you a phantom load. Out of interest, is there any metering connected to this cable? CT metering is the kinda thing that could give you confusing test results. Is there any kind of monitoring devices like anti-theft cable monitors or even an industrial earth leakage relay? My money would be on some small electronic load being still connected somewhere.
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