Warco Mill trips RCD!

MCB's and RCD's age with time, and it is quite possible that the RCD is just not good enough to discriminate loads on the supply cables any more.

If it is older than 10 years then a replacement with a slightly higher current rating might be worth trying.

If you know someone with a decent Megger, the hand-cranked type at 500V or the larger and nicer 2kV type, then get a test done and eliminate the motor and wiring.

Note that some terminals such as Klippon are not rated for more than 750V. We use a Claire Flash Tester for checking insulation up to 5kV AC, and we usually find the terminals are the first failure point in a 2kV test.

The Meggers I have are a small hand-wound 500V one and a very nice large mahogany cased 2kV one in its original leather carrying case.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

The RCD is about a year old I would say. The motor was tested with a nice looking Megger; digital readout and all that stuff. The resistance was coming out at about 80k from what I can remember. The thing I cant understand, is that I have used a 100A stick welder in the garage with not problem, the lathe and the floor mounted grinder all leave the RCD alone...

OK, so we can eliminate the RCD for now. What electrical gear is hanging off the wiring on the Warco apart from the motor?

Lighting?

What voltage was that 80k resistance at?

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

The only electrical stuff is the motor..that it. Motor and switch (The motor starter switch is now a new Siemens contactor).

That is a good question, I think is was 500v but I couldn't be sure, I didn't take much notice of it.

'Might' be a voltage-induced breakdown somewhere, and as everything else seems to be OK on the RCD, I think you'll have to look at the motor.

Worth checking any cables for chafing etc etc, usual things.

One last thing you could try, heat it up in the airing cupboard or in the oven at a very low setting (fan ovens are best for this) to drive off any moisture that may be inside. You don't need more than 120 deg F for this, any higher might cause problems for the insulation.

Then try it again.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

Cheers Peter, I'll give it a go!

Hi, I'm not an electrician so I look at everything from a mechanical viewpoint, I expect you have checked but is the mill free to rotate? If it is old and not had much use (in fantastic condition) then the oil and grease can get very sticky and make the starting load very high. Just a thought, is it possible to try running the motor with no load ie the belt removed to warm up/dry out?

Regards

Keith

Theoretically, 80K should only give 4ma, but if it was measured at DC then the AC current could be much higher; however, when we test equipment we will not accept anything less than 2M at 1000V so I would suggest 80K is way too low. As others have said, try leaving the motor in a warm airing cupboard for a few days to dry out and retest it. Martin

Just an example of how you can fall foul of moisture in used tackle - I bought a dust extractor for my surface grinder. What I didn't know, was that it must have spent some time outside in the rain! The 3 phase motor ran on my converter, but then the next time I wanted to use it, it had seized solid. Investigation uncovered a motor half full of water, which had rusted the rotor clearance up. A clean and dry, plus new grease in the bearings sorted her out. The water had run down the cable conduit to the terminal box on the top of the motor, found its way in, but had no way out until the end cover was removed. I was impressed with the insulation of the motor (Siemens) though, as I didn't experience any electrical problems.

Steve (Sheffield)

Hi Keith. You might have a point. I tried the motor this morning without the drive belt on it: the motor ran fine, with out tripping the RCD!! I switched it on and off several times, but still no problem. SO it must be something to do with the high current start. I wonder if it would make a difference if the capacitor size was changed... Failing that, I wonder if I should find a soft start controler such as an VFD, expencive I know.

Cheers Jason.

extra:

I have read some text in the "A Little Workshop Electrical Advice ", some guy had similar trouble to me, with the RCD tripping on motor startup. The solution: Connect the neutral wires together and only switch the phase side has completly solved the problem! I have been banging my head against the wall for some time because of that little device in the consumer unit!!!