Tingling from Myford Super 7

The Dewhurst switch on my Super 7 started to deteriorate to the point where the motor was struggling to start. I have stripped it down, cleaned all the contacts and replaced one with a spare, and the switch is now operating correctly.

From reading it seems it is best to leave the Dewhurst for selecting forward or reverse whilst powered off and then switch power on and of via a NVR (no voltage release) switch. I am sourcing one of these but for now am switching the lathe motor on and off via the 13 amp plug socket switch.

Question: after switching everything off this morning I touched the top of the Dewhurst switch housing and felt a very mild tingle, which disappeared after a short while. The motor is a Crompton Parkinson .75 bhp 1425 rpm 220 volt unit which is earthed to the lathe cabinet and to the domestic circuit earth. It starts by engaging a centrifugal switch which I can hear resetting itself as the motor slows down when switched off. Is the tingling I'm getting some residual current from the starting condenser and is it something to worry about?

I'm not an electrician but would say that Yes, you have something to worry about and should get the electrics checked. It could be dangerous.

Paul

The problem is poor insulation and earthing of the Dewhurst switch. First thing is to check (or more likely provide) the earthing on the switch chassis and cover.
Theses old switches probably would not meet current safety standards. A modern cam switch (proper make and suppler, not junk off ebay or alibaba) in a proper enclosure would be a better choice.

Yes it is something you should be worried about.

Robert G8RPI.

Turn it off at the mains, unplug it or isolate it or whatever it takes to make sure it can't be energised. Without wanting to get too melodramatic, this is a potential (no pun intended) emergency and you MUST isolate it. NOW.

The symptoms you describe are of two separate unrelated faults. Firstly you have an earth leakage fault of some kind, and sure that Dewhurst switch is a likely suspect, so are the motor internals.

You also have a second fault, in that the frame of the machine (lathe, switch etc) is not earthed.

My suggestion is that you get someone with experience of looking for these two faults to have a look, as you need some specialised test equipment and the knowledge to use it and interpret the results. If you are in the West Glos area send me a PM and I'll gladly help.

As Robert has said, is is very definitely something you should be worried about.

Isolate it and call for expert help.

HTH Simon

If indeed the motor, Dewhurst switch, and lathe itself are well earthed and commoned together, you might need to get the earthing in the 13A wall socket checked out.
Bill

I have used Kraus + Naimer switches successfully for a number of years . They have a huge number of switching options and have proved very reliable . Suggest you get out a qualified electrician to fully check out and test not only your existing installation a.s.a.p. but also to wire any replacement switch and then re-test once installed. . Hope this all helps .

The motors often fitted to the Myford lathes are in a rubber resilient mounting which effectively isolates the motor to which the earth is connected from the lathe. Any switchgear mounted on the lathe must be earthed or you risk the lathe becoming live if a suitable fault occurs. It will never do any harm to fit some earths to the lathe and the cabinet and indeed it is almost certain to be a requirement in the regulations. There have been a few references recently to equipment that gives a bit of a tingle and it would do no harm to make sure effective earthing is installed and tested to prove it’s effectiveness. It is worth protecting your workshop with RCD protection although this can bring problems if you run any VFDs that can often cause nuisance trips. Power tools which in themselves are usually double insulated have cables that in a metal working shop can easily be damaged and make things like metal benches and cabinets live, swarf can be embedded in the cable and cut through to the live, don’t walk on trailing cables.

Mike

Switch OFF and leave it that way, until you have had the motor, Dewhirst switch and all the wiring INSULATION tested using a Megger or similar tester. Hopefully you know of an electrician, its a few minutes job. You mention bonding, Myford motors are usually resilient mounted using rubber bushes on the motor cradle. There needs to be an uninterrupted earth connection from the motor case to the motor mount/cradle, to the cabinet, Dewhirst switch and the 13 amp plug. You mentioned using a proper motor starter fitted with NO-Volt release, that is the proper way to go, select direction of rotation using the Dewhirst , then press green button. John

If your house is wired using PME system ,there could sometimes be a potential difference between the earth you’re stood on and the earth your lathe is connected to, possibly giving a slight tingle.

Best advice is isolate it and get a qualified electrician to check it.

Many thanks everyone. Lathe is unplugged and out of bounds until I've got the circuitry checked out professionally.

Simon, thanks for the offer, but I'm in Leeds so it's a bit of a trek for you!

Edited By Colin Wilks on 07/10/2019 15:47:17

+1 especially as Colin had to repair the Dewhurst. I find that electrical faults that appear after I've repaired something are usually due to me...

Check how the Dewhurst's cover is earthed. I think it relies on the screws that hold it together, so dirt spoiing the electrical connection is a possibility. However, it may be two faults. Something is leaking current and something is stopping the domestic earth from grounding it.

The repaired switch has got to be chief suspect, it's elderly, not off particularly good design, and may not have gone back together perfectly. How old is the wiring? Rubber goes off and can leak through the insulation when a hot wire is laid against the metal work. Moving the cable harness might be enough. If it's rubber, time to replace it.

Damp?

Be good to confirm earth continuity from the lathe to the domestic earth. Wires come adrift and develop stress fractures while plugs and sockets get damaged. Sod's Law says the earth will fail rather than Live and Neutral.

Whatever the cause I hope it's trivial to fix!

Dave

Well, I'm not saying it can't happen, but it would be a strange and significant fault in the supplier's PME system to allow sufficient potential difference between neutral and earth to be detectable by the "tingle test".

The point of PME (parallel multiple earth) is that there are many paths of cross connection between neutral and earth; all of them within a zone would have to fail simultaneously to get a "tingle" as described. Not to mention a few house fires....

No. Go looking for a failed or broken earth connection. The last one I did was caused by the earth pin socket pin not making contact in the socket outlet (13amp socket at the wall).. The machine earth bonding was all connected together but the whole lot was floating from the house earth because the earth pin didn't connect. In that instance the motor was connected to a VSD so that was the cause of the leakage current. If I've understood correctly there is only switchery in this application, so there is no excuse for enough leakage current to cause the symptoms as described.

But that's not the whole problem in this case, you've got a second fault (earth leakage current) as well. Come what may you need an adequate earth connection and this is absolutely a text book example of why.

HTH, do let us know how you manage to resolve this.

Good luck Simon

Yes, been there, which is what I was alluding to in my previous post about checking the wall socket.
When I first bought my previous house, back in '84, I was aware that something in the downstairs ring main wasn't quite right.
It turned out that whilst the walls were apparently dry(ish) on the inside, they had been plastered on top of a corrugated bituminous cardboard like stuff.
There was enough dampness behind it, that the bare earth wire in the grey twin and earth had completely corroded away; I don't mean a bad connection to the 13A socket, the wire had vanished completely.

Bill

Edited By peak4 on 07/10/2019 17:51:28

'The point of PME (parallel multiple earth)'

The correct terminology for PME is Protective Multiple Earth, and is also known as Tn-C-S. (For those interested, Further detail can be found here.

As stated the lathe is unplugged and will not be used until properly checked. However, to clarify, the tingling I felt occurred some thirty minutes after the lathe had been switched off at the 13 amp socket, although the plug was still in. Prior to this the switch had been operating correctly. Slightly foxed.

It would depend on what sort of distribution board is your house/workshop; RCCD, ELCB, MCBs, wired fuses etc.

As regards the lathe, if the Green/Yellow earth isn't actually earthed to ground (on the machine or the socket) and the neutral is in contact with the metal body of the machine, then I'd expect there to be an ac voltage to ground on the chassis. Probably enough for you to feel it. This would be coming from the 13A socket, so would be the case, regardless of when you last used the lathe.
It would be the case even with your arrangement of turning off the motor at the 13A switch, as very few are double pole switches, so mains neutral is still connected to the machine. (obviously the earth should be too, as that shouldn't be switched)
If somewhere someone has managed to wire an earth neutral reversal, or maybe an earth neutral contact, but with broken earth continuity, the machine would still work live to earth when you turned it on at the wall, rather than live to neutral, as it should.

An early house distribution board might not sense this and trip the circuit.

Bill

I should probably ask, are you in the UK?

Edited By peak4 on 07/10/2019 22:01:19

Edited By peak4 on 07/10/2019 22:08:08

In the UK at Leeds. We have the builders in at present and the electrics started tripping yesterday. Got the sparks on site today to trace the fault. The garage / workshop is now totally isolated from the circuit and the general fault is still there, so this may well be a combination of a number of separate faults confusing the picture. I'll report back if I find something wrong with the lathe. I think the way forward is to test the motor and then rewire the lathe when I install the NVR switch.

Well, I'm puzzled too. The 'tingling' implies a.c., so energy stored in the motor's start capacitor can't be responsible. It was well-known, to schoolboys, decades ago, that a torch bulb could be connected between neutral and earth, and would glow, variably, providing free light. This demonstrated that the neutral line floated around a few volts from ground potential, but nothing like enough to cause a tingle. The wall switch should switch the live line. It's possible that it has been installed so that the neutral is switched instead. Surely, if the wall switch is OFF, and assuming it's correctly switching the live line, there is either a persisting high-impedance connection to the live line (because the switch is faulty) and a poor earth, or stray coupling to the neutral line and a poor earth. If you have a high-impedance voltmeter, with the switch OFF, can you detect significant volts around the machine, between L or N and earth? And can you do the same test using a known good earth?

I have seen a domestic light switch in which the insulation between the contacts had carbonised, allowing a small current to flow across the open contacts.

Never trust earth connections!

Edited By Kiwi Bloke on 08/10/2019 11:07:57

Edited By Kiwi Bloke on 08/10/2019 11:09:37

That implies an earth fault in the domestic circuit rather than the lathe and I see your next post says 'We have the builders in at present and the electrics started tripping yesterday.'

I think something is leaking gently on to the house earth, this could be entirely normal like that due to suppression capacitors, but enough flowing to trip the mains suggests a faulty appliance. (Not the lathe.) Depending on the age of the house and the standard it was wired to, it's possible for a poor mains earth to float a little above true ground and cause tingling. (As Kiwi Bloke says, tingling = AC.) Wouldn't rely on it being 'normal' though - more likely it's an earth fault.

Interesting one! Can you let us know what the electrician finds?

Dave

We isolated the problem to an old bit of wiring which the electrician had decided to reuse when we last had work done 17 years ago (it was routed under some concrete). That will be rewired in its entirety which should fix things. There was resistance between the live and earth. The builders have been using a Kango hammer, so I suspect the vibration from this has precipitated an accident that was waiting to happen.