Tingling from Myford Super 7

Hopefully, the problem will be sorted shortly (no pun! )

Howard

The mention of parallel earths brought to mind an accident which happened in the Air Force many years ago, I can’t remember the details but parallel earths had been configured which led to a lethal potential occurring and a fatality. Can’t be too careful with earths they seem innocuous but need to be regarded with respect.

Dave W

Sod's Law strikes again! At least the lathe is innocent and the builders are already in...

I hate wires and pipes buried in concrete.

Thanks for sharing,

Dave

Colin, it's good to hear that you escaped electrocution. I'm worried that there may still be a problem with your wiring.

If the insulation between live and earth had partially broken down, I think that your house earth must be poor, if earth wires' voltage was 'pulled up' enough to cause a tingle. You were, in effect, acting as a better earth (lower impedance) than the house's earth. I suggest you get the earth impedance checked.

My early childhood was in a house which was wired exclusively (as far as I can remember) without any earthed sockets - they were all two-pin. It was a 'normal' to get a tingle or a buzz, when a metal-cased appliance, such as an electric fire, was brushed against. Also, child-sized fingers used to be able to touch the plug pins when inserting or removing plugs, with expected shocking consequences. I think I'm lucky to have survived - along with millions of others...

Post WW2 it was common to find 2 pin systems in use and they remained so until at least the the mid 50s. Those were the days of rubber and cotton insulated wires some encased in a lead sheath, but many draped wherever it was convenient. Even had 2 pin adaptors to plug into lamp sockets.

As has been said before, you have two faults.

The leaky cable under concrete is fault 1, but you still have to find fault 2 which is the inadequate earth connection to the frame of the lathe - and possibly other loads. Please don't ignore the second fault!

HTH Simon

I intend getting the motor checked out and then rewiring the lathe. As I understand it there are three items that need earthing in common to the house earth, motor, lathe itself and Dewurst switch. They say things come in threes and I've just had a call from the builder saying there's a huming from the garage. On investigation he says it's coming from a battery charger which is completely disconnected! I've asked him to stow it safely away from the house just in case.

Don't focus all attention on the lathe. I suspect that it's innocent, and just allowing you to experience the fault, which is likely to be common to all power sockets in the house, because the house earth is not good enough. It's potentially dangerous - get the house earth checked!

Got that thanks. We have an electrician on site sorting the house installation.

Got that thanks. We have an electrician on site sorting the house installation.

I assumed that the lathe is on the end of the faulty cable. If so it's earthing to a point distant from the lathe and there will inevitably be some resistance between the house earth and the ground under the lathe. Any resistance will cause a potential difference and it doesn't need much to cause a tingle.

We don't know how Colin's supply is earthed - there are three or four different arrangements in the UK, including a bit of pipe or a copper plated steel earth rod banged a few feet into the ground. This old fashioned arrangement is good enough to blow fuses and take the bite out of a shock, but not good enough to keep everything in the house at ground potential. New build houses don't use this system, but there's still the possibility of slight voltage differences to ground depending on how the wiring is arranged.

Earths are a bit like lathes, they vary in quality! As earths go, domestic electrical earths are rather poor. Not a good idea to use them for lightning protection, and they're total carp in an antenna system.

I'm not worried - Colin has an electrician on the case.

Dave

Colin, you say the electrician is sorting out the house installation but earlier it was stated the lathe is in the workshop, is the electrician now going to treat the workshop as a separate installation with it's own supply, distribution unit and earthing arrangement ?

Emgee

I know I keep banging on about this, but the idea that there might be enough resistance in the earth path to allow any item within the installation to reach a voltage sufficient to experience a tingle needs de-bunking, as it flies in the face of everything the Reg's have to say about earthing.

Sure, the earth path will have some resistance, but the installation must be such that the potential reached by any exposed conductive parts (metalwork) is always below the safe touch voltage, even under fault conditions (whilst blowing a fuse). Safe touch voltage is usually deemed to be 25 volts RMS, that won't give a tingle though the actual requirement is that anyone touching an exposed metal surface must not be given sufficient shock to cause an involuntary movement.

This places some pretty tight constraints over what earth path resistance can be accepted, and rightly so.

Cows being very susceptible to electric shock there are specific additional requirements for livestock pens, farms etc.

Yes but what Simon's describing is an installation working to specification. Best practice suggests a mains earth should be less than 5 ohms, and that may have been the case when the house was wired. But who knows what's happened since then? Corrosion and damage are far from unlikely, and the actual earth resistance may be higher than ideal. Of course Colin's earth could be out of spec, but he reports an earth leak found in a cable buried in concrete. The leak may not be between the live and earth conductors of the cable, it might be between live and the actual ground, and there's no reason to assume the earth conductor is working correctly: it may be damaged or badly corroded. That could cause a tingle.

How many volts needed for a tingle? Sensitivity varies between individuals but I can detect about 10 VAC on a metal plate as a kind of rubbing sensation, nothing like a shock. I have friends who thought nothing of 70V but that's my 'involuntary movement' threshold. Perhaps I'm thin-skinned and sweaty!

I'm not suggesting tingles are acceptable, only that Colin's symptoms might well be explained by nothing more sinister than the cable fault. No doubt when it's replaced the electrician will test again and let's hope all will be well. If not the search continues.

Dave

Dave

I believe your'e a bit off the mark with a 5 ohm earthing point, that's about 10 times what you would expect from the worst case incoming supply, either PME or sheath earthing.

Of course when using an earth electrode as the installation earth it can and will be many times greater but the system shock protection will be provided by an RCCB (Residual current circuit breaker)

Emgee

Emgee wrote: "Colin, you say the electrician is sorting out the house installation but earlier it was stated the lathe is in the workshop, is the electrician now going to treat the workshop as a separate installation with it's own supply, distribution unit and earthing arrangement ?"

The garage/ workshop is part of the house so is spurred off the ground floor socket circuit. The lathe is the far point from the distribution board, which was a new installation 17 years ago. There would be a benefit to having an isolation switch for the whole workshop, but I am not sure about the benefits of having a separate supply as Emgee suggests?

If there is a benefit, now is the time to do it of course.

Edited By Colin Wilks on 12/10/2019 13:29:45

This may sound a silly question, but has your water main been replaced? A lot of older houses are earthed by the rising main which isn't much good when the water board replace it with plastic!

Brian

Whoops! Double posted... 

Brian

Edited By Brian G on 12/10/2019 13:47:01

I can't advise you what to do, as all our circumstances are different.
When we moved in 3 yars ago, I had a brand new garage/workshop built.
We, the builder/electrician and I decided the best solution was to run an armoured cable (probably not needed in your case) from a spare way on the distribution board.
This was fitted with its own MCB, but wasn't rccd/elcb protected at the house end.
I then had a new distribution board fitted in the garage, which has its own ELCB/RCCD protection.

That way, if I do trip something in the garage, it doesn't take the house out too.

Also it's allowed me to have more than one circuit in the garage, e.g. its own ring main ,+ 16A outlet for the compressor, + 16A for the permanent 110v transformer etc.

Mine's a larger installation than yours, but you get the idea. Also, this allows each of the individual circuits to have its own appropriate curve on the MCB, e.g. my 110v transformer has a large inrush current, so has a "C" Curve breaker, as has the 16A compressor outlet, but the ring main is a normal "B" type.

I'm only working in there on my own, so the overall power consumption calculation, was down to my own projected load at any one time. I just make sure I don't trip the house end C40 MCB by welding, whilst boiling the kettle, with a 3KW fan heater running, and having a mate washing his hands with the electric water heater; You get the idea.

I now need to re-wire the existing workshop on the same principal.

Bill

Just for the record, the earth loop impedance of a given supply, at the supply point (the meter) should be no greater than 0.8R for a TNS supply and no greater than 0.35R for a TNCS supply. It can be and often is lower, the princpal variant being the distance from the substation.

Brian G makes a very valid point, the house previous to my current one was built in 1922 and used the lead rising main as the earth electrode which was fine until the supply pipe was replaced by an MDPE one. It took 18 months to persuade the DNO to provide an earth which took 5 minutes to implement. It's certainly worth checking in an older property that there is an earth at all.

I would agree with Emgee that a separate supply to the workshop is sensible, not least because you not limiting the current availability to the rating of the circuit it is currently spurred from and as in this case a ground floor ring final might well have a washing machine, dishwasher, tumble dryer all potentially in use at the same time before considering the workshop needs. If it can be done easily I would.

Regards

Martin