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Myford super 7 lathe tripping consumer rcd

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Nick Clarke (Suffolk)01/01/2021 22:35:42
7 forum posts
12 photos

Hi. I am a newbie with no lathe experience at all although I do have a fair amount of aircraft engineering time under my belt.

I have just picked up a Myford Super 7 lathe which was all working fine when I bought it. I am so excited but now it’s installed in my workshop it won’t switch on. It trips the house rcd as soon as forward or reverse is selected on the Dewhurst switch. I have cleaned and rewired the Dewhurst but no improvement. Everything else in the workshop works fine with no trips.

Any suggestions to fix this?

cheers

Nick

not done it yet02/01/2021 08:30:01
5596 forum posts
20 photos

Does it trip ‘as you select the dewhurst position’ or if the dewhurst is in forward or reverse when the machine is switched on at the mains? If only the former, it seems that there may be a make-before-break occuring within the dewhurst or leakage to earth as it is changed over?

Most Dewhurst switches are just that - for change-over - not as a starter. They are (mostly?) not rated for starting duties. A dedicated starter should be incorporated in the supply to the motor.

SillyOldDuffer02/01/2021 09:43:06
Moderator
6861 forum posts
1538 photos

The RCD is detecting an earth leak, ie somewhere along the wires, switch, or in the motor, live electricity is dribbling enough current through the insulation to signal an electrical fault.

Old-fashioned fuses are far too insensitive to react to small earth leaks so it's possible the lathe ran OK in it's previous home and would be happy in your workshop too if you had a pre-RCD consumer unit. But earth leaks are at least a little dangerous - if the earth fails, as they do, the lathe becomes a shock hazard.

Although the Dewhurst is suspect, and should be operated as NDIY describes, never used to power the lathe on and off because that damages the contacts, it's probably innocent here. (By modern electrical standards Dewhurst's aren't a good design.)

First step is to carefully inspect all the wiring for damage. Although a well maintained Myford made in 1947 could be mechanically excellent, the same can't be said of ancient electrics. Rubber insulation perishes and over the years physical damage due to vibration or cables being trod on or pulled etc. If the wiring is rubber or looks in any way tired, replace it. Damaged switches are a possibility, but more likely candidates are:

  • Suppressor capacitors - if fitted. These can be removed without effecting electrical safety; if disconnecting them lets the lathe run without popping the RCD, replace them. The lathe can be used while they're in the post.
  • The motor. More complicated, possibly as simple as drying out damp windings, but... The exact type of motor fitted will need to be identified. Can you post a photo so a motor expert can advise?

Don't panic - in the event of very bad news Myford electrics and motors are easily replaced. Could even be a good opportunity to modernise the machine with a 3-phase motor and VFD. (Note how happy I am to spend your money!)

Dave

Edited By SillyOldDuffer on 02/01/2021 09:43:28

Robert Atkinson 202/01/2021 10:32:34
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902 forum posts
17 photos

SOD and NDIY have pretty much covered it, but I have an additional comment / question.

Do you have access to any kind of test meter? While an insulation tester "megger" is required to do a proper test, a multimeter on resistance will show any gross faults.
With the lathe unplugged but swtitches set to run and the meter set to it's highest resisance range check the resisance between tha earth pin and both live and neutal pins connected together. Any reading less than 1 megaohm is cause for concern.

Robert G8RPI.

Ady102/01/2021 10:47:03
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4224 forum posts
593 photos

House RCDs can be really sensitive

In the last house if I changed a lightbulb it tripped the circuit out

The light switch was off but it still tripped (it must incorporate a low voltage test circuit system kinda thing)

I have an off-board isolated RCD for my garage supply which has done welding lathes shapers lawnmowers etc and never tripped out once

Edited By Ady1 on 02/01/2021 10:49:23

Simon Williams 302/01/2021 10:59:13
578 forum posts
80 photos

Don't forget that a neutral to earth fault will trip an RCD just as much as a live to earth fault. it's worth checking that the neutral and earth are not connected together.

Martin Cargill02/01/2021 11:12:47
153 forum posts

Its worth remembering that the RCD is probably looking for 30ma of earth fault current, but this can be a cumulative 30ma. As an experiment try switching off all of the other breakers in the distribution board and then try the lathe. If it trips the RCD then the lathe is probably faulty. If it runs then add the circuit breakers back in one at a time until it trips. This may show that something else is causing the problem. The usual culprits are heating elements (kettles, electric cookers, water heaters etc).

To add to the complication it may be a neutral to earth fault that is causing the problem so you may have to disconnect ie unplug or switch off items to find the source of the earth fault.

Martin

Robert Atkinson 202/01/2021 11:18:09
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902 forum posts
17 photos

The test I suggested, resistance between Earth pin and Live and Neutral tied together will detect both live and neutral faults.

Robert G8RPI.

not done it yet02/01/2021 11:24:52
5596 forum posts
20 photos

‘Fraid I disagree with SOD (yet again!🙂 ). I think the first step is to check as I suggested. If the motor starts snd runs without the RCD tripping that would exclude leakage from the motor or other wiring - even if the insulation is rubber and ancient.

His suggestion of a three phase 220V motor, with VFD, is a good one, however. Nearly all my machines have been modified in this manner. Soft start, variable speed and other options are well worth the extra cost over a simple single phase motor replacement, IMO.

Paul Kemp02/01/2021 11:26:13
600 forum posts
18 photos

I had a similar fault develop on the ML7 after turning a lot of cast iron. It has the I assume original Brook Compton motor with air vents and it sucked in a load of cast iron dust which accumulated in the terminal box. Opened it up, blew it out, problem solved.

Paul.

Oily Rag02/01/2021 11:48:27
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310 forum posts
126 photos

I have a separate consumer unit fitted in my workshop with its own RCD. When first fitted it tripped every time I switched on a VFD, but was OK the second switch on event of the same VFD following the trip, it did this for the first switch on of the day. I was told by a good electrician that the problem is due to the VFD charging the capacitors up and this causes a drop in the return line power measured by the RCD, he said that industrial RCD's when situated at a significant distance from a 'power sink' will regularly trip RCD's - the reason is the delay in the return power which causes the RCD to then assume that there is an earth leakage and as a consequence then trips. The time function of the RCD can be changed to accommodate a longer delay time function; this allows time for capacitors and inductors to 'charge up' without continually tripping the system. I believe the domestic time function is around 10 micro seconds, my RCD was changed to 30 micro seconds and I have not had a problem since. 30 micro seconds is still ample time to protect humans from a fatal electric shock.

Ady102/01/2021 11:54:21
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4224 forum posts
593 photos

I've done loads of lightbulbs and fuses with no problems at all on the garage one, so they are not all the same

noel shelley02/01/2021 13:45:30
356 forum posts
9 photos

If you take ME, No 4643 smoke signals may help. At this time of year dampness can cause trouble. RCDs can become weak. But a good plan is to have the shed or workshop circuits wired round the house RCD then have a dedicated RCD in the shed with it's own good earth, it will save domestic strife when all mains clocks Etc need resetting. Good luck Noel.

old mart02/01/2021 15:20:09
2670 forum posts
176 photos

You could dissconnect the motor connections and then try the on off switches. While the motor is disconnected, use a multimeter to see if there is any leakage. This is only useful if there is a very large fault. Some lathe motors are in a position where they can ingest particles of swarf if there is air flow through the motor. A 1/2hp replacement motor is not too expensive new.

Andy_G02/01/2021 15:48:05
111 forum posts
Posted by Nick Clarke 5 on 01/01/2021 22:35:42:

... it’s installed in my workshop it won’t switch on. It trips the house rcd as soon as forward or reverse is selected on the Dewhurst switch. I have cleaned and rewired the Dewhurst but no improvement. Everything else in the workshop works fine with no trips.

Is everything else in the workshop on the same circuit as the lathe? If the lathe is on a dedicated circuit, it is possible that the neutral for this has been wrongly connected in a split load consumer unit in the house (if that is what you have - if the neutral from an RCD protected circuit is connected to the non-rcd neutral busbar the circuit will appear to test OK, but the RCD will trip when a load is connected.).

Les Jones 102/01/2021 16:04:50
2214 forum posts
153 photos

Andy. That is a good theory.
Nick, You could prove this by connecting a lamp or some other load to the lathe circuit. If that trips the RCD it probably proves Andy's theory. Tracing neutrals in a consumer unit can be difficult if it is fully populated.

Les.

Maurice Taylor02/01/2021 16:24:43
172 forum posts
30 photos

Hi, another simple test to try ,is use an extension cable and plug into house socket

I don’t mean permanent ,just to test the lathe.

Maurice

Edited By Maurice Taylor on 02/01/2021 16:26:50

Nicholas Farr02/01/2021 16:30:23
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2622 forum posts
1225 photos
Posted by Oily Rag on 02/01/2021 11:48:27:

I have a separate consumer unit fitted in my workshop with its own RCD. When first fitted it tripped every time I switched on a VFD, but was OK the second switch on event of the same VFD following the trip, it did this for the first switch on of the day. I was told by a good electrician that the problem is due to the VFD charging the capacitors up and this causes a drop in the return line power measured by the RCD, he said that industrial RCD's when situated at a significant distance from a 'power sink' will regularly trip RCD's - the reason is the delay in the return power which causes the RCD to then assume that there is an earth leakage and as a consequence then trips. The time function of the RCD can be changed to accommodate a longer delay time function; this allows time for capacitors and inductors to 'charge up' without continually tripping the system. I believe the domestic time function is around 10 micro seconds, my RCD was changed to 30 micro seconds and I have not had a problem since. 30 micro seconds is still ample time to protect humans from a fatal electric shock.

Hi, as far as I'm aware, an RCD only detects an earth leak on either live or neutral, in actual fact they are both live wires, it's just the neutral one is held to earth at the distribution and the earth, well goes through the earth, so you get a small potential difference between the two in your property due to the resistance of mother earth. I don't think an RCD can detect how long it takes for a capacitor to charge up and don't forget that the current flow is swapping sides 50 times a second, i.e. you get a point of zero volts 100 times a second. A circuit breaker on the other hand, will detect an inrush of higher than rated amperage over a few seconds and you can get circuit breakers that will allow a high inrush for a little longer without having one that has a nominal breaking current higher than is needed. An RCD will detect a very small current flowing through your body faster than you can take your figure off a live wire and it will trip, (assuming it is working correctly) a circuit breaker will not. Hence; an RCD will probably save your life but a circuit breaker probably won't in such a situation of being in contact with a live wire.

Regards Nick.

old mart02/01/2021 16:35:44
2670 forum posts
176 photos
Posted by Andy Gray 3 on 02/01/2021 15:48:05:
Posted by Nick Clarke 5 on 01/01/2021 22:35:42:

... it’s installed in my workshop it won’t switch on. It trips the house rcd as soon as forward or reverse is selected on the Dewhurst switch. I have cleaned and rewired the Dewhurst but no improvement. Everything else in the workshop works fine with no trips.

Is everything else in the workshop on the same circuit as the lathe? If the lathe is on a dedicated circuit, it is possible that the neutral for this has been wrongly connected in a split load consumer unit in the house (if that is what you have - if the neutral from an RCD protected circuit is connected to the non-rcd neutral busbar the circuit will appear to test OK, but the RCD will trip when a load is connected.).

I misswired a neutral wire in a split load consumer unit and the fault was hard to trace as the rcd didn't trip every time. My garage spurs come off the non rcd protected side of the consumer unit and each have an rcd socket. The en suite shower and heater have their own consumer unit with rcd and there is also a spur into the bedroom which I fitted at the same time. If I want to test something, in the house without tripping the lot, I use that spur.

Andrew Johnston02/01/2021 17:28:37
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5924 forum posts
666 photos
Posted by Oily Rag on 02/01/2021 11:48:27:

I believe the domestic time function is around 10 micro seconds, my RCD was changed to 30 micro seconds and I have not had a problem since. 30 micro seconds is still ample time to protect humans from a fatal electric shock.

That seems very fast? I was under the impression that standard domestic RCDs normally broke in 100ms, or 40ms as needed. Note that's milli, not micro.

Andrew

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