By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

RCD Tripping

All Topics | Latest Posts

Search for:  in Thread Title in  
Raphael Golez25/02/2021 17:22:05
167 forum posts
153 photos

Hello everyone, needed your input on what might be the possible problem on my S7 motor. Its been ok since I have my lathe over 5 years now and it never failed me once, ok a few times it tripped my RCD but after reset all is well until 2 days ago. My lathe is in a well ventilated shed and I doubt moisture is the cause as all my other machines are ok and its been in my shed for the last 5 years.

When I turned it on there was a "clicked" and everything went dead. So I went to reset my RCD knowing it will be ok but when I tried to switch it on it tripped the RCD again and not even a click on the motor. Wires are all ok, plug fuse is new and i could not smell any burning wires. The rest of my tools are ok so the wire to my shed is all intact.

The motor is the original Myford motor. I do have a spare stored away but I need to access it once weather is ok as its buried deep and I don't want to take things out yet. Motor is a single phase by the way.

I was wondering did I fried the motor?

Is the start capacitor the problem?

Is it moisture?

Hope you guys can help me out as I'm useless at electronics. I tried to search the web but I gave up at the end, guess this is the easiest way out. If you guys think its inappropriate for me to ask a silly question then mods can just delete this thread. Thanks in advance!


Edited By Raphael Golez on 25/02/2021 17:22:41

Speedy Builder525/02/2021 17:31:27
2408 forum posts
191 photos

Hmmm! I had a similar problem on my mill the other day. 3/4 Hp single phase capacitor start. After two resets of the RCD, no further problems. It was cold in the workshop, 4 degrees C, but not unusual winter temperatures.

Emgee25/02/2021 17:34:36
2156 forum posts
265 photos


When you "turned it on", does that mean when you switched the power on supplying the lathe or when you operated a starter or other means of starting the lathe ?


Raphael Golez25/02/2021 17:54:49
167 forum posts
153 photos
Posted by Emgee on 25/02/2021 17:34:36:


When you "turned it on", does that mean when you switched the power on supplying the lathe or when you operated a starter or other means of starting the lathe ?


Hi Emgee,

When I reset the RCD and turn on the lathe via the dewhurst switch it keeps on tripping the RCD. The motor does not make any noise at all. So I unloaded the motor from the pulley to test it. Everytime I turned the switch the only thing that is happening is the RCD tripping off. Surely if i fried the motor then subsequent reset of the RCD won't be affected as it shorted out, or is it the case that its just cold?



Martin Connelly25/02/2021 18:09:54
1891 forum posts
203 photos

After a period of cold weather followed by a few days of warm weather moisture condensing onto cold metal can cause problems. It will sort itself out after some time but poorly ventilated areas will take more time than well ventilated ones. Since motors and switches tend to be poorly ventilated they can take a bit longer than expected. Given the recent weather condensation would be my prime suspect.

Martin C

Thomas Cooksley25/02/2021 18:12:32
55 forum posts

Hi Raphael, I would recommend that you disconnect the wiring at the motor and make the ends safe, either connectors or insulating tape. Then power the lathe up, if the RCD trips then the fault is in the wiring or the dewhurst switch. If the RCD doesn't trip then it looks like the motor. Unbolt the motor and take it indoors, put it somewhere warm and dry for a few days. Refit the motor and try it again if it still trips then you are going to have to get the motor looked at. Hope this helps Tom.

Jeff Dayman25/02/2021 18:16:11
2176 forum posts
45 photos

Re condensation possible cause for problem - might be an easy thing to find a small fan driven electric portable heater and let it play warm air on the motor / switch for a day or two, then re-try, before doing disassembly disconnections motor purchasing etc.

larry phelan 125/02/2021 18:16:43
1089 forum posts
14 photos

Could there be a problem with the switch ?

Why not disconnect the motor and supply it without using the switch ?

Dont be surprised if it works ! If the motor was on its way out, you would have known about it before now, I think.

Raphael Golez25/02/2021 18:32:38
167 forum posts
153 photos

I see, thank you all for the input. I will dry things out and see what happens. I wish I could easily disconnect the motor from the switch Larry, but my lathe is position very close to the wall as to save me space. I need to take everything out of the way in my shed in order to leverage the lathe myself for access to the back and easily take the motor out and check the connection from the motor to the terminal block. Hopefully its just moisture as you all suggested and i will patiently wait for it to dry up once the weather improves. I feel that the motor is still ok but im no expert on these hence seeking all your expert opinions. Worse case scenario the motor is fried, I have a back up 1 Hp single phase but I have to dig it out from storage and reconnect as it has its own switch. Hoping that the motor is still ok.

Thank you all! If there are any other thoughts on this issue please share it here as I really need all the help I can get from all of you.


noel shelley25/02/2021 18:44:21
759 forum posts
19 photos

Dampness is the most likely, so a small fan heater for a few hours may solve it. Remove the terminal cover and blow the air in there ! If that doesn't do any goog slip the cover off the Dewhurst and give that 1/2 an hour. Disconnecting the wiring when the unit has a dewhurst switch is not so simple and the Op has said he is not good with electrics, so This may not be a good idea and in view of the number of questions we get on single phase motors and their problems and owners who know very little about them KISS seems a good addage. Try the fan heater first, if that does not help come back to us. Good Luck Noel.

Emgee25/02/2021 19:03:03
2156 forum posts
265 photos


As said the problem could be in the switch but I believe more likely to be the motor.
If there is a short circuit live to earth or neutral at the motor repeated closing (switching ON) of the switch will cause further damage to the switch contacts.

If the trip is a balanced load type even a neutral to earth fault will operate the trip.
Check all wiring for damage and tight connections.
Is there a capacitor fitted ?

Ideally you need an insulation resistance test meter (Megger or other make) to locate the fault without causing further damage by making the faulty circuit live.
You may be lucky and locate the fault with an ohm meter.

Good advice given given re condensation in motor or Dewhurst, check the Dewhurst for signs of tracking between contacts and earthed metal.


Raphael Golez25/02/2021 20:18:48
167 forum posts
153 photos

Emgee, Noel. Thank you. I will try to dry things up and hope I won't go through difficulties. I will be patience and wait for good weather to keep the moisture off the motor. I will also check att the wires and connection as all of you suggested as well as the switch. I will report back the outcome and let you all guys know. Again thank you all for the quick responses. It really helps me to think that I still have hope and a straight forward solution.

regards to all,


Dave Halford25/02/2021 21:10:42
1729 forum posts
19 photos

Raphael, the only places that damp could cause trouble is inside the motor electrical cover plate (two screws) and the centrifugal switch both of which are at the same end of the motor > would have thought the Dewhurst was too well sealed to get enough moisture in it.

If you normally switch the lathe on by the Dewhurst the metal splash from arcing may be causing enough earth leakage to cause the problem.


Edited By Dave Halford on 25/02/2021 21:14:37

DMB25/02/2021 22:07:13
1164 forum posts
1 photos


Dewhirst switches are for motor reversal, not starting. Set Dewhirst to forward and use the wall switch to start. Constant use of the Dewhirst for on/off will damage the contacts. As Dave H and Emgee said, the large spark could cause earth leakage sufficient to make the RCD jump out.If after drying out and all wires securely clamped and motor still not going, then maybe the start capacitor has 'retired'. Replacement avail. on the internet, so I understand. It's under a casing with 2 screws on top of motor, just unbolt and replace. Dont do this for sometime since last live, high voltage shock risk. Serial no. on side of capacitor, to replace like for like.                                  When my original Myford motor packed up, I took the opportunity to modernise with a new vfd from Newton Tesla, so side-stepping single phase faults.Cost far more than original cost of new lathe, stand, 3 and 4 jaw chucks and other accessories put together when purchased many years ago. Wonderful! Wish I'd made the change over years ago. Of course, I realise that you may not wish to afford such a large expenditure on a new motor drive, only praising up my experience of the vfd.




Edited By DMB on 25/02/2021 22:19:01

Raphael Golez25/02/2021 22:22:00
167 forum posts
153 photos

Dave, John. Thank you. I will follow all your advice. I will dry things up and take it from there. I will check the capacitor if all fails and follow your advice. I will try to check this weekend and let you all know of the results. Again thank you for chipping in.


noel shelley25/02/2021 23:26:32
759 forum posts
19 photos

Dave, you may be supprised to find that the Dewhurst has NO sealing, just a tin cover, but yes there may be traces of vapourised metal that could form a track to ground. Since this unit has a dewhurst switch all 4 wires from the motor, 2 start and 2 run there is NO live to the motor ! L1 and N are fed to the switch which then dictates the direction of rotation and switches the run windings in. We will look at faulty capacitors in the next chapter ! Noel.

Dave Halford25/02/2021 23:57:23
1729 forum posts
19 photos

Noel, I considered the small tin cover would not hold enough air, nor would there be a noticeable change of air to condense enough moisture either.

It would be interesting, but not possible to see how/if the motor ran without an RCD

John Paton 126/02/2021 11:43:45
304 forum posts
17 photos

Just to add another possibility as I have experienced it a couple of times - insects getting into switchgear.they like to0 lay up for winter in hidden holes like switch boxes. On one occasion an earwig gave sufficient electrical path in a plug top to actually blow the fuse (he ended up as a carbon track!)

I have also heard of people getting earth faults from fine brass dust getting into mischievous places. You will know best how clean you machine is but it might be worth taking a look under cover plates to electrical connections before rushing out and changing the motor.

If it trips only when you try and start the motor (as opposed to when you switch on the machine with the motor stationary) it could be the start capacitor failing. Any capacitor over 10 years old will be a suspect and as others have said arcing from the contractor or the motor's centrifugal capacitor start switch can also trip RCDs.

Journeyman26/02/2021 11:53:27
1033 forum posts
199 photos
Posted by Raphael Golez on 25/02/2021 17:54:49:

When I reset the RCD and turn on the lathe via the dewhurst switch it keeps on tripping the RCD. The motor does not make any noise at all. So I unloaded the motor from the pulley to test it. Everytime I turned the switch the only thing that is happening is the RCD tripping off. Surely if i fried the motor then subsequent reset of the RCD won't be affected as it shorted out, or is it the case that its just cold?



I notice that Raphael says he is turning the lathe on and off via the Dewhurst switch. I was under the impression that such a switch should only be used to set the rotation direction, the actual starting and stopping to be done through an NVR or similar. Perhaps the contacts in the Dewhurst switch have been damaged.


Raphael Golez26/02/2021 12:21:03
167 forum posts
153 photos

Thanks Noel, Dave, John Paton and John.

John (Journeyman), Yes Im guilty of doing that as Im unaware of its true purpose. DMB mentioned it and I just have a sinking feeling after his explanation and yours. I thought that its ok and I have been doing that for 5 years. Maybe I have caused damage to it. If only I knew its true purpose then I might have stop long time ago, guess its payback time now. If everything fixed, where would the NVR switch be located on the line? Just asking directly but if its difficult to describe I will try to research it at some point. I will try to see over the weekend and will follow all your advices.




Edited By Raphael Golez on 26/02/2021 12:21:53

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
walker midge
Eccentric July 5 2018
rapid Direct
JD Metals
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest