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

A smoking M300

Problem with main switch

All Topics | Latest Posts

Search for:  in Thread Title in  
Gareth Owen 219/05/2021 21:58:39
9 forum posts

Hi all

Ive had my Harrison M300 for about 15 years. I’ve always run it from a Tanswave static converter. I bought myself a lovely Transwave Rotary converter and it ran for a while fine. Then there was a calamitous bang and then another. Couldn’t really tell where it came from but then I noticed smoke from the electrical enclosure of the lathe. I am a fairly brave (but stupid) soul and tried it again.
everything was fine and away I went.
to cut a long story short after intermittent bangs and smoke (my reasoning being that maybe condensation was the cause and after it cleared it ran fine) I’ve had a good look at the main switch after ascertaining by smell and found that there is charing/ blackening on the Moeller T20b-1 switch.

Will this just be a failure due to age or is the rotary converter at fault

ps please be gentle with me as I know I am a fool


Thor 🇳🇴20/05/2021 07:50:20
1397 forum posts
41 photos

Hi Gareth,

Welcome to the forum. Smoke from electrical equipment is usually not good, if you have little experience with electrical equipment I suggest you try to get someone experienced to help you. If you give an indication of where you live there may be some member of the forum that lives near you.


Chris Evans 620/05/2021 08:00:17
1952 forum posts

Scary thing to happen, if your electrical knowledge is like mine do as Thor suggests and seek help. I run my M300 Taiwanese copy from a Transwave static inverter but would not delve into the wiring.

Welcome along to the forum, a lot of talented people on here so I am sure good advise will be had soon.

Gareth Owen 220/05/2021 08:46:30
9 forum posts

Thanks for the welcome Thor and Chris. Never doubted that a model engineering forum would be welcoming.
If there happened to be a potential “ life saver” that lived in the York / Selby area I would certainly welcome some advice. If this isn’t the case does anyone know of a machine electrician in this neck of the woods?

My blood pressure isn’t faring well having to touch the lathe with the back of my hand just to make sure ... not that it would help with 415v I suppose

i am generally a very prudent person!

SillyOldDuffer20/05/2021 09:30:23
7487 forum posts
1658 photos

Hi Gareth and welcome to the forum.

Sounds like the switch has failed - nothing lasts for ever! The thin layer of water left by condensation is unlikely to cause flash-bangs directly, but corrosion caused over time by condensation inside the switch might. Or maybe the contacts arc intermittently due to old-age mechanical disarrangement.

I doubt the rotary converter is faulty, and the switch may be victim rather than root cause. Check the lathe's wiring carefully for loose connections and damaged insulation throughout. Rubber insulation ages badly and should be replaced. Though much better than rubber, plastic insulation can be damaged by heat or vibration chafed by edges - have a good look near grommets and cable clamps.

Ideally fuses should pop before equipment starts smoking, but an arcing bad connection might not draw enough current to do it. Arcing within fuse ratings is a common cause of electrical fires. Won't be the cause, but whilst checking the wiring confirm the fuse or circuit breaker is sensibly sized for the machine.

A visibly heat damaged switch must be replaced pronto. There's a good chance switch failure is the problem and all will be well after changing it.

Health and Safety Warning! Think carefully before tackling anything you don't understand! The risks are high: working on a live lathe on a damp concrete floor can be fatal. When working inside machines, the safety earth becomes a major hazard - touching live while the other hand is grounded via the lathe body is very nasty. Never work on machines unless absolutely certain the power is OFF. Unplug it.

Replacing the switch should be straightforward.

  • Disconnect the power.
  • Open the machine
  • Make sure the new switch matches the old one - same contacts in the same place. Double Check. Not unknown for the design of a switch to vary over time, for similar part numbers to be used for different switches, and for otherwise compatible switches to have other contact layouts. If an old switch is no longer available it may be necessary to fit a modern alternative.
  • Photograph the existing wiring, take copious notes and label the wires as necessary to make absolutely sure the new switch will be reconnected identically. Don't assume memory or the back of a fag packet will be good enough. Take pains.
  • Remove the old switch. (Ranges from easy peasy to downright awkward depending on how the machine was originally assembled.)
  • Install the new switch.
  • Stand well back and reconnect the power to test. All being well, disconnect power and make safe by refitting covers etc.
  • Reconnect power and cautiously confirm normal operation.
  • Take a break if the job becomes complicated or frustrating for any reason.

The really dangerous bit is what happens if the machine still doesn't work. Even experienced electricians are prone to lose their heads and poke about thoughtlessly inside live equipment with a screwdriver or multimeter when a fix fails. Stay calm. Better to walk the dog than indulge in angry bodging.

The forum can help, especially if you can post photos as described HERE but it can't cover big skills gaps. Don't be afraid to spend money on an electrician!


Gareth Owen 220/05/2021 10:03:43
9 forum posts

Thanks Dave

that was possibly the most comprehensive post I’ve ever read. Really appreciate it.
I suppose nothing lasts for ever and your explanation makes a lot of sense.
there is no way I would even look at the inside of the cabinet with the power connected and without the main board trip off.

I will try and source a new old stock moeller T20b-1

or get an electrician to sort it out with a newer type unless someone knows of a modern direct replacement

Once agian thanks a lot

Stuart Smith 520/05/2021 10:12:15
229 forum posts
27 photos

Another vote for leaving well alone and getting an experienced person to look at it.

Daves post and suggestions may be ok for someone with electrical fault finding knowledge, experience and test equipment, but otherwise not.

The problem needs to be identified before rushing in changing parts.

Better to spend a few quid than not be here!


Andrew Johnston20/05/2021 10:45:25
6237 forum posts
676 photos

Posted by SillyOldDuffer on 20/05/2021 09:30:23:

Even experienced electricians are prone to lose their heads and poke about thoughtlessly inside live equipment with a screwdriver or multimeter when a fix fails.

Been there, done that. Trying to fix an open frame contactor on the repetition lathe I got frustrated when it still chattered after the second disassembly and clean. So I took it apart for the third time - and then discovered I hadn't disconnected the 3-phase supply. embarrassed

The switch on my M300 is still going after 40+years although it's possible that old age could be the cause of the failure. I doubt the problem is condensation. My lathe is in an unheated garage and condensation can be an issue in terms of flash rusting, but never had a problem with the 3-phase supply. I don't see why the converter should have had an effect either. Technically the item is a disconnector and isn't intended to make or break large currents like a switch.

The item no longer seems to be manufactured. My first port of call would be Harrison, although you'll need to sit down before getting a price. Alternatively it shouldn't be difficult to find an appropriately rated alternative.


Emgee20/05/2021 10:54:36
2148 forum posts
265 photos

Hi Gareth

From your description of the bangs and charing + blackening on the switch I believe you have experienced an electrical flashover, could be caused by condensation or build up of dust on the switch.
The fact the lathe ran OK after the smoke was because the short circuit cleared the fault but left it's mark, once this has occured it is best to change the switch and possibly fit an anti-condensation heater in the switch enclosure to prevent a repeat performance.


Gareth Owen 220/05/2021 12:04:36
9 forum posts

Thanks Andrew

No problems 40+ years so far!!
At least if I sort it out then we will all know about flash bangs.

I will definitely not let this be another unresolved thread by a newbie

Thanks for the reply emgee. I actually had a hairdryer in there yesterday just to confirm or repudiate my ONE theory and hence why I have come here .

Gareth Owen 220/05/2021 12:05:26
9 forum posts

It didn’t work

Gareth Owen 220/05/2021 14:58:35
9 forum posts

Just to let all you kind posters that I have contacted a professional!

I will update this post just in case it happens to any one else. Seemingly it could involve the higher voltages present in the switching on early Harrison’s of 415v. Hope I said that right!
anyway thanks

mgnbuk20/05/2021 16:15:58
1031 forum posts
69 photos

This Ebay listing shows a KM switch with the same part number. It appears to be a straightforward 3 pole cam switch. Handily the listing shows views of the original swicth instruction sheet & various views of the switch itself. As this particular switch is in the USA, shipping is probably prohibitive.

But this similar item from RS components should do the job. The RS site has datasheets & an installation guide showing the mounting hole layout, dimensions etc.

A 4 pole version is available on Ebay for £12.25 delivered. Chinese manufactured - UK supplier. You would just use 3 poles & leave the extra one blank.

A known brand name unit on Ebay is £23 delivered if you would rather not trust Chinese quality (though the RS Components swicth is probably made in China).

Nigel B.

Forgot to say that Klockner Moeller are now known as Eaton. RS Components list Eaton 3 pole switches that may be a direct replacement - run out of time to look further into this now.

Edited By mgnbuk on 20/05/2021 16:31:02

Gareth Owen 220/05/2021 17:09:43
9 forum posts

Having just re read my earlier post regarding calling a professional it may appear that I am am being rude. I certainly didn’t mean it that way.
I went down to a local engineering workshop and asked if they knew anyone. They did and I am in contact. He even said he worked for Colchester/ Harrison and probably has a spare switch in his spares. Looks like I’ve lucked out.
Thinking about it it wouldn’t be a bad idea to have a once over inspection just to make sure it’s safe anyway. Insulation does degrade... and those sudden bangs make you realise how sudden and powerful electricity is.

and Nigel... are you vying for the most comprehensive reply to a post? Wonderful info and thanks very much for posting. Like I say these posts stay up forever and someone may glean some knowledge like I have.

Clive Foster20/05/2021 18:01:34
2817 forum posts
101 photos

As Andrew says this sort of cam switch is not intended to be used on powered up loads. Especially at high voltages.

The actual current carrying capacity is impressive given the small size but live load swiching ability is miserably small.

My experience is that the root cause of this sort of episode is past, often long past, episodes of someone using the disconnector as a switch on powered up loads. Even if the mishandling is not taken to the stage of almost wrecking the switch the contact damage sustained increases contact resistance causing the switch to run hot leading to further damage. Eventually damage progresses to the stage where it is unable to carry the load leading to the magic smoke escaping with appropriate sound effects.

Once sufficient damage has been done in the initial episode of abuse the switch will, eventually, fail. The reversing switch on my Bridgeport, also Klockner Moeller, lasted about 18 months of proper use in my hands. Forensic examination of the failed switch revealed it to be so seriously distressed that I reckon it should have died several years previously.

I've seen switches failed in this way that were known to be at least a decade past the sort of abuse that is the root cause.

Used with appropriate care these switches last for decades. I guess that eventually the internal springs weaken leading to contact issues.


Gareth Owen 220/05/2021 18:22:44
9 forum posts

Thanks Clive it just goes to show you that we never know the abuse suffered by the machines we buy. There must be thousands of years of hard won experience on this forum. It really is an eye opener. I ambled on here and posed a seemingly innocuous question and people have taken the time to respond with some great advice. I wish that investment site had done the same for me when they all recommended Bitcoin haha

Gareth Owen 222/05/2021 10:53:39
9 forum posts

Just to update if anyone needs the info I have had a response from the manufacturers.

Hi Gareth,

The T20b-1 were dedicated 3-pole ON/OFF switches, they were not true cam switches as they were only limited to this single function, refer to the attached catalog extract.

The T20B-1 switches are obsolete long years ago. Today these are represented by the ‘P’ line of switches. so you have to upgrade the entire switch with P series cam switches.

For example, if do you have T20-1/EA/SVB then you have to go with PN # P1-32/EA/SVB covers all of the old ratings and is the closest replacement however the pad-lockable handle and mounting hole pattern is different. below are the dimensions of the current P1-32/EA/SVB for comparison.

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
JD Metals
walker midge
rapid Direct
Eccentric July 5 2018
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