|Phil Whitley||25/08/2019 16:30:51|
1250 forum posts
This one had me scratching my head, and could have been very dangerous. just shows the importance of giving the electrics a once over inspection before using a machine for the first time.
Just wired some three phase sockets for some machines that have been repositioned, plug in the Grafton drill press, and what do you know, it runs backwards! No problem, reverse a couple of phases in the plug and all will be well, so whips off the plug top, reverses red and white phase, refit and test again. the machine is now running slowly and growling a bit(two phasing/ phase missing) so I shut off and take off the plug top again, thinking I have botched one of the connections, but no, all are correct and tight. It is only on close inspection that I notice that the green wire (earth) is actually connected to a phase, and the white wire which I swapped with the red, is also connected to a phase!. Took the lid off the starter to find that red green and blue have been used for L1 L2 and L3, and white used as earth!!! The problem is that the pin designations are moulded into the red plastic, but are not coloured, so the whole thing is just red! later plugs have a green sticker by the earth pin.When I reversed the red and white connections, the motor was running on 2 phases and earth, and the chassis of the machine was connected directly to a phase! Luckily I was wearing safety boots, and as soon as something like this happens, I go into hyper vigilant mode until I have sorted it out, and part of that is not touching any metalwork, so I had pressed the starter button very carefully. At worst I would have got a 240v shock, which whist unpleasant, is rarely fatal unless you are standing in water, or grasping earthed metalwork with the other hand, which is why we were all trained to work with one hand in the pocket when working live. For the observant, yes, that is asbestos dust in the starter, which has come from the insulators on the overloads, and will be vacuumed out, and the asbestos replaced with glass fibre tape, using suitable PPE! I would fit another starter to replace this antique MEM Startet, which has no NVR, but £45 for a starter + £16 for the suitable overload set seem a little excessive for a fractional hp motor.
|Brian Wood||25/08/2019 18:33:12|
|2244 forum posts|
Well done Phil, it could have been a lot worse.
|Mark Rand||25/08/2019 21:59:32|
|918 forum posts|
All operatives will keep one hand in their pocket at all time when working on open switchgear. BTH/AEI/GEC-Machines Rugby safety book.
|Stuart Bridger||25/08/2019 22:05:24|
|464 forum posts|
100% agree, that was what i was taught when did my apprenticeship many years ago
|jann west||26/08/2019 06:39:36|
|66 forum posts|
so ... stupid question time. I understand what the issues are in the original post, but this has me confused: "All operatives will keep one hand in their pocket at all time when working on open switchgear. BTH/AEI/GEC-Machines Rugby safety book."
Why one hand in their pocket? Is it so you can't form an earth to something you are touching with your other hand?
|Stuart Bridger||26/08/2019 07:34:05|
|464 forum posts|
The answer is to prevent current flowing across the heart, resulting in a cardiac arrest.
|Anthony Knights||26/08/2019 07:51:36|
|419 forum posts|
I was given the same advice when working as a TV service man, on the old LIVE chassis televisions.
|Mike Poole||26/08/2019 08:16:21|
2699 forum posts
I was trying to reroute a cable to a light switch I had moved, after pulling on the cable I thought it was which was in a bunch I decided to call it a day as it was getting dark and turned the power back on, still thinking about why this cable was being difficult I went back to retrace the cable and make sure I was on the right one. Working through a hole in the floor I could put my head in the hole but my shoulders rested on some copper heating pipes, running my finger along the cable to trace it as it was not visible I found a live bit where I had damaged the insulation with my previous efforts, the electric shock was my best ever, I bit both sides of my tongue and burnt a small hole in my finger, definitely felt a bit wobbly after that one and had to lie down for a while.
|Alan Waddington 2||26/08/2019 08:39:03|
|505 forum posts|
A friends son has recently been electrocuted at work, cardiac arrest at the scene but fortunately revived by a quick thinking colleague. Entry wound to his chin and a massive exit wound to the back of the head........nasty stuff electrickery, play safe boys
|Nicholas Farr||26/08/2019 09:05:35|
2405 forum posts
Hi, yes the one accross the cheast can be fatal. I had one when I was about 30, both hands would not let go and I had to throw myself backwards to arrest the situation and my right hand foot landed where the only floor board I had up and through the cieling below. It shook me up a bit and I decided I didn't want to try it again. I think I was very lucky that day or the good lord had pity on me, but I had no lasting affects of it. As they say on TV, "Don't try this at home" Had two jobs after that, one to finish the job I was doing and two, fix the hole in the cieling below. Just to say I did seek medical advice after the event. One thing that a lot of people don't know is that when mains electricity is flowing through the palm of your hand, your fingers close up and you can't open them.
|Stuart Bridger||26/08/2019 09:30:14|
|464 forum posts|
Had a few minor "belts" over the years. Nothing too serious and 100% caused by lack of concentration. Only ill effects being muscle strain in the arm closest to the contact. It's certainly not a pleasant experience.
|Alain Foote||26/08/2019 09:48:27|
|39 forum posts|
Phil, if you PM me I might be able to help with some second hand switchgear for you. Alain
|Dave Halford||26/08/2019 14:15:55|
|870 forum posts|
Makes you wonder why a separate machine earth cable was not used, that would stop colour blind sparkies trying to kill you.
|Phil Whitley||26/08/2019 16:19:24|
1250 forum posts
That sounds like HV, very nasty, glad to hear he is ok. A mate of mine just round the corner works for powergrid, and told me last year he had a cushy job on, replacing the safety "bagging" on a 36kv overhead line just outside a substation at Wansford, where the line crosses the canal, which is done live, from a cherry picker with an all insulated boom. They put it up empty and touch one of the conductors, and if it doesnt explode, they go up and do the job, he is a senior linesman, but not an electrical engineer, and his original trade was in precision engineering. Personally, as an electrical engineer, I simply would not have the bottle to go up and do that job, and I admire his lack of concern, and his classification of "cushy"
Dave Halford, Seperate earth wires were used up to about the late seventies and the flexible metal conduit glands had an outside connection for that earth wire, but machines seem to have gone over to armoured flexible cable with all lines and earth included, especially in food processes where machines are subject to daily washdowns.
Comments about colour blind sparks noted! I am not colour blind, but my father was, and I cannot remember if he put this cable on, or if it was on when we got the machine from another guy in the village, but whoever wired it needs his backside kicking...............................
needless to say I will be rewiring it all!!
Edited By Phil Whitley on 26/08/2019 16:21:50
|Dave Halford||26/08/2019 17:13:48|
|870 forum posts|
All telecoms cabinets still get an independent earth back to an earth bar regardless of AC or DC supply. The shield in flexible cables should be grounded at each end and therefore fulfils the double earth. Unfortunately if it gets unplugged and someone snags a live cable on it you can get Mike Poole's issue just by touching the machine.
954 forum posts
Had an electric experience last year, I was changing over two light fittings on the upstairs lighting circuit at home, I switched the circuit off at the distribution board and removed both fittings and swopped them from room to room and fitted the first unit. Went to distribution board and switched the circuit on and checked the lights came on, great that one worked. So now to tackle fitting the second fitting in the other bedroom, pulled down the exposed cables prior to fitting the new light and Oh B———r got a fair belt up my right arm, as the current started to flow I remembered that I hadn’t isolated the circuit after checking the previous light. Easy done, not the first belt I have had and probably not the last I shall get. Thoroughly endorse the one hand in pocket especially when working on open live distribution board, particularly as the latest revision calls for metal enclosures on boards now, probably more forgiving when the enclosures were plastic.
|Phil Whitley||08/09/2019 20:37:11|
1250 forum posts
Sometimes, when you realise you have opened a can of worms, you are too far in to back out, and the only thing to do is put your head down, charge, and "break on through to the other side".............................
So, I remove the starter and motor unit from the drill (easy, just take of the belt and slacken the adjustment, and it slides off as a unit) and I look over the electrics, and find that the flexible conduit twixt starter and motor wants remaking at both ends, and the connection box on the motor is hanging loose, so I correct the main wiring fault, so that green is now earth, and connected to the chassis, and red,white and blue are the phases, remake the flexible conduit at both ends, and add an extra earth wire between starter and motor, and then notice that the motor tail leads insulation is cracked and crumbly, so ok, need to take the connection box off anyway, because the two fixing screws behind it are loose, so I will solder new tails on to the windings. Disconnect the motor, remove it from the frame, and strip it down, fit new motor tails and fit them up with shrink sleeving, and then check the bearings............................which appear to be full of black treacle! Remove the bearings, wash through with petrol, and dry and there is a definite catch in the drive end bearing, but the other "looks" fine. No worries, jumps in car and belts off to local bearing shop for a replacement, winces a bit at the £14.00 price for a 5/8 x 1-9/16, but coughs up, and back to the shop. Good thing the rear bearing is ok, because it is a double row self aligning bearing @ £35, and they dont have it in anyway!
Rebuild the motor with repacked bearings, reassemble and refit to the drill and test...............and it is fine when running (well not fine, but fairly quiet, although I never noticed the bearings were particularly noisy before), but when I turn it off, a horrible noise comes from the bottom bearing!!! Strip again, clean and another visual on the bottom bearing, repack, rebuild, and refit, and test aaaaaand,
well it all happened so fast! I hear a tell tale ting ting ting as I start the motor, and I was a mite to slow! As I got to the stop button there was a flash and a pop, as the fan on the rotor cut through the insulation on one of the NEW motor tails, and the breaker tripped out! So, I gather my thoughts, and after running through a very colourful list of words I keep handy for these occasions, it's motor off, strip it again, more heat shrink over the tiny hole in the wire, and then realise that although I have tied the tails back to the windings very neatly, tying them back holds them in such a position that when you rotate the end caps to line up the through bolts, the tails will either move outwards, out of harms way, or inwards, and into the fan! I obviously got away with it the first time! The next problem is that when you short a phase out to the rotor, the currents path to earth is through the bearings, and sure enough, the new bearing now has a catch in it! As it is now nearly home time, I stick the bearings in my pocket, with the intention of ordering them on the net over the weekend. I order up two identical bearings, as I cannot see any reason why this motor (BTH) in this application would need a self aligning one, rebuild the motor AGAIN, and refit, and it runs very quietly indeed, but on two phases!!, so I look in the starter, and sure enough, it has taken out one of the overload coils, so I look in my collection and I actually have enough .010 nichrome wire , so I rewind the overload, service the starter while I am in there, and also replace the hairpin spring on the stop bar, which was taken out by the overload wire as it blew up. and now it runs perfectly, and is so so quiet and smooth. and its only take 3 days to do three hours work!
On examining the lower two row bearing under a magnifying glass, the balls, although perfectly shiny looking to the naked eye, are covered in tiny striations, which is where the noise was coming from, the treacle thick grease must have been masking it. It is now looking good, working fine and is oh so quiet and smooth, and electrically safe. I will finish the belt guard tomorrow, and post up some finished pics , but I have drilled holes with it, and TBH it is like using a completely different machine. A ball handle is on its way for the swivel lock, and I have also cured the loosenes in the quill pulley which caused an intermittent rattle as well, happy days, and back to the Covmac next week!
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
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.