Earthing Issue on new machine

On another general thread a DRO issue was traced to a faulty safety earth on a new machine (not the DRO). The earth connectin was a ring tag onto a painted panel resulting in no earth. The machine came from a known UK supplier.

It is clear that in this case there was a design and or manufacturing issue as well as lack of testing.

This type of issue is hidden until another fault occours. then it could cause electrocution, fire or injury caused by pulling a hand away due to a mild shock and contacting a rotating part or sharp cutter.

While the other case was a particular make model, this could apply to any machine tool new or used.

Even if a check carried out with a multimeter shows low resistance to earth, this does not assure adequate earthing in the event of a fault. Ad a proper earth path has not been designed in, a stray connection through a burr of single wire strand will show low resistance but could fuse in the event of an insulation failure leaving the machine live, The only adequate test is a high current bonding test. This measures the resistance with a high current, typically 20A for equipment with a 13A plug, appled for at least a second.

A check with multimeter will pick up gross errors and I strongly recommend that owners of small machines carr out at least this basic check before first use, if they have been moved or modified and ideally every couple of years.

BASIC EARTH CHECK.
1/ Unplug machine, including any accessories like lights, DRO's etc.

2/ Using a multimeter select the lowest Ohms range (or auto)

3/ connect the test probe tips / clips together and note the reading. It should be less than 1 Ohm. For a analog meter zero the meter.

4/ Connect one meter lead to the earth pin of the mains plug.

5/ For each main exposed metal part of the machine connect the ther meter probe to it and check the meter reading. If an auto-ranging meter make sure it is still reading Ohms and not kiloohms or Megohms

6/ subtract the reading noted in 3/

7/ If the result is more than 0.5 Ohms the machine may be faulty and should not be used until properly inspected and tested by a competent person.

The main parts to check are the control box, Bed or main casting, Motor casing if exposed and spindle. Items like control handles should be checked but only ones that might become live are critical. For rsample a mill Quill feed wher the drive goes into a housing that has wiring in it must be grounded, but a cross-slide handle is not critical because athe bed and slide, or chuck, would have to be live first. If getting someone else to check, make sure they are competent, in my experience most individuals offering "PAT" testing services are not competent to check a machine tool .

Robert G8RPI.

Thanks for this post - the sort of thing which is very helpful to an electrical ignoramus like me.

I think PAT stands for portable appliance testing.

If carrying out a Class 1 PAT on a machine tool it is important to totally electrically disconnect any electronic equipment on the machine to prevent damage to said equipment.
Denford are 1 of the manufacturers that post warnings do not PAT labels on their machines.

Emgee

I've had to remove the original thread for a number of reasons.

I'm letting this good advice stand as it covers what people need to know.

Neil

I'd like to add to Robert's suggestions and recommend that the new user of anything machinery related should visually check the earth connection for robustness and security before going on to do a circuit check. Robert rightly makes the point that a whisker of wire contacting the frame of the machine will permit enough current to flow to satisfy a multimeter; one way around this is to be sure that a robust physical connection is present.

That probably means taking covers off etc; that in turn begs the question of invalidating the warranty. I'd hope that a responsible commercial vendor would be able to get himself past that hurdle and not get all hoity toity about it. The user needs to know of his own certainty that the earth is satisfactory.

HTH Simon

Edit - for clarity

Edited By Simon Williams 3 on 28/05/2020 12:11:40

Yes, PAT is Portable Appliance Testing. Don't get me started on that topic

There are two reasons for unplugging accessories, to protect them during some tests and to stop any possible parallel earh connection masking a fault until the day you run the machine with the accessory unplugged.
An earth bond test shold not damage an machine. The potential problem is when people do insulation tests intended for 3 pahse motors on 240V electronic equipment.

Robert G8RPI.

I remember our Safety Officer getting very upset at contractors who applied a PAT test to fixed heaters.

They said they were correct but as he said if you need a tool of any kind to assemble the connection/supply then it is not portable! The only tool you can use to connect/disconnect is your hands!

As a Class 1 PAT includes a 20A test of earth continuity and insulation resistance it is likely to damage any connected electronic equipment, although most small machines in the home workshop will be powered by a 13A plug they are not portable so therefore do not belong in that category of testing, they need testing as Fixed Equipment to ensure that all appropriate tests are conducted and the machine is safe to use.
Any earth continuity testing on electronic equipment should use the 25ma test function provided on the tester.

Emgee

Alistair post, I agree with the Safety Officer if the heaters were fixed equipment.

Edited By Emgee on 28/05/2020 14:03:33

You need to remember that machines are sent out with little or no inspection despite what the seller tells you.

You have only to read some of the posts on this Forum to see that.

Cars are not much better. It pays to check EVERYTHING before using any machine.

The suppliers goal is to Get it out---Get the money in ! Full stop !

You don't believe it ? Read a few posts on this Forum and you might see things in a different light.

Don't even ask how I know.

There is no such thing as a "Class 1 PAT" defined by regualtion. Indeed there is no such thing a defined PAT.

There are Class 1 APPLIANCES these are those with a protective earth.

Portable Appliance Testing is normally considered to cover any electrical applance which plugs in. This includes industrial plugs, not just 13A ones. Fixed equipment regarding electrical testing applies to the supply, not the mounting of the equipment. Thus a Mill that is bolted to the floor but has a CY cable and a Commando plug going ito the wall is "portable" but pistol drill wired to a switched fused outlet is not.

A 20A earth bond test on a piece of electronic equipment will NOT damage it unless the equipment is faulty or test is carried out incorrectly. I've done or witnessed dozens of these on "portable" equipment that contained lots of electronic including servo drives.

A insulation test on a piece of electronic equipment will not damage it as long as the equipment is not faulty and test is conducted properly including selection of appropriate test voltage. A "Hi-Pot" (high potential breakdown test) does have a good chance of damaging electronic equipment but they are not appropriate to routine in servce testing.

There is NO legal requirement to carry out PAT.

Robert G8RPI.

I said don't get me started on PAT (especiallty not " PAT Testing" which is testing a test)

Edited By Robert Atkinson 2 on 28/05/2020 17:49:28

This is a interesting question , If your motor is not attached to you lathe IE Mounted as a seperate unit it is quite possible that the only connection to the lathe may be your drive belt, This willl not provide any earth connection very easy to over look? You can buy a Plug for 13amp socket which has its own built in protection together with a manual reset in the event of a power failure , If you are using 3 Phase then more important because off the extra voltage to get a Electrician to check and test with a Megger for a Insulation test of either 500 volt or 1000 volt , Your Life is worth more than this simple test .

Take Care and check your equipment is safe for all .

Once remeber asking someone where is your earth connection its that lead there in the bucket of earth.

Take care Bee M

Robert

Couple of points to raise with your post, I stated Class 1 PAT, clearly that means a "class 1 portable appliance test".

Fixed equipment does refer to the item, not the power supply to it, the supply to the power point comes under the electrical installation testing regime.

Emgee

Hi Emgee,

1/ I knew what you ment, but "Class 1 PAT" is not clear.

2/ I sver said anything aout the supply (that does come under installation test) I described the means of connection. If the connecton to the supply has a plug it is considered "portable" as far asr PAT goes, if it hasn't got a plug it's fixed and PAT does not apply. Typically a physically fixed machine with plug would have longer times between inspections than something movable with a fixed connection rather than a plug (these are rare but not unknown).

If the lathe tool has NO electrical equipment, including switches, lights DRO's etc, mounted on it and the only connetion to the motor is an insulating belt there is no requirement for the Lathe to be earthed. It is still good practice though. It's the fault current path that matters. If the motor was mounted on the same metal stand as the lathe but not directly on the lathe then both the motor case AND the stand must be checked for earth continuity, but the lathe would not have to be. If bothe the motor and stand are earthed there is no series fault path to the lathe. In practice of course the lathe would almost certainly would also have some earth path to the stand due to it's mounting.
Plug top earth current protetive devices are usefull but a modern consumer unit "fuse box" with protection built in is best. A socket with built in RCD is better than a plug type. Exmples are

https://www.mkelectric.com/en-gb/Products/WD/surface/MetalcladPlus/Switchsocketoutlets/RCDprotected13amp/Pages/K6231ALM.aspx

https://www.mkelectric.com/en-ar/Products/WD/white/logicplus/switchedsocketoutlets/RCDprotected/Pages/K6211WHI.aspx

Note that there are two types of RCD socket, Active and Passive. You should use an Active type for machine tools etc as the trip on mains failure so act as an additional no-volt release preventing unexpected re-starting.

Passive types should be used for things like freezers that must re-start automatically.

Robert G8RPI.

Edited By Robert Atkinson 2 on 29/05/2020 08:14:45

I have two vintage AEG corded power tools. The packaging of the first, a two-speed hammer drill, bore the dual insulation box-within-box symbol, but the tool itself (no such symbol on plate) had a twin and earth lead. The second tool, a jig saw, did carry the symbol and the legend "DO NOT EARTH" and, of course, had a twin only lead. Both tools had exposed metal gearboxes.

I recall querying the drill anomaly with the seller and coming away with the impression that I had bought an item on the cusp of the move to dual insulation. This must have saved a lot of copper but I wonder whether one system is actually safer than the other?

Hand held appliances with properly implemented Class 2 (double insulated) protection are safer than similar Class 1 (Earthed) appliances as an undetected insulation breakdown is less likely than an undetected earth conductor fault. A second breakdown of the basic insultion is required in either case to cause a hazardous condition.
For Class 2 items with exposed conductive parts like your drill there has to be a minimum amount of insulation between these and any live part. It is possible that your old drill was indeed a crossover product that incorporated the Class 2 levels of insulation but retained an earth connecton. A "belt and braces" approch.
Safest of all is the "110 Volt" (actually 55 V - 0 - 55 V) power tool system which give a maximum of 55 V to grorund and 110 V between terminals.

Robert G8RPI.

Robert Atkinson 2:

Thank you. I get the impression that corded tools are fast disappearing in favour of cordless and I suppose the inconvenience of the 110V transformer has contributed to that.

I’ve just been looking at this and another post about running an inverter on a Colchester lathe and there is a fair amount of talk about earthing. Now I have started using a rotary converter to supply 415 3 phase and it got me thinking maybe I should look a bit closer at all my machine earths. I have checked continuity of the earth from the plug pin to bits of the machine and it’s all ok, but should I really have a separate earth wire from the machines to a stake outside in the ground or is this just not necessary?
Thanks

I bought a lovely piece of equipment from a reputable american supplier, nicely finished, painted etc. but wouldn't power up and on inspection found that the lovely paint finish was also a lovely HR for the earth conn. Easy fix but just proves that a 4 figure priced piece of equipment was never powered up on completion, which they confirmed, they only test the component parts before assembly!!!!

Buffer

Any additional earth bonding in your installation will depend on the type of electrical supply and construction of your workshop,
Does your workshop have it's own distribution board ?
Is there a RCCB (RCD) included protecting all final circuits. ?
What is the type of supply ? (SWA, overhead cable, spur from house ring main)
Is there any exposed metal frame accessable to touch ?

All of the above have a bearing on your question but it is unlikely you need to bond each or any individual machine to an outside earth stake, the earth fault protection should be included in your final circuits.
Checking with a multimeter does give an indication of an earth being present but it does not prove the connection is able to withstand the fault current required to rupture any supply fuse or trip an mcb, for this you need a test instrument that injects a suitable test current through the cpc source (plug ?) to the earth connection on the machine.

Emgee