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Current leakage om CNC

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fizzy24/08/2019 17:11:59
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1624 forum posts
111 photos

If I touch the inside of my wrist against my cnc bed I get a highly unpleasant shock. Ive experienced the same sensation with laptop computers but given that there are just 3 stepper motors and a drive motor I wasnt expecting this to occur. I tested the bed to earth and got 120V @ 5 uamps so virtually no current to speak of. Is there anything I can easily do (other than not touching it) to reduce this? Its all on wheels on a concrete floor. Thanks

David Jupp24/08/2019 17:26:19
701 forum posts
17 photos

Make sure the metallic parts are earthed (as required by IEEE wiring regs) ?

Joseph Noci 124/08/2019 17:54:41
549 forum posts
836 photos

More than likely the capacitors from live and neutral to ground, in the mains input connector/filter somewhere causing the slight, and correct, leakage, if if they are present. HOWEVER!! the system should be earthed - ALL metalic parts connected together and to mains earth. If not, and one of those caps goes short circuit, you are DEAD!

Joe

Robert Atkinson 224/08/2019 18:15:12
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400 forum posts
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Commercial CNC machine or homemade?
5uA is enough for a shock. Assumuing you are in the UK or EU with 240V mains the 120V you are measuring is caused by the inut filter capacitors acting as voltage divider, The common of the filter appears to be connected to the machine metalwork but not the mains earth. This could be a fault in the machine or the workshop wiring.

Robert G8RPI.

Dave Halford24/08/2019 19:02:48
489 forum posts
4 photos

Had you not found 120V to earth I would have suggested static build up on you. What earth did you use for the test?

fizzy24/08/2019 23:24:16
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1624 forum posts
111 photos

its a chinese set up but the plug has earth. I connected the bed to the trolley, not realy n earth but makes part of the circuit. I dont think there is a workshop wiring fault as it is installed to Iee regs.

Vasantha Abey25/08/2019 04:22:50
16 forum posts

You need to fix an Earth wire to the body of your cnc, then at the end have a piece of flat metal like copper with a rubber sponge pasted to the bottom and wet the sponge with a bit of salt water.

this will allow the charges to go to the earth. This is because you have the machine on wheels is the problem. You can also change the rubber wheels to steel wheels that can solve the problem.

Joseph Noci 125/08/2019 07:05:42
549 forum posts
836 photos
Posted by Vasantha Abey on 25/08/2019 04:22:50:

You need to fix an Earth wire to the body of your cnc, then at the end have a piece of flat metal like copper with a rubber sponge pasted to the bottom and wet the sponge with a bit of salt water.

this will allow the charges to go to the earth. This is because you have the machine on wheels is the problem. You can also change the rubber wheels to steel wheels that can solve the problem.

I cannot see how this can be a substitute for a properly earthed system!

Fizzy, as you measured 120v to earth, there is OBVIOUSLY a missing earth connection. Use your meter on ohms scale, unplug the machine from the mains and measure between the machines cable end earth pin on the plug, and all over the machine's metal parts. If you do not get close to zero ohms then the machine's earth connection is faulty and NEEDS fixing.

If you do find close to zero ohms, then the problem lies either in the wall socket, or the machines power plug-to-wall-socket connection, ie, maybe the earth pin does not make good connection to the earth socket.

If you are not comfortable fiddling in the mains wall socket, get an electrician to do it for you - the earth socket receptacle is normally a stamped form sheet metal receptacle the envelopes the earth pin - sort of split tines that make contact. If these are spread apart and no longer make contact, then you have no earth. Or maybe the earth wires in the wall socket was not inserted properly.

Can you try another wall socket, preferably on a different breaker circuit even? And please, use you voltmeter to do the '120v measurement to ground' , NOT your wrist!

You need to get to the bottom of this - if this leakage is caused by the noise suppressor capacitors on the mains input, that current MUST be drained to ground/earth. I repeat, if those capacitors are present on your machine, should any one develop a shorts circuit, the body of your machine will be fully live, at 220VAC and will kill you if a proper earth connection is not present..The present setup circumvents the purpose of the distribution board's earth leakage trip switch and you have NO protection at all with this setup. Salty wet sponges will not save you life!

Joe

Johnboy2525/08/2019 07:23:49
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213 forum posts
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Fizzy... listen to what Joe has written.

As a retired Electronics & Electrical Engineer I total endorse what Joes written.👍

John

XD 35125/08/2019 09:18:14
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1362 forum posts
118 photos

A question i would like to ask is what voltage do the steppers operate at ? If it is around 40v then you have a serious power supply problem , i would try isolating the power supply from the machine and see if the problem disappears.

Les Jones 125/08/2019 09:28:04
2097 forum posts
144 photos

I second Johnboy25's comment to do exactly what Joe has advised. Do not use the machine until this problem is resolved.

Vasantha Abey's comments are total nonsense

Les.

Edited By Les Jones 1 on 25/08/2019 09:32:14

fizzy25/08/2019 09:29:13
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1624 forum posts
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Indeed the stepper motors are under 40v but the spindle motor is 110v. I have a new, unopened motor somewhere so I will swat it out and retest. Thanks all.

JasonB25/08/2019 09:58:35
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16528 forum posts
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By way of comparison my KX3 gives 0.4ohms testing between table and earth pin on the plug as Joe suggests.

Have you got rubber tyres on your casters?

Edited By JasonB on 25/08/2019 09:58:46

SillyOldDuffer25/08/2019 10:13:04
4839 forum posts
1017 photos
Posted by fizzy on 24/08/2019 23:24:16:

... I dont think there is a workshop wiring fault as it is installed to Iee regs.

May have been OK when installed but there's always the possibility of wear and tear later. I've had the neutral connection inside a well-used wall socket lose enough spring to spark due to a fatigue crack.

A socket tester would confirm all is well with the workshop wiring, this example is £7.99 from Screwfix.

Though I trust nothing I agree faulty workshop wiring is unlikely. A more likely cause is damage to the CNC machine's power lead, perhaps the earth connection in the plug has been strained or the wire pulled off inside the machine. Or a manufacturing problem left the earth disconnected.

Easy enough to test with a multimeter as Joe described. There should be close to zero ohms resistance between the plug's earth pin and all the CNC machine's exposed metal work.

Most likely the cause is simple.

Dave

Joseph Noci 125/08/2019 11:16:16
549 forum posts
836 photos
Posted by fizzy on 25/08/2019 09:29:13:

Indeed the stepper motors are under 40v but the spindle motor is 110v. I have a new, unopened motor somewhere so I will swat it out and retest. Thanks all.

Also from XD 351:

A question i would like to ask is what voltage do the steppers operate at ? If it is around 40v then you have a serious power supply problem , i would try isolating the power supply from the machine and see if the problem disappears.

Actually, it matters little where the leakage is from in terms of safety. The current seems happy to find a way through Fizzy's wrist, arm, heart, and whatever route it finds to some or other ground somewhere, anywhere, EXCEPT via the mains plug earth pin! Fix that first then worry about what is causing the leakage, if you need to. 5uA leakage or even more is very common with the mains input filer caps, so its not a crisis, IF you have a proper earth in place.

Joe

Robert Atkinson 225/08/2019 11:21:04
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400 forum posts
21 photos

+ one for Joes comments. and as Dave says fault can develop, you arn't using an extension lead by any chance?

All metallic parts of the machine that might come into contact with a voltage ove 50V in the event of a fault MUST have a path to ground (unless the machine is double insulated which is unlikely and not the case ith yours as you have an earth). This path must be designed in and able to carry the full fault current. Just continuity becuase conductive parts hppen to touch .s not good enough. Unfortunatly many machines do not meet this requirement.
The use of a 110V spindle motor may indicate a more serious fault/ design issue if the main supply is 230V.

Sponges and saltwater have no place in any safety earth.

Robert G8RPI

blowlamp25/08/2019 11:22:47
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1210 forum posts
82 photos

I had a similar thing happen with my CNC lathe a while ago.

See page 2 of this discussion for what was wrong with my machine.

Martin.

Ian P25/08/2019 11:53:40
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2246 forum posts
90 photos

Posted by fizzy on 24/08/2019 17:11:59:

If I touch the inside of my wrist against my cnc bed I get a highly unpleasant shock. Ive experienced the same sensation with laptop computers but given that there are just 3 stepper motors and a drive motor I wasnt expecting this to occur. I tested the bed to earth and got 120V @ 5 uamps so virtually no current to speak of. Is there anything I can easily do (other than not touching it) to reduce this? Its all on wheels on a concrete floor. Thanks

Posted by fizzy on 24/08/2019

its a chinese set up but the plug has earth. I connected the bed to the trolley, not realy n earth but makes part of the circuit. I dont think there is a workshop wiring fault as it is installed to Iee regs.

Posted by fizzy on 25/08/2019 09:29:13:P

Indeed the stepper motors are under 40v but the spindle motor is 110v. I have a new, unopened motor somewhere so I will swat it out and retest. Thanks all.

About twelve replies so far, some absolutely on target and contain warning of serious risk, some others are irrelevant. To be fair to some of the less useful repliers, the information Fizzy gave is somewhat fuzzy.

Initially he said he measured 120V to earth. The meter will have two probes, one we know was on the metalwork of the machine, but what 'earth' was the other connected to?

Stepper motor and spindle motor voltages, concrete floor, rubber tyres, wheels, Chinese made is all irrelevant!

Unless this machine is of double insulated construction (which is very unlikely) then there should be continuity between the machine metalwork and the earth pin on the end of its mains lead. If there is low ohms continuity then the fault lies in the workshop wiring.

Fizzy, you say connecting bed to trolley form part of the circuit, I hope not! What circuit would that be anyway?

Absolutely no point in swapping motors whatsoever, test the earth wiring and connections first

Ian P

old mart25/08/2019 16:33:34
780 forum posts
76 photos

I have just bought a VFD made by Schneider for the rebuild of the Tom Senior mill. Reading the full manual, there is a mention of an earth leakage caused by the filters fitted to it. I don't know what the value is.

Phil Whitley25/08/2019 16:41:18
934 forum posts
130 photos

Take heed of the above! unfortunately it is very common to find that Chinese equipment has not had the earth connected at the appliance end! see Bigclivelive.com on you tube. Using a continuity tester, test from the earth pin on the plug to the chassis of the machine, and test different parts of the chassis! I have had some 240v led lighting units supplied in metal cases where I found earth wires unstripped and unconnected inside the case, although there was an earth fixing point right next to the cable entry!

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