I.R Test on an Isolated Motor on a live working Industrial Machine

[jbingham]

Hi Everyone, Im a new poster.  My question regards IR testing a 'locally' isolated 3ph motor on an industrial machine, where the rest of the machine is still powered up.

Scenario: This question could apply to numerous machines, but lets say this machine is an oven line with 3ph motors driving various conveyor tracks.  The machine has numerous 24VDC sensors, and its PLC controlled.  A 3ph motor on one of the conveyors has stopped working and I suspect its 'down to earth'. The motor has a local 3 pole isolator, and I've switched this off.  I'd now like to remove the motor terminal box lid and IR test between windings and windings to earth.

Q1) Is it ok to IR test (500VDC)? Or is there a risk of the test voltage damaging the sensors elsewhere on the machine or the PLC.  If not ok, then why not (i.e how would the test voltage cause damage?).  I'm somehow worried about the test voltage going down the motor circuit cpc (or earthed metalwork), and then into earth/neutral busbars inside the control cabinet-where all the PLC and 24VDC circuits are connected. 

For all I know, industrial sparks may do this all the time.  Personally, Ive always powered down the entire machine before IR testing.  I want to avoid throwing the main isolator, so that the rest of the machine can continue in process.  Also time and hassle would be saved by not powering down.

Two more related questions:

Q2) Safety: Is it the done thing/necessary to ensure people stand clear of all metalwork on the machine before IR testing - to avoid contact with the test voltage appearing on earthed metalwork?

Q3) If I should come across a motor with thermistors in the windings. (therefore, e.g additional 'T1' & 'T2' terminals inside the motor terminal box);  What must I do (if anything) to protect the thermistors from the 500VDC test voltage? 

If there is anything else I need to be weary of when IR testing isolated motors or parts of otherwise live working machines?  Any related tips/advice would be appreciated.

Thanks In advance, John.

 

[Marvo]

You won't cause damage to the controls if the motor is isolated locally and you IR test it. Connect the 3x lives together in the motor termination box and IR test between them and the frame of the motor. Earthed metalwork will by definition remain at 0V wrt earth or ground.

If the chassis of the motor is earthed ie the CPC is still connected then people in the area cannot get shocks from faults or testing voltages.

The thermistors are in physical contact with the windings but they're not in the path of the running current so test as I mentioned above and don't apply IR test voltages to any thermistor wiring in the terminal box.

If there is running machinery or live voltages in the vicinity of where you're working do a quick method statement for your proposed work then a risk assessment. Just be aware of the additional risks and manage them appropriately.

 

[roys]

I have certainaly IR tested motors on production lines with just the local isolators off, and motors proved dead, with no issues . A lot of the time inter posing relays are between the PLC outputs and the output functions anyway.

Q2 there is no way any other person should be able to touch your IR tester connection points, no worries about people touching the general metal work.

Q3 again no worries with the thermistors they are insulated / isolated between them and the windings.

 As I am sure you are aware great care has to be taken when working near adjacent live parts or moving plant, sometimes this is achieved by physical barriers. I would have thought that in these circumstances Risk Assessments and Permit To Work safe systems of work will be followed.

Don't know your experience but our RA's always had a line in them about persons must be competent to carry out task.  Also never stick your fingers on an electrical terminal that hasn't been proved dead first, remembering that on production lines a motor may be dead one minute and live the next due to the control logic.

Watching the rugby just now so I hope my answer makes sense as  am a bit distracted

 

[Sidewinder]

Hi Everyone, Im a new poster.  My question regards IR testing a 'locally' isolated 3ph motor on an industrial machine, where the rest of the machine is still powered up.

Scenario: This question could apply to numerous machines, but lets say this machine is an oven line with 3ph motors driving various conveyor tracks.  The machine has numerous 24VDC sensors, and its PLC controlled.  A 3ph motor on one of the conveyors has stopped working and I suspect its 'down to earth'. The motor has a local 3 pole isolator, and I've switched this off.  I'd now like to remove the motor terminal box lid and IR test between windings and windings to earth.

Q1) Is it ok to IR test (500VDC)? Or is there a risk of the test voltage damaging the sensors elsewhere on the machine or the PLC.  If not ok, then why not (i.e how would the test voltage cause damage?).  I'm somehow worried about the test voltage going down the motor circuit cpc (or earthed metalwork), and then into earth/neutral busbars inside the control cabinet-where all the PLC and 24VDC circuits are connected. 

For all I know, industrial sparks may do this all the time.  Personally, Ive always powered down the entire machine before IR testing.  I want to avoid throwing the main isolator, so that the rest of the machine can continue in process.  Also time and hassle would be saved by not powering down.

Two more related questions:

Q2) Safety: Is it the done thing/necessary to ensure people stand clear of all metalwork on the machine before IR testing - to avoid contact with the test voltage appearing on earthed metalwork?

Q3) If I should come across a motor with thermistors in the windings. (therefore, e.g additional 'T1' & 'T2' terminals inside the motor terminal box);  What must I do (if anything) to protect the thermistors from the 500VDC test voltage? 

If there is anything else I need to be weary of when IR testing isolated motors or parts of otherwise live working machines?  Any related tips/advice would be appreciated.

Thanks In advance, John.

Q1.

OK, yes you can IR @ 500V, the live conductors are isolated from the rest of the machine, thus here is no way for the voltage to feed back into the remainder of the machine via the phase lines to the motor.

The IR tester whilst generating 500V between the probes, has negligible current, the spec on my Fluke is 1mA @ 500k Ohms load resistance.

Thus with the machine correctly designed and built the bonding and earthing of the machine will ensure that there will be negligible potential across the machine.

Just think, in the event of an earth fault it would be required to keep the potential differences very low to ensure operation of the protective devices, and that would have a LOT more than 1mA of current flowing.

The test current, will flow between the lowest resistances which are likely to be the faulty motor, there is technically no current path down the live conductors past the isolator anyway.

There will be a connection from the motor earth back to the main machine earthing system, but, where will the current flow path be to cause any current to flow down a path to even be conducted through these items?

Q2.

How can the test voltage appear anywhere on earthed metal work elsewhere on the machine, it would need current flow, see above, what would be the current path to cause the earthed metalwork to rise to any sort of potential above that of earth?

Q3.

You will probably find that the official OEM guidance is to earth both sides of the thermistor at the motor earthing connection.  That was certainly what our instructions said when I worked for a motor OEM.

However, again, think about where the potential will exist and the current path.

Is there anything else...

LOTO, safe working, making sure that you can’t be injured by anything else on the machine if it were to go into failure, or during normal operation.  Do a suitable and sufficient risk assessment, and a method statement and stick to them, you don’t need necessarily to write anything down, just do it mentally.  Engage in dynamic risk assessment & mitigation as you work.

 

[jbingham]

Thanks gents, for taking the time to reply.  You've answered my questions perfectly. 

With the thermistors, my thinking was - if there was a weak point of insulation on a thermistor (or the thermistors circuit wire insulation within the windings), then the test voltage would leak from the winding into the thermistor damaging it.   

 

[Phoenix]

Thanks gents, for taking the time to reply.  You've answered my questions perfectly. 

With the thermistors, my thinking was - if there was a weak point of insulation on a thermistor (or the thermistors circuit wire insulation within the windings), then the test voltage would leak from the winding into the thermistor damaging it.   

It is I suppose possible if the windings have got very hot that the thermistor leads could be in contact with part of the windings, if this were the case then putting 500v WRT earth onto the windings would put 500v WRT earth on whatever the thermistor is connected to. This would be why the guidence by the manufacturers stated by SW is to tie both ends of it down to earth to that it cannot rise to anything WRT earth.

However, this is quite an unlikely situation and damage may have already occurred as the supply voltage would have been applied there when the motor was powered up

 

[Rob_the_rich]

Not sure if this helps but it might be worth doing a continuity test between live conductors and earth first, then insulation test at 250VDC, then at 500VDC, if you are worried.