Afternoon Fault Finding

[LankyWill]

Got a call from work to go and visit a signage factory, they manufacture road signs, lights etc. One of their machines is used to buff/sand/polish cylindrical stainless steel pipes. This is a belt sander which rotates via a motor inside the unit, the belt sander is then spun around via a secondary clutch drivin motor.

The machine is plc run. They were complaining of getting shocks off the machine when using it. Their is a visible electric arc between the pipe and the guard and as the belts are nylon its obviously excess static.

So i do a EFLI tests on each Line(3 phase machine) and all readings satisfactory so i delve into the machine and have a high resistance Earth-Line on the 24v system. I found this, can you guys explain the logic? I removed the link Between a transformer and the earth block. Cable 24-0 and the din rail terminal and retested and it was clear. The loose earth as pictured was inside the same terminal as the red 24v outgoing cable the linked to the din rail earth.

 

[Richard Burns]

I can't really make a lot of sense of the post, but it sounds like someone had linked the 24V supply to earth, which does not seem totally correct, though it could be possible for some arrangements, but I think not in this case.
But it could have been a misguided attempt to reduce static by ensuring that all ELV equipment was connected to earth, to give the charge somewhere to go?
Possibly more effective to have earth wipers on the belts, but difficult to apply.

 

[davesparks]

I think control panel supplies are required to have an earth reference in some situations under the various machinery rules and leaving it disconnected may be either dangerous or leave you in serious legal trouble. But it'll need someone who knows the rules to confirm.

 

[Brightspark2]

It's too give it it's ref point to earth.

So it gives it 24v to earth. Not 12v and 12v. Which looks like you can have configured on that transformer.

Youll find it on 110v systems too, instead of having 55v down each line, you link one side of transformer to earth to give you 110v to earth.

You do not want split voltages down each line, under fault conditions this can leave you in a dangerous situation where relays and contactors can hold in when the fuse in the line side blows but the voltage remains on the other line as you will not have double pole protection.

Don't know how well I've explained that...

A copy of BS60204 is essential here.

 

[Brightspark2]

So you'll need to put the link back.

 

[Lucien Nunes]

i delve into the machine and have a high resistance Earth-Line on the 24v system

Can you explain this more fully? Which line? How high? Why were you testing around the 24V system?

 

[LankyWill]

they were complaining about getting shocks from the machine when polishing or grinding. I say shocks like a lighter starter shock not a belt of 230 or 400. I gathered it down as static from the nylon belts and the nature of the machine. They rest the pole on a plastic rest whilst using the machine so its not at the same potential as the machine itself. The bloke wanted me to look inside so i did, i tested the earthing of the machine and checked for any shorted cables to earth. I then found the link between the outgoing 12v transformer cable connected directly to the earth din rail bar. I thought it could be a mixture of static and the link causing the shocks so i disconnected the link. The schematics were terribly assembled and not to mention in french.

 

[LankyWill]

Can you explain this more fully? Which line? How high? Why were you testing around the 24V system?

From the control transformer to the earth din rail. Around 0.89 ohms

 

[Lucien Nunes]

I thought it could be a mixture of static and the link causing the shocks

I've no idea why you would think that. Unless they are getting 24V 'shocks' from exposed conductors in the 24V control wiring, I can't see why the 24V circuit is implicated at all or what good could come of removing its earth reference connection.

Static shocks involve tiny currents (in this case triboelectrically generated i.e. by rubbing) charging up an insulated object's stray capacitance to earth, then discharging that capacitance to earth suddenly. You won't prevent people discharging themselves via the control wiring (e.g. when they press buttons on an insulated panel) by floating the 24V supply if that is what you are after, the stray capacitance and insulation breakdown voltage of the 24V parts will present little obstacle to the tens of kilovolts of a typical static discharge.

0.89Ω from 24V line to earth? Sounds like the transformer secondary, nowt wrong there methinks, or did you mean the earth reference link had that resistance between the rail terminal and the transformer terminal. If so, tighten the screw...