Today's fault

[ElectroChem]

Thought I might post up a small exercise based on a service call I attended today. Called to a factory, operator says he came on shift and found the following:

- Vacuum pump, connected by plug lead to freestanding panel.
- Pump operated by foot pedal at machine, but won't start.
- All other functions of machine working ok.

Trainees (and mentors too if you like), how would you approach this problem? If we get a decent discussion I'll post what I did and we can compare.

 

[Pete999]

Fused plug was it?

 

[pennychew]

With it being connected to a panel i assumed it would be a proper connector but thats just what im used to seeing!

 

[pennychew]

I typed an answer earlier but obviously never posted it! try again

First thing i would do is open the panel and check none of the overloads have tripped, after that i would check for voltage at the pump with the switch pressed then voltage at the switch and work back from there

 

[Pete999]

Come on OP was it a fused plug?

 

[ElectroChem]

Pete - No, it wasn't a fused plug.
Penny - No overloads or breakers tripped, couldn't test voltage at the switch but there was 400V at the contactor line terminals. There was not, however, 24VDC at the coil.

So far Penny and Pete have covered the first steps taken, proceed.

 

[pennychew]

Got to be a control/switch problem then, I would check for voltage from the PSU then look for the switch cable going into the panel to confirm it had voltage present going to the switch and check for voltage coming back when the pedal is pressed

 

[Bobster]

Pennychew, why are you testing for voltage, I'm guessing this fault will be solvable with dead tests aka a continuity test.

Although I have no problem with live testing, it's often easier with permit structures on certain sites to undertake fault finding dead.

First thing, find some drawings if any.

Second, continuity test from the source of the 24V control to the foot switch, if confirmed test continuity across contacts within the foot switch.

I'm guessing the fault lies there. Unless there are other interlocks.

 

[pennychew]

Usually just check for voltage first as it means you dont have to disconnect anything, if there was voltage present then continuity test on the switch (which i'll admit i never thought of!)

 

[Bobster]

You shouldn't have to disconnect anything for just doing a point to point continuity test.

 

[ElectroChem]

Unless there are other interlocks.

The plot thickens...

 

[Bobster]

The plot thickens...

If there are other interlocks, carry on point to point continuity testing, to find where the break in the interlock circuit. This is made a lot easier if you have a set of drawings to hand. Although if we had the machine in front of us/knowledge of how it works. I'm sure you could almost pinpoint the interlock causing the fault without testing.

 

[pennychew]

The plot thickens...

I reckon some dozy sheila has left her bag in front of a light beam

 

[ElectroChem]

The final verdict was that the previous shift's operator had left a maintenance switch in the off position, interrupting the 24V line to the contactor.

From what I've heard of this client, I'm not surprised. My boss tells me they once had a machine running at half capacity for 6 months because the main operator retired and no one else knew the program properly.