3-phase motor question

[mtfpuk2011]

Can someone give me some advice please?

I sat my C&G 2394 exam this week. Passed the practical, failed the multichoice by 1 question. Written paper next week. My question relates to one in the multichoice paper.

What is the correct sequence for safe isolation on a 3-phase motor? Can't remember the answers but none had the neutral - earth test.

Would appreciate some help on this one.

 

[iamelectric]

identify circuit
permission to switch supply off
lock off
retain key
approved voltage tester
test on proving unit
test between all live conductors including N
test between all live conductors and E
test back on proving unit

 

[mtfpuk2011]

identify circuit
permission to switch supply off
lock off
retain key
approved voltage tester
test on proving unit
test between all live conductors including N
test between all live conductors and E
test back on proving unit

Thank you for that. In the answers Neutral was not listed!

 

[iamelectric]

some three phase motors dont have a neutral but i would include it anyway

 

[guest119]

And how do you know the circuit was live if you locked it off before testing if it was live in the first place?

 

[mtfpuk2011]

And how do you know the circuit was live if you locked it off before testing if it was live in the first place?

It was not part of my practical but the multi-choice paper. But Neutral was not listed as one of the possible answers which I thought was odd.

 

[guest119]

You could be killing the wrong circuit if you dont test first then switch off and test again.
Circuit charts and even drawings can be er........... inacurate, to be polite about it.

 

[johnboy6083]

I completely agree with the above! Also with motors, a lot of them are automatically controlled. You may happen to test at a time when it's off, but think you've isolated it. It then may start up with you in danger.
dont take chances.

 

[iamelectric]

but thats why you would identify the circuit and lock it off

 

[ipf]

Maybe for control purposes if required (at the starter), but no neutral at the 3 phase motor.

 

[guest119]

Yea, not trying to split hairs, somebody could get hurt out of this.
Identify circuit, if that is what you mean as prove live, needs to immediately preceed lock off so should be the second item on your list not the first.
Not slaggin anybody, just trying to help the OP

 

[johnboy6083]

but thats why you would identify the circuit and lock it off

The point is that to identify the circuit to lock off, removing power, then brief re-energisation before locking off, proves beyond all doubt that the circuit is in fact the intended one.

I work on borehole pumps that work intermittently. If I locked one off, and then started to work, I may find that in fact I had isolated the wrong one. This is an easy mistake to make in some circumstances.

 

[iamelectric]

yeah i know what yous are saying but for the purposes of the 2394 and 2395 exams which he was asking about it usually only ever says itentifiy and lock off and i would include the N in the written exam as it wouldnt do any harm and if its not in the online answers you would know there was no neutral