is it a 3 pole or 4 pole isolator.  If 3 pole the N could be reverse fed.

Notwithstanding faults on the motor with which others will have more experience, I would start the batting with an accidentally swapped conductors resulting in unswitched line and switched neutral, followed by capacitor discharge if fiddling with/exposed to any innards.

(I assume that this is for electrical maintenance)

Roland Grodkowski said:

the contractor said he doesn't need to because if the motor "running" then that means there's no voltage.

You mean that the other way round?

I take it the contractor is mechanically skilled and not electrically skilled? Else why would they need you to isolate for them? How do the other technicians test for voltage, with a volt stick? Do you go on the roof and test for dead before you allow the technicians to go up there?

Sorry for all the questions, I don't work on big sites and wondered how it all works.

Motor seized up is always a good one.

Yh it can happen with a few things like motor seizing, drive set to 0hz, or even being wired incorrectly in multiple ways such as the first reply.  I just wanted to see if anyone else could think of more examples.

To answer your questions, he was an electrical contractor however the company still requires the isolation to be done for them as they don't know they site and we do, so once they have given us their work order we perform the necessary isolations. All I asked the contractor, was to do his own voltage test before touching anything just as good practice and to be on the safe side.

Clearly the person who stated that has no idea of the requirements of the Electricity at Work Regulations 1989 and safe isolation procedures and the use of appropriate test equipment. They are not a competent person and should no be working on electrical systems unless under direct supervision of a competent person. Also the employer of this person does not appear to have a Safe System of Work derived from a Risk Assessment and Method Statement as required by the Management of Health and Safety at Work regulations 1999.

I would suggest Roland if you are in the UK you would be liable to be prosecuted for you failure to comply with your Statutory Duties.

John Peckham

I think you have blown things out of proportion by making assumptions. 

Firstly, both my company and the contractor's company require work permits, method statements, risk assessments and a work order is the signed once the task is established and necessary isolations have been performed. So no, neither the contractor or I can be prosecuted.

These are strictly monitored by the Health and Safety Managers as well as Engineering Managers to strictly abide by the legal UK regulations, this is a short post not going into detail of legalities so I'm unsure where you've arrived to these far fetched conclusions/assumptions.

The debate was simply regarding a theoretical question about motors, not regulations which you have assumed have been broken which they have not.

Rules and permits aside UK or abroad, I'd not be happy letting someone open any box that had terminals in it, if he thought that seeing a motor was not spinning was the same as ensuring it was safe to work on.

It just isn't any guarantee of anything at all. After all how do we know it is not going to power on at any moment, or the motor itself may be broken but the power going to it is on.

Anyone working in the box with the wiring in it,  should be confirming electrically that the power really has gone off when they get to the roof, just for the day that either the wrong switch was isolated or the wrong roof wiring box has been opened.

It may never have happened to him yet, but the day it does may be the last.

There are a few choice words for folk who have this sort of attitude.

Mike

Once again I think you're missing the point of the post and going into irrelevant semantics. We DO know the motor will not be powered because we literally isolated it as per company and legal procedure with the necessary isolator and rotary supply breaker (lock, key, tag).

The point of the post is just to debate motor theory and point out other scenarios where a motor could not run whilst still having voltage applied. All necessary rules, permits, isolations, confirmations etc etc etc have been performed by qualified people who actually do this job day in day out so I'm unsure where these unwarranted assumptions are coming from.

However, yes the last part I agree, the contractor's attitude should've been better as simply requesting an extra test by themselves just gives peace of mind whilst they work although isolation was already definitely confirmed. Having said that he did not do anything illegal and he had signed and received the necessary work order with risk assessments and isolation confirmation which was of course signed.