Isolating A Pump Or Motor

[Mr Average]

Hi, I am going for an interview next week and I have been given a heads up on a few questions I will be asked.

One of the questions will be about isolating a 3 phase motor, I know the answer is locked off at the mcc panel but why is that important compared to isolating from the local isolator, is it because the secondary circuit may still be live?

Cheers in advance

 

[NozSpark]

I'd have thought that isolating and locking off at the local isolator, then prooving dead (safe isolation) would have been the right answer as the pump might only be a part of the system, the rest of which might need to remain live..

But what would I know.......

 

[Mr Average]

Thanks for the reply Nozspark, dunno mate, the bloke who gave me the heads up had been through a similar interview and was asked this question (it was a HR thing) and he gave the answer- lock off at the mcc- and the interviewer told him that was right but then asked why there and not the local isolator....

 

[Andy™]

might be something to do with not wanting the rest of equipment its connected to trying to run without it?

 

[Evans Electric]

Why don't these people explain what they mean with these questions , totally ambiguous if you ask me .   

Local isolation is for what it says ...... anyone needing to make the device safe will see the isolator in the obvious position ,  most non- electrical people would not want to open Dist. Bds etc. to search for MCBs that might or might not be marked up, and would not know where the board was.     Big switch next to motor...locked off.. is the way to isolate THAT particular device.  Your question is ...isolate a 3ph motor ...   answer , big switch , on wall , next to motor.                College lecturer , NIC assessor  or other never worked in the real world seat polisher who never opened a toolbox but can rattle off regs. numbers for England type person ...............who knows what they may come up with .  

To isolate the CIRCUIT is different question with a different answer .

I'd say they are just trying to trick you , to show clever they are by saying the answer is ........ switch off at dist bd. and LOCK OFF the breaker.

I was quized on this not long ago by an assessor,  when I said we would often disconnect within the board because we don't have an infinite supply of locking devices..... he came back with .. Ah but someone could reconnect it and switch back on !!!   Well theres no answer to that  other than stripping out the wiring  FFS . !!  

 

[Andy™]

by that logic Deke, then even if you do lock it off, someone could take the cable out of the MCB and stick it in another and switch it on. going on like that, its not really much of a surprise how much assessors are hated

 

[Mr Average]

Thanks chaps, Canoeboy I think you've nailed it, I should have said isolate to work on, if I only isolate at the isolator then someone sees that it's off on the BMS because a fault has appeared on screen and try to get it going remotely.

 

[NozSpark]

But I have seen systems where they have two pumps... An operating one and a backup fed via a changeover switch. So you could effectively work on one pump while the system continues to operate normally.. in this case local isolation is the answer.

So the only real answer is that there are many differennt ways dependant on the system and situation.. there isn't a one shoe fits all answer

 

[Mr Average]

Your right NozSpark, it could be a twin set, but I was under the impression it was more a health and safety question really, I'll bear it in mind though.

Thanks again everyone.

 

[kerching]

there will always be some ill informed twonk trying to trip you up!

an assessor told me it was not acceptable to isolate, lock off , label and keep the fuses in my pocket as someone could fit another fuse!

yeah fricking right they could....with 2 broken arms after wrestling me to the floor and taking my padlock key AND the fuses out of my pocket. PRATT!

he was less than amused with my non christian reposte

just intolerant and Twonk averse

 

[steptoe]

I think you need to cut the seal, remove the main fuse, ring the DNO and report a cut seal, then they will come out, cut off the supply as suspected theft,

I wonder if you could get them to lock off their Tx, would that be acceptable? or would you need to get the powerstation shut down too?  :shakehead

 

[Mr Average]

Well I've cheated on you chaps, so desperate am I to get this job I joined another forum to get maximum feedback, a fella on there did say that anyone could come along and push the delta contactor in, and that a de energised coil is not really isolation, looking at a wiring diagram for star delta I can see what he means, one side of the motor connections would liven up.

 

[Blue Duck]

Are we over thinking this?

Lol@steps

 

[kerching]

Well I've cheated on you chaps, so desperate am I to get this job I joined another forum to get maximum feedback, a fella on there did say that anyone could come along and push the delta contactor in, and that a de energised coil is not really isolation, looking at a wiring diagram for star delta I can see what he means, one side of the motor connections would liven up.

So how did 'they' tell you to do it? Please

Just curious

 

[Mr Average]

I always knew how to do it Kerching, the question was why isolate at the mcc panel rather than the local isolator, I have been told to expect that question, the fact that someone can notice a fault and go down to the panel, when i'm away from it working on the motor, and start having a play and push the delta contactor in sounds a plausible reason as well as the BMS thing I mentioned earlier.

I have to say, as an installation (commercial) spark, I've not had to think of any of this since college days and even then I'm not sure I got it entirely.

Can't thank you all enough, I may be back

 

[Andy™]

if its isolated at a local isolator, and it can still run by someone pushing in a contactor, then 'safe isolation' isnt the problem, its been wired dangerously wrong.

 

[roys]

As I understand it the EAWA 1989 states that isolation means the disconnetion and seperation of the electrical equipment from every source of electrical energy in such a way that this disconnection and seperation is secure (regulation 12).

So if the local isolator achieves this then EAWA must surly be complied with.  Now in saying this we preferred to do the isolation in the switchroom at the MCC's which is where we considered to be the best and a central place to carry out out isolations.  We did have exceptions to this for example where we had a bank of motors that had a row of lockable isolators and in this particular circumstance local isolation was more practical, after of course the motor was stopped.

So the answer for me is:

Comply with EAWA regs.

Comply with local rules (which may say local or switchroom isolation points)

Comply with local Isolation and permit procedures.

 

[Evans Electric]

You can click my scoob button for thanks :slap

Just trying to beat the spark of the year ROTFWL

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