When to isolate incoming supply to DB

[GeeWizz12]

I always turn off main switch before removing cover or doing any work in a DB, but when should incoming supply be isolated ?
When placing a clamp-on ammeter around an incoming phase conductor in a 3 phase board a spark to board casing occurred. Later found to be pre-existing damage to insulation on this conductor, movement of which caused the arcing. Would you normally isolate incoming supply before placing ammeter around insulated conductor, then re-energise to take readings ?

 

[pc1966]

Certainly switching off before removing a DB cover is a very sensible thing to do, as you might get a nasty surprise triggered by the act of removing it!

I guess the answer really is "it depends". If there is any sign of damage or what you are doing is likely to place you or others in danger if something minor goes wrong then it would be the correct thing to do. Of course, in some situations if critical stuff is being powered and what you are attempting ought to be safe then you might not, but in those cases wearing suitable PPE would be sensible, so gloves, flame-retardant overalls, helmet and face shield rated for arc-flash, etc.

 

[GeeWizz12]

Certainly switching off before removing a DB cover is a very sensible thing to do, as you might get a nasty surprise triggered by the act of removing it!

I guess the answer really is "it depends". If there is any sign of damage or what you are doing is likely to place you or others in danger if something minor goes wrong then it would be the correct thing to do. Of course, in some situations if critical stuff is being powered and what you are attempting ought to be safe then you might not, but in those cases wearing suitable PPE would be sensible, so gloves, flame-retardant overalls, helmet and face shield rated for arc-flash, etc.

Thanks for the reply, the install looked in very good condition, relatively new, with no obvious damage. So no reason to suspect damage. I think you’re basically saying air on the side of caution, which is accepted, but in a domestic DB these conductors would be fed from supplying companies fuse locked with leaded wire. At home I have similar ammeter clamps in electric cupboard which solar panel installers fitted to mains to monitor incoming vs solar electricity being used, without isolation.
I’m facing disciplinary action due to this incident so am looking for something to support my argument, that when placing a clamp-on ammeter around an insulated conductor it’s not required, or normal practice to first isolate. If I’m wrong I’ll accept it, but would mean in future, even if required to work near incoming cables, isolation required. Such as feeding cable of a new circuit behind/around them, from the place they enter DB to reach MCB, neutral and earth connections.

 

[snowhead]

The argument for must isolate first in this case could easilly fail if the supply was isolated first, then the cables moved to get the clamp ammeter on, power restored only to find the damaged cable may well have still touched to earth with a flash bang or whatever.

It would be difficult to say that all cables must be isolated before a clamp ammeter was fitted and the same would apply before removing it again.
So for a 3 phase DB that's on and off 6 times, 8 if you're clamping the Neutral or alternately use 3 or 4 clamp ammeters which may not all fit in.

Cables are normally well insulated, clamp ammeters are also insulated so there should be no need unless there are obvious signs of cable damage, which in this case there probably weren't, or if there are exposed parts within the D.B.

If the outcome of this is that all cables/ Dbs need to be isolated first, then what would normally be a few minute process of checking current with an ammeter in a commercial environment could turn into an extensive operation where permission to switch off machinery or computers etc was needed.
This could be hours / days / months / or up to a year process.

Where I worked for my final 25yrs of employment we had to give 6 weeks notice to interupt power to the computer systems and even then it may not be have granted.

 

[James]

Thanks for the reply, the install looked in very good condition, relatively new, with no obvious damage. So no reason to suspect damage. I think you’re basically saying air on the side of caution, which is accepted, but in a domestic DB these conductors would be fed from supplying companies fuse locked with leaded wire. At home I have similar ammeter clamps in electric cupboard which solar panel installers fitted to mains to monitor incoming vs solar electricity being used, without isolation.
I’m facing disciplinary action due to this incident so am looking for something to support my argument, that when placing a clamp-on ammeter around an insulated conductor it’s not required, or normal practice to first isolate. If I’m wrong I’ll accept it, but would mean in future, even if required to work near incoming cables, isolation required. Such as feeding cable of a new circuit behind/around them, from the place they enter DB to reach MCB, neutral and earth connections.

Disciplinary action is a bit rough in this case I think (from the info provided)
assuming that before the DB cover was removed, the power was switched off at the incoming switch/breaker.

obviously doing a current measurement on the incoming tails has got to be done with the power on,
so at some point when the cover is off and as best you can see, it is safe to energise with the cover removed, then the power goes back on and the loads are fed so you can measure current.

If you were asked to take current measurements, then either
A. the company has a procedure that they want you to follow to do it safely
or
B. they expect you to make a professional judgement on each job to ascertain if the test is safe to do.

it could even be both, the test procedure should be followed but stopped if at any point the tester deems it to be unsafe or unwise to continue.

If you are concerned about the disciplinary, I would firstly decide who from the company or union you would like to sit in the interview with you (never go in alone)
next I would be asking for a copy of the company hand book and the training / documented procedure for carrying out the test you were asked to do.

If they are looking for a scape goat, don't give them the chance, when asked about how and why you did things a certain way, make sure you have a good reason.

 

[snowhead]

Was this on company premises or customers?

And were there any issue with whatever is supplied from that 3 phase DB?

And presumably the damaged cable has been repaired / replaced?

And did anyone get the current readings?

 

[GeeWizz12]

Disciplinary action is a bit rough in this case I think (from the info provided)
assuming that before the DB cover was removed, the power was switched off at the incoming switch/breaker.

obviously doing a current measurement on the incoming tails has got to be done with the power on,
so at some point when the cover is off and as best you can see, it is safe to energise with the cover removed, then the power goes back on and the loads are fed so you can measure current.

If you were asked to take current measurements, then either
A. the company has a procedure that they want you to follow to do it safely
or
B. they expect you to make a professional judgement on each job to ascertain if the test is safe to do.

it could even be both, the test procedure should be followed but stopped if at any point the tester deems it to be unsafe or unwise to continue.

If you are concerned about the disciplinary, I would firstly decide who from the company or union you would like to sit in the interview with you (never go in alone)
next I would be asking for a copy of the company hand book and the training / documented procedure for carrying out the test you were asked to do.

If they are looking for a scape goat, don't give them the chance, when asked about how and why you did things a certain way, make sure you have a good reason.

Thanks James, useful points there, company are saying I should have used company procedure of LOTO, lock off tag off, I’ve said I don’t think it to be relevant as power has to be on to conduct set up and testing of current monitors. Also the equipment is specifically designed so that they clamp around an insulated conductor and no disconnection is required. Electrician on tech support line for company who provided equipment also said he wouldn’t expect isolation to be necessar.

 

[plugsandsparks]

I think there does need to be clarity between you and your employer for working practices. I dont think it really matters whether you can or cannot connect an clamp on ammeter on a live cable.

For instance, some companies mandate a strict no live work, period, this means to remove a DB cover you will need to isolate and lock off upstream of the DB.

I had an interesting one around 10 years ago when i was EICR testing (which is live testing with the cover off), there was a huge amount of dust and debris in the bottom of the panel board and the necessary N_E link was missing. The site would not allow their own electricians to clean out the board and fit the N_E link without isolating upstream. Considering what work i was doing, there was no real risk using PPE but again it was policy, strictly applied. Yes, loads of paperwork and scheduling whilst i hung around drinking coffee, lol

I think the principle needs to be clear and then everyone is in no doubt regardless of the work being done,
So either the DB needs isolating upstream or not ?

 

[snowhead]

The company are going to have give in on this one then clarify it in the future and adjust their procedures accordingly.
Either to say absolutely no exceptions to LOTO for D.Bs, which would also mean banning the use of clamp ammeters and throwing all the existing ones away,
Or exceptions allowed for use of clamp ammeter but following a stated agreed written procedure.