Debate over rating of switch gear

I had a debate today with another, not like me!

Quick description:

3ph balanced load cct. 40A type 3 3871. Industrial cct. SWA.

Machine local isolator has been replaced after I issued an EDN on it for not "switching off".

Client would not allow me to sort. :_|

I was there on the machines not the fixed install.

Isolator has been replaced with a 32A device, can't remember what was there before.

I'm not gonna say who said what, but the 2 sides of the "argument" were:

1 - The isolator must be rated at or above that of the CPD.

2 - The isolator is fine as the machine is only rated at 21A FLC.

BTW, this is only the main motor, there is another small motor & the control system / DC injection brake on top of this main motor load.

Opinions please?

1 or 2.

And your reasoning for your opinion if you don't mind.

Thanks

id go 1 because if a fault happens or a over load between phases and neutral or earth happpened across the contacts of the switch gear and the mcb/cpd was rated higher ie 32 amp switch gear with protection from a 40 amp mcb/fuse the fuse would not trip on overload/fault current therefore the machine/fault would be able to draw more current through the rated switch gear than it was rated to handle.

I'll chuck my hat in the ring.

Option 1. apart from what Badge has said what if something as simple as the machinery is changed for a load closer to the ocpd & above the 32A of switch gear.

---------- AUTO MERGE Post added at 19:21 ---------- Previous post was at 19:20 ----------

I'll chuck my hat in the ring.Option 1. apart from what Badge has said what if something as simple as the machinery is changed for a load closer to the ocpd & above the 32A of switch gear.

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OOps pushed the wrong one sorry:Blushing....Bloody hell trying to do too many things at once & got in a mucking fuddle.

I ignored my option and chose option 2. This has to be bad design if nothing else. If the load is fixed and only 21A WTF have they fitted a 40A breaker?

We all know that a 40A breaker will let more than that through for a period of time so if that is what's protecting the 32A isolator from melting then I'm not a happy bunny.

Downgrade MCB or upgrade isolator.

OK,

Patch you are along the right lines, but the mcb may be a type 2, not a 3 not completely sure as I could not get access to the DB.

The load is "fixed" but it is more than 21A as it says in my description, there are some clues in there also to the type of load.

Mind & I'm not havin a go or taking the p*55, they are probably a bit subtle for a vet!

---------- AUTO MERGE Post added at 21:19 ---------- Previous post was at 21:13 ----------

In fact patch being a vet you may have some input with respect to your experience believe it or not on the other thread I have started tonight.

Now there's a clue to the use!

http://www.talk.electricianforum.co.uk/showthread.php?t=12651

I'll throw in the 13A rated switched sockets you get on a 32amp ring which is always deemed safe. The socket may only draw a max 13a but whatabout wheen in fault

As for your scenario, I'd up the isolator or down the mcb. Can't guarantee the load will be the same this time next year.

I`ve got to agree. For fixed load (which could be changed at some point....) the iso should be capable of handling the MCB current at least.

Why not have a 32, 30 or even 25A MCB if FLC doesn`t reach that level?

In short, completely agree with sellers & `Pache on this, mate.

OK,

My view to client was - "I see you've had that isoator changed, scuse me a sec", wanders over to board, hops up on machine in front of board, opens cupboard covering board, looks at MCB with do not switch on machine dangerous tag on it & its a 40A & on!

Comes back & says, "yep but that isolators too small" its underrated for the job.

Phone call etc. well other spark says motor is only 21A so 32A is fine.

I say OK, his insurance!

As I have not "worked" on the job since I'm OK.

The machine is a band ressaw with 36" blade pulleys Y/D starter, run up time around 35/35 seconds. It also has another small motor & a DC brake.

LRC around 180/200A on start!

Prob why a 40A breaker, think the board is a C50 so thats an issue to get breakers for only type3/C available. I would normally put a machine like this due to the start up inertia on a D, plus the sudden load from the DC brake on stop, as the m/c has to run down within its run up time.

I'm quoting for a new coontrol system coz the existing dc brake install is a bodge & has been really badly integrated into the existing machine control & has nearly caught fire. It also does not comply with PUWER.

There is a load of other stuff to quote as well.

The reason patch for the 40A breaker is for the inrush, the guy originally fitting it obviously did not have a clue.

They had some tripping problems there a little while back & the spark they had in told them that the problem was the new Tesco the other side of town using up all the lektrik!

I can't get the go ahead to do any works at the moment until all is quoted and it goes before the board for approval as they are closing 3 mills & rationalising so machines may move etc. etc.

There are also guarding issues I have to price up for & uplifting machines moving stuff etc. so it will be a while.

Mind the "sparks" that fitted the small isolator are still in the running but they did realise that there was no up front RCD on the mill mind!

Machine could be changed but it's about half a day to get it off the floor as parts of it have to be stripped off before it can be lifted! Its as top heavy as hell then too! They are a real pain to move & it looks like I'll have 5 to do soon!

Changing the fixed load is not quite that easy in this situation!

However it is possible, but, the spark connecting it up should verify the adequacy of supply then should they not!

Thanks guys, just verified my side.

The more you do PIR inspections on workshops the more you come across this problem.

To overcome high surges it is common for inexperianced electricians to up the rating of the mcb instead of trying to match the in surge curve to a suitable type.

Thanks for the lesson Sidewinder I enjoyed following the debate.

The more you do PIR inspections on workshops the more you come across this problem.

To overcome high surges it is common for inexperianced electricians to up the rating of the mcb instead of trying to match the in surge curve to a suitable type.

Thanks for the lesson Sidewinder I enjoyed following the debate.

Only just come upon this Thread..... So here's my 2 pennies worth!! lol!!!

I think this just shows what happens when you cut-out what would normally, ...would have been in place maybe 20 years ago, ...''A factory maintenance dept!!!''

What many forget, is that in industrial environments, a supply would be sized/designed to the needs of a dedicated machine. That can only be done by an experienced industrial electrician or superior, that understands the equipment they are working on.

Far too often these days, Companies such as this, farm out their electrical and mechanical work, to those that DON'T understand!! And the results are, ...too often exactly as SideWinder describes on this heavy band-saw machine.

The modern day company/factory accountants have much to answer for, they only see ''saving money'' They have absolutly no interest in the engineering side of things, ....Which in most cases is where the company makes it's money!!!!

Only just come upon this Thread..... So here's my 2 pennies worth!! lol!!!I think this just shows what happens when you cut-out what would normally, ...would have been in place maybe 20 years ago, ...''A factory maintenance dept!!!''

What many forget, is that in industrial environments, a supply would be sized/designed to the needs of a dedicated machine. That can only be done by an experienced industrial electrician or superior, that understands the equipment they are working on.

Far too often these days, Companies such as this, farm out their electrical and mechanical work, to those that DON'T understand!! And the results are, ...too often exactly as SideWinder describes on this heavy band-saw machine.

The modern day company/factory accountants have much to answer for, they only see ''saving money'' They have absolutly no interest in the engineering side of things, ....Which in most cases is where the company makes it's money!!!!

Click to expand...

besides being our resident Joker, you speak some electrical sense:Salute