New supply to factory

Just wondering what people thoughts are on obtaining a supply to a part of a local mill where I am now a maintenance engineer. The existing installation is terrible with many switch fuses that have been added over the years, the oldest part is from 1946.

The total supply capacity in question is 1000Amps and enters a 1000A Dorman Smith main switch then off to bus bars. Lead tails are taken from these bus bars to the old 1946 busbars via a oil filled Pot breaker (stamped at 600A)

A local contractor is proposing removing the second oil filled pot breaker fitting a switch fuse, the busbars do not look big enough to be rated at 600A however they are proposing taking tri rated tails onto the output of the main breaker.

Personally I would much prefer a new switchboard to be installed and the 600A feed to the new factory be taken from there, this also allows for any emergencies if there are any issues where the feeds could be re routed to this switch board.

My concernes are with the reliability of this old switch and using the existing busbar chamber as a cable way, also the capacity of the lead cable feeds as there is no indication of what the oil filled breaker is turned down too.

The issue is I’m not sure what to propose, I’d like a new feed taken from the 1000Amp Dorman Smith to a new switchboard, is this a feasible idea? The existing load is around 400A however this will reduce once the new plant is running.

What’s everyone’s thoughts? I know it’s difficult to suggest but I just think what’s being proposed is a terrible idea, it may be functional at the start when there’s hardly any load but it will be increasing over the next 7 years. This also has to be done while the place is running as it cannot be shut down for weeks obviously. My aim would be to remove as much load from the old part as possible.

When you say lead cables do you mean PILC. So they are going to resupply the old busbar with tri-rated from the main 1000A mccb, leave the existing switch-fuse in place then take a supply from the old busbar through a new switch-fuse.

Rocboni said:

I am now a maintenance engineer.

Rocboni said:

The issue is I’m not sure what to propose, I’d like a new feed taken from the 1000Amp Dorman Smith to a new switchboard, is this a feasible idea? The existing load is around 400A however this will reduce once the new plant is running.

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IMO, I think you are biting off more then you can chew.

Hi, sorry it is quite hard to describe, but ill try my best.

No, they are leaving the exiting set up alone with the old PILC singles feeding the oil filled breaker (this is fed from busbars protected by the 1000A dorman smith MCCB)

Paper wrapped Cables then come out of the oil breaker to supply the right and left hand side busbar chambers. They are installing a switch fuse in the bottom of the old busbar chamber and supplying it via tri rated through the busbar chamber to the 630A switch fuse as the busbars do not look like they are rated at 600A, i presume around 300A/side. The dash pot circiut breaker with the do not switch on sign will be removed (this is an unkown rating) and used to supply the building that was demolished for the new factory. Technically the busbars should be able to supply 600A per but im not sure of regs in 1946! and whether they go on the presumption that you would'nt load one side to the complete capacity of the breaker

I would much prefer to see a new supply taken from the main 1000A MCCB to a new switch board, where sub mains could be moved to, one by one as and when required. It would also allow a future proof
supply to the new extension.

There is a lot of un knowns with the plan as well as some non compliance issues.

buzzlightyear said:

IMO, I think you are biting off more then you can chew.

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Totally agree but i am not and will not be undertaking the work, but i am questioning what is getting done at the moment

What scope of works are they working to?

If it's just for a new supply to the new factory then that's what they appear to be doing.

They wouldn't be preparing for replacement of all existing old, but probably serviceable, switchgear unless the scope suggested it.
Obviously the replacement of existing would have significant cost implications.

It is only for the supply of the new extension however i am questioning the way in which it is getting done as i don't think the 1940 switchgear is capable of supplying reliably 600+amps, it is already drawing 170A and the main dash pot breaker is rated at 600A this could be turned down inside, the mill estimates requiring 630A supply.

It has been known to trip occasionally as well!

The only way i see feasible is to install another switchboard and take this new supply from that, this would enable spare ways to be facilitated and which should have been highlighted when they asked for the new supply.

The fact of the matter is that the old switchgear will never be removed and nor am i suggesting this however i do question adding to it to the extent of it's full capacity at this stage in it s life, surely it is unpredictable.

I'd be looking at your budget allowance and time scales before making any decisions. Shutdown time is essential.

Money is not really an issue to be honest, shut down would be carried out a night.
The new extension is not populated with machinery and wont be for another year or so.

The worry for me is when the extension is fully kitted out with milling equipment they start getting random tripping, who knows if it will reset!

Money no object, look at Schneider panel boards.... but it's not going to be an overnight job. You'll need shutdown to completely remove and replace.

I'm not proposing removing the old gear, just installing new to supply new extension as well as future issues. Possibly take off some of the lager switch fuses and supply via new switch board thus minimising impact if there is an issue, eventually the old questionable oil breaker could be removed and busbars supplied via new MCCB or fuses.

The new unit could be installed and tails could be made ready to connect and done at night, minimising downtime.

It is a Schneider 630A powerpact unit going in the extension.

Rocboni said:

where I am now a maintenance engineer.

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You need to update your Avatar m8, it says apprentice.

Make sure that the design takes into account the prospective short circuit fault currents. Ie: The busbars are rated and cabling is installed such that they are both physically secured strongly enough to withstand the mechanical forces created by electromagnetic forces until the over-current clears the fault.

Hi Marconi,

I am leaving it to the contractors, I know the tri rated tails will just be thrown in to the busbar chamber and onto the main switch already knowing that that capacity of the main oil pot breaker will be exceeded one day, however I have informed my superiors and they are not concerned therefore there’s not much i can do.

I was proposing single core parallel awa cables cleated in trefoil formation on ladder to a new location opposite the old set up, with a new switchboard running in parallel with the old system where gradually switch fuses could be removed and the armoured feeds to the various lines re-routed to the new switchboard but I can only tell them so many times!

It will most likely come to a ‘I told you so’ situation in around 3 or 4 years time when the factory is fully loaded and the other facility is shut down and scrapped and they can’t get that old switch to reset!‍

How long is the run

I had to get a new 400a supply in a few months ago.


It was 100m from the switch room.

I asked for 185mm cable. The contractors wanted to pull in 2x95mm SWA in parallel, he said it would be easier managed. I agreed.

They used cable tray and an MCCB panel board made by Eaton.