Switchboard Breaker ties

[TDrake]

I currently have an existing 3000 amp service rated switchboard. I am adding a second utility service as well as another 3000 amp service rated switchboard. I would l like to provide a tie breaker between the two switchboards but I do not know if this will nullify the UL rating or even if this can be done. The reasoning for this is so that if maintenance or new work needs to be done on either of the switchboards, the other one can carry the building load. If possible, I would like to avoid purchasing two switchboards.

 

[charlie b]

I think it can be done. But I think a representative of the manufacturer has to be involved. I would give them a call, and ask for their opinion. As the designer for such a project, it would be easy enough for me to show the new tie breaker on the drawing, and to put a note calling upon the contractor to work with the manufacturer to get the new tie breaker and associated components added. But we should not presume that the contractor will have the ability to achieve this. I think it is part of our jobs as the engineers to make sure that what we put on drawings is always constructible.

Welcome to the forum.

 

[mayanees]

I too say it can be done, as long as it's evaluated for fault current withstand, especially if you plan to have a closed-transition between the boards.
And if it won't be paralleled, then some sort (of) kirk-key interlock should be installed to prevent it.
I'm in your general area. If you want more specifics, or if you would like me to analyze it, PM me.
John M

 

[jim dungar]

This may not turn out the way you think.
When the two busses are tied together, you actually restrict your ability to work on them. Main-Tie-Main systems are really only good for accommodating loss of a source.

 

[GoldDigger]

If the switchboard is nothing more than a service disconnect and the tie is installed downstream of that, then you have a small amount of flexibility to work in one service at a time.
But, as pointed out, if your switchboard contains any combination of feeder and branch breakers you will not be able to work on those circuits whether you supply them from the main or from the tie.

Tapatalk!

 

[lielec11]

This may not turn out the way you think.
When the two busses are tied together, you actually restrict your ability to work on them. Main-Tie-Main systems are really only good for accommodating loss of a source.

I agree because the point of the tie is to be able to transfer the load from one service to the other. However, you cannot say that just because the busses are tied together you cannot work on them. You can work on them, but only if one main is open and the tie is also open.

TDrake, basically you aren't helping maintenance with adding a tie between the boards. If you want to work on either board, you will have to shut down it's service and isolate it from the other by opening the tie. Either way, you will lose power to those loads.

 

[mayanees]

M-T-M does indeed have the problem that you can't completely isolate either piece of gear. Data Centers tend to use M-T-T-M arrangements, but of course that adds the cost of another breaker.

 

[Tony S]

One other thing to take in to account. Due to the utility?s set up you may not be able parallel the two incoming feeds. This occurs when the utilities MV supplies are separate.

We had two intake substations, each with 2 x 20MVA 33/11KV transformers.

Nearly all of our panels had trapped key systems (two keys, three locks). With a bit of suicidal ingenuity to was possible to parallel two of the utilities 60MVA 132/33KV transformers via a 433V ACB.
I did have master keys to enable paralleling of parts of the system but they were only used after consulting the utility company (UKPN). It was quite an involved process but at times it had to be done

Two feeds were taken off the 33KV bus to feed the local town.