Double Ended Switchboards

[Daggerdale]

The NEC doesn't speak much about double ended switchboards however they are used in many critical applications. What govern's the design as far as load is concerned on a double ended switchboard? It would seem to me that there would need to be some type of load shedding feature if the total load for both sides would exceed the bus capacity when a main is open and the tie is closed.

 

[charlie b]

What governs the design as far as load is concerned on a double ended switchboard?

Not the NEC. It is not a design manual. There are rules about not overloading a switchboard or a breaker, but I don?t think there is anything else on this topic.

It would seem to me that there would need to be some type of load shedding feature if the total load for both sides would exceed the bus capacity when a main is open and the tie is closed.

I agree that it would be a good thing to have, and I usually design in some type of load shedding scheme (manual or automatic, depending on circumstances). But the NEC does not explicitly require it.

Welcome to the Fourm.

 

[mdshunk]

Aren't both feeds to a double-ended switchboard normally designed with the capacity such that either one can serve the entire load of the board, with the tie closed and one of the feeder breakers open? Not saying... just asking.

 

[charlie b]

It?s really a cost issue. To design a Main-Tie-Main switchgear (or switchboard) such that each side has the capacity for the total load on both sides would be considered, by some, an excessive expense. The amount of time you are likely to have the entire facility on one side or the other is very small. The extra cost for what is essentially a double-sized service gear would be hard to justify.

 

[mdshunk]

Thanks, Charlie.

How, then, is the typical load shedding arrangement designed? A Kirk key arrangement, with certain non-vital switches having to be in the open position before the tie can be thrown? By electronic means? By no means whatsoever, and relying solely on the fuse protection of the feeders at whatever end?

 

[nvpowerdoc]

I believe it all depends how the facility wants to implement its M-T-M scheme. We have a major casino resort here in Vegas that has over 50 M-T-M subs that at any time the entire load of the D-E'd switchboard can be carried by a single transformer at either end. In this case, design load for the D-E'd switchboard is such that each half is only loaded to 50% capacity. It's the cost of an N+1 scheme in a 24/7 world.

If one loads their D-E'd switchboard past 50% capacity per side, then N+1 can not be implemented without load-shed. One must determine what is most important for carrying critical loads - load shed or N+1.

 

[rcwilson]

Main- Tie -Main Design

Main- Tie -Main Design

Double ended switchgear is often used in industrial plants where continuity of service is important. On the process side, there will be two pumps, an A & B in the same service, but only one will run at a time. All of the A loads are fed by the A bus and the B loads by the B bus to provide redundancy and better reliability. Theoretically we can maintain all of the equipment, including the substations, without shutting down the process by shifting loads. (The tie breaker cubicle can't be maintained easily.)

One advantage of this layout is that the equipment does not need to be sized to handle the A & B loads together when the tie is closed. The loads are redundant and a diversity factor can be applied in the sizing. If both pumps run at the same time, they split the load and run at 50%-60% rating. This saves a lot of unused transformer capacity and overkill.