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Is a second disconnect really required?

Is a second disconnect really required?

Is a second disconnect really required?

I came across this in an engineering book on substation design, and I have to admit I do not see the purpose, but the book seems to indicate that 4 disconnects per bay are required for a single breaker, double bus substation. My understanding is that only 3 disconnects are needed per bay, as the disconnect between the substation breaker and other two bus (selection) disconnects ends up being purely redundant.

However because the book is based with IEEE/ANSI systems in mind, I am lead to think that the other knows something that I do not (ie, NESC requirement).

RE: Is a second disconnect really required?

On gas-insulated gear the reason may be down to the location of earth switches and the breaks between gas zones. You can't easily put a drain earth / portable earth on GIS the way you can with air-insulated gear.

RE: Is a second disconnect really required?

But to be honest I've never seen GIS literature showing or mentioning it. The book is older- so I am guessing it has AIS in mind.

Although I think I am starting to see your line of thought in terms of gas partitions and increasing availability in GIS.


RE: Is a second disconnect really required?

Does the book in question show a profile view? I'm guessing the two bus disconnects are located higher up where there is not a clear association with the breaker in question from the ground. When taking a breaker out of service, the lines to the disconnects should be easily traceable. Also, using three disconnects for isolation may be a complicating factor for a worker that is accustomed to two.

RE: Is a second disconnect really required?

Is there a maximum length of wire that can be connected to a circuit breaker while performing maintenance? Perhaps the distance between breaker and the bus disconnects is long enough that significant induced voltage may be present?

Most of our substation outages require more than two isolation points. In our stations, we typically use both bus disconnects as the isolation points rather the bus side breaker as the isolation point.

RE: Is a second disconnect really required?

@stevenal: There is no profile, just that of main & transfer and breaker-and-a-half. In substations designs which I have seen there are only two disconnects- breaker - and then the line disconnect.

@Bacon4life: you are on to something. That might be a reason, but can't seem to find anything. What design are your substations?

RE: Is a second disconnect really required?

Double bus single breaker with breaker bypass. With the breaker bypass, it does require all 5 disconnects.

The protection issues with running allowing terminals to switch busses have been challenging enough that we just run it like a main and transfer scheme. The breaker bypass has become less important as we have moved to SF6 breakers which require fewer and shorter maintenance outages as well as can be replaced much faster than oil breakers.

RE: Is a second disconnect really required?

Ahhh, in that yes I would agree, you need all 5 for that beast. This design was very popular for some POCOs back in the day, but the bypass is becoming rare. In fact nearly all new installations of single breaker double bus no longer have any type of bypass. Makes relaying and breaker substitution (or lack there of) easier by leaps and bounds so I can certainly understand operating it as M&T. For those few new substations that still need bypass, often a 3rd dedicated transfer bus is added to keep relaying simple:


Its possible the author had a SBDB with bypass in mind, but failed to include the bypass in his diagram. Other than that he knows something... is it possible the strain wire from the bus to breaker gantry could become live if it falls onto the bus bars?

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