Switchgear M-T-M vs M-T-T-M

[philly]

I'm curious what some of the advantages of Main-Tie-Tie-Main (M-T-T-M) have over M-T-M Switchgear? I have seen both LV and MV Switchgear used with both so I'm curious what some of the advantages/disadvantages are with both?

The only one that I can see is that with a M-T-T-M you can completely isolate both sections of Switchgear for maintenance purposes however with M-T-M you can never completely isolate the Section of Switchgear which contains the tie breaker because the load side of the tie breaker will always be energized by the side without the tie breaker.

This point about maintenance makes sense if sections are separate but I'm not sure if it makes much of a difference for sections that are close coupled.

Curious to hear others thoughts.

 

[jim dungar]

The only one that I can see is that with a M-T-T-M you can completely isolate both sections of Switchgear for maintenance purposes ...

:thumbsup::thumbsup:

 

[Jraef]

Yep, that's it, although I would put it that the ONE side of the Tie is always energized, because which side is line and which is load is ambiguous in a Tie breaker / switch. But we all knew what you meant.

This is an issue that a lot of people forget to consider when ordering M-T-M only, especially now in our era of heightened awareness of Arc Flash risks.

 

[philly]

Yep, that's it, although I would put it that the ONE side of the Tie is always energized, because which side is line and which is load is ambiguous in a Tie breaker / switch. But we all knew what you meant.

This is an issue that a lot of people forget to consider when ordering M-T-M only, especially now in our era of heightened awareness of Arc Flash risks.

That makes sense. Thanks for the responses.

 

[rbalex]

If you ran a full FMEA (Failure Mode and Effects Analysis) you should first ask why you have a "T" or "T-T" in the first place. For reliability, it is more important to have redundant loads than redundant sources.

• If the necessary critical loads can be shifted to a single bus, there is no need for either a "T" or "T-T".

• Both schemes run the risk of running afoul of Section 110.9 for closed transition transfers.

• There will always​ be potential single-point failure mode with one of the "Ts".