Single Phase Transfer Switch in 3ph Distribution

MJS123 · Sep 5, 2023

When there exists a single-phase transfer switch within a 3-phase distribution system, is it necessary for both feeds to the transfer switch to be connected to the same phase on the two sources?

Take for example the image below, note that the transfer switches are connected to the power sources on different phases. Is this OK? Why/why not?

[ElectriciansForums.net] Single Phase Transfer Switch in 3ph Distribution

James · Sep 5, 2023

the voltage between any 2 phases are seen as a single sine wave by the load similar to any phase and N but at a higher voltage and not referenced to earth.
so no it will not make any difference.
obviously the correct type of transfer switch must be used so the 2 supplies are not connected to each other even briefly during the switching process, as this could lead to one supply back feeding into the other and/or a big BANG!

pc1966 · Sep 5, 2023

The other aspect which I am not sure about is the OP is from the USA and they often have the "high leg delta" transformers so while L-L voltages are the same for any 2 out of 3 phases, they may not be the same to N / ground.

@Megawatt is probably best placed to comment here.

Personally I would need to have a damn good reason for having an unexpected cross-over of phases at any sort of transfer switch like this!

MJS123 · Sep 5, 2023

pc1966 said:
The other aspect which I am not sure about is the OP is from the USA and they often have the "high leg delta" transformers so while L-L voltages are the same for any 2 out of 3 phases, they may not be the same to N / ground.

Assume that line to line and line to neutral voltages are the same for both power sources. I don't understand your point about high leg delta (vs. wye?)... what difference does it make? note the original diagram specifically was connecting across (any) two line conductors... would it make any difference in the case of (any) line to neutral connection as shown below? why/why not?

MJS123 · Sep 5, 2023

pc1966 said:
Personally I would need to have a damn good reason for having an unexpected cross-over of phases at any sort of transfer switch like this!

can you elaborate on this a bit?

pc1966 · Sep 5, 2023

MJS123 said:
Assume that line to line and line to neutral voltages are the same for both power sources. I don't understand your point about high leg delta (vs. wye?)... what difference does it make?

Here in UK/EU it is always star=wye connection for LV so we have 230V L-N and 400V L-L as nominal values. In that sense all L are interchangeable and it is only if you are looking at more than one phase does the difference/rotation direction matter (and for electronic 3P loads probably not even that, but 3P motors direct are still a major aspect).

The high-leg delta has two of the 3 phases at (L-L)/2 from N/E and the 3rd at sqrt(3)/2 of L-L (e.g. 120-0-120V on two and 208V on 3rd to N) so if you interchange them you get difference voltages to N. In most cases that makes no difference at all, but i have seen 3P equipment for the USA with a specific requirement as to which line has to be on the "high leg".

pc1966 · Sep 5, 2023

MJS123 said:
can you elaborate on this a bit?

The principle of least astonishment really.

Most folks in the future will assume you preserve L1-L3 order through the system, this does not and while it might not have any functional change, it can make future fault finding a problem as someone might, say, report a low value on L2 and yet L2 on the other side is fine, etc.

MJS123 · Sep 5, 2023

pc1966 said:
Here in UK/EU it is always star=wye connection for LV so we have 230V L-N and 400V L-L as nominal values. In that sense all L are interchangeable and it is only if you are looking at more than one phase does the difference/rotation direction matter (and for electronic 3P loads probably not even that, but 3P motors direct are still a major aspect).

The high-leg delta has two of the 3 phases at (L-L)/2 from N/E and the 3rd at sqrt(3)/2 of L-L (e.g. 120-0-120V on two and 208V on 3rd to N) so if you interchange them you get difference voltages to N. In most cases that makes no difference at all, but i have seen 3P equipment for the USA with a specific requirement as to which line has to be on the "high leg".

oh, yes, I see your point... when considering line-to-neutral, one has two different voltages to contend w/ on a split/high leg delta, vs. a single voltage in star/wye. That said, as long as the voltages match on the source connections to the transfer switch, it seems the phases don't necessarily have to match. However, in order to avoid miscommunication related to the failure of a given phase, it is a better design principle to avoid such mismatch.

Megawatt · Sep 5, 2023

MJS123 said:
When there exists a single-phase transfer switch within a 3-phase distribution system, is it necessary for both feeds to the transfer switch to be connected to the same phase on the two sources?

Take for example the image below, note that the transfer switches are connected to the power sources on different phases. Is this OK? Why/why not?

View attachment 110373

It really doesn’t make a difference because the actual single phase load doesn’t care what 2 phases are being used as stated above. Hope this helps and welcome to the forum.