M-T-M Transfer Scheme with Line Side PT's only

[philly]

Is it typical to implement a M-T-M transfer scheme on a LV or MV switchgear lineup with only PT's on line side of breaker (No Bus PT's). Are bus PT's typically only required if a closed transition scheme is being implemented? If only an open transition scheme is implemented are only line side PT's required with necessary breaker interlocking.

The application I'm referring to here is relay based transfer schemes implemented with relay control.

 

[paulengr]

Is it typical to implement a M-T-M transfer scheme on a LV or MV switchgear lineup with only PT's on line side of breaker (No Bus PT's). Are bus PT's typically only required if a closed transition scheme is being implemented? If only an open transition scheme is implemented are only line side PT's required with necessary breaker interlocking.

The application I'm referring to here is relay based transfer schemes implemented with relay control.

You need line side PTs to detect phase loss to initiate a bus transfer, close tie. If you had load side PTs you can’t detect loss of the feeder to the main.

What good do load side PTs do?

That’s with a typical MTM.

If you locate the mains on the high side of the transformers (for arc flash/current reasons) it may make sense to have load side PTs or even full CTs and PTs (87 relaying).

 

[philly]

You need line side PTs to detect phase loss to initiate a bus transfer, close tie. If you had load side PTs you can’t detect loss of the feeder to the main.

What good do load side PTs do?

That’s with a typical MTM.

If you locate the mains on the high side of the transformers (for arc flash/current reasons) it may make sense to have load side PTs or even full CTs and PTs (87 relaying).

Yes I agree PT's on line sided needed for detection and initiating bus transfer.

I guess my question was more when PT's were also needed on the load side (in addition to line side). I guess that would only really be a requirement if you were doing closed transition transfer with tie, or syncing multiple power sources to the bus (generators, etc...).

On some Switchgear I've also seen a relay on the tie breaker involved in transfer scheme. But I guess this is just more MV gear where there is a relay there anyway along with CT's to detect current and control tripping of tie. On LV Switchgear applications I'm not seeing the need to a relay on the tie breaker for transfer scheme unless I'm missing something?

 

[ron]

You can use load side PT's to check for dead bus with a sync check relay to be sure it is open transition if it is electrically operated.