REF Relay (Restricted Earth Fault) Location

What is/are the Transformer connection and winding configuration?

WYE (HT) to DELTA (LT) => REF relay in/on HT side because of the neutral physical location.

DELTA (HT) to WYE (LT) => REF relay in/on LT side

WYE to WYE => REF relay in/on LT side

The ideaology is to protect the transformer which is normally critical to plant/system operation and it is very expensive to replace.

By tripping the LT breaker the magnitude of the fault current reflected through the transformer windings is minimized.

Tripping the HT breaker will allow the fault current to reach maximimum value.

you can install REF in STAR or DELTA both side..... in Delta side residual current of three phase CT used to ref relay

Your scheme is a bit vague, but it will depend on the zone you are intending to protect.

If you were trying to protect the LV windings and connections between a HV/LV transformer and an LV switchboard, then that board is the logical location (near the CT's) and it will normally then trip the upstream HV breaker if a fault is detected. In this case tripping the LV breaker alone would not remove the supply to the fault. All this assuming power is feeding from HV to LV but is widely used in UK.

This is still just one typical useage.

The widely accepted practice is to protect the most.expensive part of the circuit, while not causing any fires.

Are there any codes in your part of the world.

What is the name of your. Consultancy?

Most of the transformers are delta/star configuration . For these , it would preferable to mount the REF relay on the LT panel as it will be convenient to connect the CTs. For tripping , both HV side and LV side breakers to be tripped.

I agree with Ram's reply

It is convenient to fix Relay in LV panel as the CT wiring will be easy . Because, Neutral CT only fixed in the transformer. Other phase CTs are fixed at LV panel incomer Tripping can be done LT and HT together or LV/HT. If you have easy access to HT side then either tripping in HT or LT doesnt make any change.

Both should be tripped

Why should be?....if incomer breaker at LT panel is tripped, then Transformer is on No load. Some client/consultants ask for only LT side tripping .

REF relay operates only for Internal faults of Transformer. Any fault occurring between Transformer secondary winding and CTs located in the LT Panel.

Therefore, the source feeding the fault has to be cut.

Sources are HT supply and Motor loads connected to the LT side.

Therefore, both HT and LT Circuit breakers are to be tripped.

"Why should be?....if incomer breaker at LT panel is tripped, then Transformer is on No load. Some client/consultants ask for only LT side tripping ."

This shows a total lack of understanding of the protection a REF relay provides. Removing the LV load does not remove an internal transformer fault. Both the MV and LV breakers should open at the same time.

If I'm wrong I've been connecting the relays incorrectly for the past 40 years.

Yes...Tonys and Ram is correct. It is my mistake. I took it as High temperature trip. Yes, REF trip both HT and LT.

Thanks everyone for the replies. The transformer is Delta-Wye. HT on delta side and LT on wye side. Name of my firm I will not be able to reveal since I am participating in this forum in my personal capacity and not as a company representative. In the schematic there is an inter-trip between LT and HT breakers, as suggested by MR. Ram