Power Transformer Trip

The detector circuit of the RTD is based upon a Wheatstone Bridge, and looks for the difference in resistance between the two legs of the RTD wiring. So a shielding issue is unlikely to have any impact on the reading.

To prove it, connect the shield to ground/earth and have another go.

The RTD itself is made of fine wire, which may have become cracked or nearly broken, either within itself or at an attachment to one of the lead wires. If so, vibration could result in these apparent temperature spikes.

As with comments from others, hope this will also assist you.

The answer to your question may depend on your DCS. While most manufacturers design their RTD board to work as a Wheatstone bridge, some just look at the voltage drop across the terminals where the RTD is connected. In the latter case, an ungrounded shield would definitely make a difference.

I would also check the DCS diagnostics to see if you had a scan failure or power supply dip at just the right time to cause this problem. Other possible causes:

- Intermittent failure of the analog to digital (AD) converter for that DCS channel

- Cracked RTD board in the DCS

- Improperly seated RTD board where it plugs into the backplane of the DCS

- Loose connection or bad joint at the RTD head

I'm surprised that the transformer safety is wired through the DCS. I've always insisted on discrete temperature & level switches wired directly to the breaker control circuits to ensure reliability.

As queried , the protection is not wired through DCS , the high temperatue trip is directly wired to upstream circuit breaker .

In the electronics lab , tested the RTD for any noise pick up keeping the shield wire un-grounded . No effect , RTD is working well . However , to avoid nuisance trips , now RTD has been changed in the trafo , with a new one, its shield wire is grounded now.Transformer is charged now and under observation.

Thank you all for your valuable inputs .