Why would the compensation leads on multiple RTDs be tied together in the field J.box

Never noticed this before and no-one can explain the reason why to or why not to tie the compensation (2 white leads) all together to ground on multiple RTDs. I have new installation with a large motor with 6 100 ohm rtds and was surprised to see a grounding terminal block on the 3rd lead (white) of 3 lead (1 red, 2 whites).

Maybe this is a stupid question buut why or whynot?

Brian.

(I do know that the two leads with the same colour are used for lead length resistance compensation)

I think you have a problem.

As you point out, the third lead is for lead length compensation. It is probable that the RTD sensing device is for two-wire RTDs. This is common, and should be easy for you to check. The proper thing to do in that case is to leave the second white lead "floating" - that is, not connected to anything.

By tying all of the ends of the RTDs together at a single point I don't belive you will get any temperature reading. Have you checked the temperature display on the motor protection relay or the PLC that these leads are connected to?

My guess is that the electrician didn't know how to wire up the RTDs so he just faked it!

Thanks Tom,

I just got back from reporting the 'problem' to my supervisor (elect eng.) who asked the project guy (elect technologist) who thought it was ok. I drew it out on paper and talking to the project guy who wasn't now so cetain, he called the motor shop where the RTD were installed and they called the RTD supplier and back down the chain to me came the order "replace all the grounding t.b.s with standard ones."
Guess it wasn't such a stupid question after all.

By the way there are 3, 100 hp motors wired like this that have been in service for 6 months. 2 have been having ongoing rtd faults, but they have been getting some good rtd temp readings as well.

Thanks again guys,

Brian.

2 wire RTD input cards are also those most likely to be single ended and have very poor ability to reject common mode noise.

By tying the 'extra' white wires to a ground, rather than letting them float, the ground loop problem could easily magnify and get out of hand.

I actually find it surprising that you haven't experienced more more serious difficulties getting good temp readings.

I'm always loathe to cut off the extra wire, just in case the input card is ever replaced with 3 wire inputs.
Dan

Thanks Dan,

These RTDs go to a temp control module with 3 wire inputs.
Just a matter of the motor shop sending the motor after installing the RTDs with a junction box containing every third terminal block being a grounded one. So the electrician wired by the numbers. Can't be too hard on him, it was a rushed startup as I remember and as I am also a maintenance electrician, I may do the same or worse some day. Glad I noticed it.

Thanks,
Brian

2 wire RTDs are not as accurate as 3 wire due to the fact that there is no resistance compensation taken in to account by the reading device. The longer the run the greater the error, dependant on cable size.

A 3 wire RTD temperature controller, for example, reads the resistance between the red and 1 white wire. It also reads the resistance between the 2 white wires and subtracts the value from the first reading (R/W). If the subtraction does not take place the reading is in error by the amount of resistance in the original red/white wire reading.