Three Wire RTD Connection

If you could post a drawing, I might be able to understand what you're getting at.

regards,

Vulcan

Figure showing connection of an RTD at a distance of 1metre

A current of 1mA flows from SC to RTD Hi through W1. At eHi, voltage is higher thatn at RTD Hi by the drop in this wire. (RTD Hi has a voltage of Vrtd +elow.

The same current returns through W3 to common of SC (which provided the excitation current)

W2 just provides the same voltage as available at RTD Lo to signal conditioner! No voltage drop in this wire!!This voltage is Iexc*Rw3.

Er, and?

Current source operated and bridge configured RTD's with

3 wire connection :

I've found that people find it easier to understand when you present them with a simpler concept. I usually use this one:

Just measure the resistance across A and B (R_AB) and then measure the resistance across B anc C (R_BC). Then subtract the two resistances and the result is the resistance of R_rtd.

This is valid only if R₁, R2 and R₃ are all equal, which is why they always say that the three wires in an RTD circuit should be the of the same length to ensure that they have equal resistance.

regards,

Vulcan

Now, that makes a lot of sense.

And?

Come on. Where's the punch line?

Ahhh . . . I finally figured out that venku isn't asking a question! He's just spreading his knowledge around for the rest of us!

Thanks!!!

Hi ,

We have a 12.5 MVA 34.5 kv / 4.16 kv Power transformer , which has a 3 Wire RTD for Oil temperature monitoring . The transformer is working fine about 65 % only loaded , no abnormal current raise , no faults on the load side recently , the temperature of oil is about 54 Deg C for the last two weeks , as displayed in the DCS trend.

The problem is the transformer tripped on Oil temperature high ( Trip set at 90Degs C ) , though the DCS trend couldn't capture it but the DCS alarm summary has a record stating that the oil temp went high to 106 Deg c suddenly for a fraction of a second and returned back to normal temperature .

During the investigation found all ok with the trip circuit loop , no abnormality of loose termination or a malfunction of Temp monitor controller etc.,. Only thing which was noticed is that though the 3 wire RTD ( 10 Copper ) is connected properly , the cable shield is left ungrounded .

The question is , Can this signal wire pick up stray noise and make the temperature raise to 106 Deg C momentarily and that resulted in a trip ? Is it necessary to ground the shiled of the signal cable ?

Can we conclude the non-earthing of the RTD cable could be the cause for the trip of transformer ? Otherwise , the electrics are perfectly ok .

Request those experienced to offer valuable feedback .

Thanks and best regards,,,,,

Are you selling something, or is this just a random outburst?

Thanks Venku about RTD Informartion...........

Wow! Thanks. That makes the whole effort worthwhile.

A point to note is that this technique is useful not only for unknown (but equal) lead resistances but also for their temperature variations. Generally copper wires are used, and copper has a positive temperature coefficient of about 0.4% per degC. Since all wires will have a higher value at a higher temperature, temperature effects are also minimized by this philosophy.

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