Are Shielded T-Leads Enough

I have a VFD driven motor of 125 HP that has a separate conduit run for the T-Leads but they go into the PLC thus they are 24 VDC an OEM design not mine.

I was thinking that these shielded 2 conductor cable for the T-Lead back to the control cabinet and into the PLC may be enough but it's on the same 24 VDC supply as all the other PLC I/O.

I can maybe add an interposing relay but I prefer not to if it's not necessary as they is not much free cabinet space at all from the OEM.

Bump

Or is there a such thing as a signal conditioner for a 24 volt dc digital signal?

Would that help?

You are talking about n/c temperature switch leads from a motor? Pipe them back separately for best results, spaced at least a couple of feet away from the motor leads. If you must bring them back in the same pipe with the motor leads, use an interposing relay at the PLC or relay panel to protect the input from the induced current which can sometimes just cause false inputs but can also fry stuff. You may even find that you need the interposing relay even if you follow the best wiring practices, so design one in if you have the choice.

What can happen will be that all seems to be well then one day the motor overheats and lets out the magic smoke, but the noise on the wiring makes the PLC think all is dandy until it is too late. So in testing during commissioning, open the circuit at the motor, cover the bare wires and run the motor to make sure that any noise along the wiring does not cause a false input.

Even without the VFD, I would do this, but being VFD driven, you will for sure have noise riding on any conductors running alongside those leads. One of the best learning experiences for me with noise and drives and wiring was to go probing all over a machine with poor wiring practices with a Fluke Scopemeter and just look at the junk that these drives are putting out.

We use this cable on a lot of our drives.

http://www.lappusa.com/Spec_Template.asp?nGroupID=20305 but not on the one in question.

When using VFD cable with the T-Leads it's shielded so would it still be a must have to use an interposing relay when using this type of VFD cable?