SLC 5/03 RTD Temp Problem

[ceilingwalker]

What about your shields? Did you leave them disconnected on both ends?

Yes sir, I had to land the shield wires at the RTD Modules. Before, I had so much noise in and around the mag panel that the shield was acting like a antenae (probably didn't spell that correctly). That still puzzles me, I haven't seen that before.

 

[Lancie1]

Maybe you need some of these (my invention to provide better cable shielding even at the terminal blocks). Collaborators wanted, any profits shared!

 

[shooter]

check if shields are connected at the sensor end if yes then leave them open at plc side otherwise connect them on bottom of cabinet.

 

[Bit_Bucket_07]

Yes sir, I had to land the shield wires at the RTD Modules. Before, I had so much noise in and around the mag panel that the shield was acting like a antenae (probably didn't spell that correctly). That still puzzles me, I haven't seen that before.

Shielded cable is nice, but it provides limited protection from noise. If your wire is exposed to a very strong EMF field, then you'll need to install it inside a steel conduit and avoid routing it in near proximity to AC conductors inside your control panel. Terminating your shield on both ends will lead to conduction of eddy currents in your shield due to different ground potentials in the different termination point locations.

 

[Lancie1]

Shielded cable is nice, but it provides limited protection from noise.

That depends on the quality of the cable. If it has an enclosed cage of foil or wire, then it is as good as steel conduit. If it has only a straight grounded wire running along with the signal conductors, then you are right.

Terminating your shield on both ends will lead to conduction of eddy currents in your shield due to different ground potentials in the different termination point locations.

Yes, that can happen if your ground system is poor and one part is really not at earth (0) volts. Use a Ufer Grounding System to avoid this problem. But even if your grounding points are both at 0 volts, tests have shown that how many ends to connect depends on the frequency of the noise in the area. In miliary bases where 400 Hz communication noise is common, the best results are found with both ends of the shield connected to ground. For 60 Hz and below, one end works best, and it is usually the end at the power source, NOT at the power-user end.

 

[Lancie1]

In my many experiences, the noise gets in at the nodes, where the cables terminate and the unshielded 1 or 2 inches are exposed to induced and transmitted noise from other circuits in the box. This happens with cables in steel conduits too, so that method does not always work. It is the breaks in the protection, whether metal conduit or shielded cables, that causes the noise problem. These unprotected areas occur at every connection point where the box or conduit fitting is not metal, or where the cable shields are not extended all the way up to the terminal screws - a difficult job and often impossible if the box is not designed to allow it.

 

[DwSoFt]

In my many experiences, the noise gets in at the nodes, where the cables terminate and the unshielded 1 or 2 inches are exposed to induced and transmitted noise from other circuits in the box. This happens with cables in steel conduits too, so that method does not always work. It is the breaks in the protection, whether metal conduit or shielded cables, that causes the noise problem. These unprotected areas occur at every connection point where the box or conduit fitting is not metal, or where the cable shields are not extended all the way up to the terminal screws - a difficult job and often impossible if the box is not designed to allow it.

unless you patent that shielding terminal block. haha