In transfering 4-20mA sensor signals, what considerations should be taken to avoid various noise and interferences?
Thanks
Reza Hasseli

 

while transfering 4-20 ma signal you should use seperate cable for that. Do not mingle with other control cables. In my experience,4-20 Ma does not required screened cable, but for connecting field RTD and Bus communication screened cable is must.

We use, but in general practise, we are using screened cable for 4-20 Ma signal also. One end of The screened should be earthed to avoid interference. for more information you can mail me for the details

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Hi,
The Shielding of the Cable and the Grounding of the Shield are the ways to avoid noise and Interferences.

regards,
S. ELAVAZHAGAN.
Engineer - I & C.
TCE Consulting Engineers Ltd.

 

A few good "rules of thumb" are: keep the 4-20mA lines away from high power lines such as motor feeds (about 2 in per 10 amps of wire current, or
enclose wires in steel (ferrous) conduit, should be good and if you have to cross unenclosed wires do so at 90 degree angle) ; provide isolators on one end (I prefer the source end) of any line going to noisy/power handling devices such as motor drives and pumps; do not connect cable shield leads to ground at both ends of a cable, in fact unshielded cable is usually preferable to shielded cable.

In my experience, these 3 rules should keep you from most noise problems seen in industrial work.

 

Sometimes the best route is to eliminate your sources of noise. (Not always practical, and Never easy, but the best none the less.) Surge suppressors on relays, etc.

With that said... Shielded cable is the way to go. Ground the shield at one point only. Most suggest doing this where the signal is generated (i.e. the sensor), but grounding it at the receiver (i.e. PLC) is also acceptable and generally easier. DO NOT GROUND AT BOTH ENDS OR AT ANY JUNCTION IN THE CABLE!! This can cause ground loops which will make matters worse.

Finally, keep the cable away from high power AC and DC conductors such as motors, overhead lights, contactors etc. If these cables must cross, do so at right angles to each other.

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Hello
incidentally, recommend me a sites in Internet with the subject "Noise and interference elimination"

Regards
Igor