Is there a lot of interference from relays?
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Dear experts, I want to use the normally open signal from a relay to display the motor status on the DCS. How far can such a normally open signal be transmitted? Is 500 meters sufficient? Is the interference high?
Then the DO card for your DCS should be powered. It mainly depends on whether the resistance of your wires is high; if it is, the voltage drop will be significant, which will affect the signal
500 meters is definitely no problem; the voltage for DCS cards is usually 24V. I can’t remember the drive current, but it’s probably similar across different manufacturers. I’ve seen transmissions over a distance of one kilometer as well. Regarding the issue of interference, in accordance with the installation specifications for instruments, as long as shielded cables are used and the system is properly grounded (grounding carried out in compliance with the specifications), there will be no problems
There are many types of relays, and their principles vary. Relays with electromechanical components are generally prone to causing interference, whereas relays that use only semiconductor components generate very little interference. As for how far this digital signal can be transmitted, it depends on the impedance of the circuit as well as the magnitude of the driving current and voltage.
This distance issue is mainly determined by the pulling force required for the relay at the DCS DI point; if the distance is too great, the resistance becomes too high and the current insufficient, then the relay may not be able to engage； Generally speaking, if you use a 1.5 gauge wire and its quality is good, 1000 meters is no problem
Using the dry contact signal from the relay to display the motor status on the DCS is no problem; we do this often. A distance of 500 meters isn’t an issue either. The power supply for this is provided by the electrical system to the instruments, so interference should be minimal – it’s just a digital signal. It depends on which path you take.
Dry signals are basically unaffected by distance; I’m not sure about those over extremely long distances. The DI signal should not be placed near areas with strong alternating magnetic fields; proper shielding and grounding are necessary. When programming with DI, apply digital filtering to eliminate the 50HZ interference.