Relay issue

Generally speaking, the DO relay outputs of DCS are primarily driven by 24VDC, and their contact operating voltages can be 24VDC, 220VAC, etc. At least regarding the transmission distance, it seems that this has nothing to do with relays; instead, it depends on the quality of the wires and whether there will be significant voltage drops. Relays merely provide mechanical contacts.

Found it in Baidu Wenku; I hope it can help solve your problem. The difference between DC relays and AC relays: There is no such thing as a DC or AC coil per se; the coil of a relay is simply insulated copper wire wound around the core. Relays are divided into DC and AC types, and the difference lies in their cores – AC coils have short-circuit rings in their cores to ensure that the magnetic circuit does not go to zero while the current does go to zero, whereas DC coils lack such rings. Electromagnetic relays come in AC and DC types. In principle, when a DC voltage is applied across the coil, the current generated is determined by the resistance of the coil. Since the resistivity of copper is very low, in order to prevent the current from becoming too high, the coil must be made with thin wires and a large number of turns. In contrast, for alternating current coils, the magnitude of the current is determined by the reactance; therefore, such coils must be made with thick wires and a small number of turns. In an AC relay, the relay releases when the voltage reaches zero, but the relay’s short-circuiting ring allows the zero point of the alternating current to be bypassed, enabling the relay to remain closed. 1. They cannot be used interchangeably. 2. When direct current flows through the coil, there is only the resistance of the wires used to wind the relay coil; whereas when alternating current flows through the coil, in addition to the coil’s own resistance R, there is also reactance XL. As a result, for relays with the same rated voltage, the number of turns in the coil of a direct current relay is much higher than that of an alternating current relay. A 3.24V DC relay cannot engage properly when used in an AC circuit, while a 24V AC relay will get damaged when used in a DC circuit

I agree with you; that’s absolutely correct.

Additional note: This is a DI signal. If the design calls for a 220V DC relay, but in reality 220V AC is being used, can it still drive the relay contacts? The purpose of the above issues is not to switch between AC and DC, but rather for external interference to affect the operation of the relay; it is assumed that this interference comes from a power supply.

I’d like to share some of my personal experiences. The distance from DCS to MCC is approximately 500 meters. The DO channels from the DCS are connected to the MCC (with a signal source); the relay switching cabinets are located within the MCC. The OMROM relays use 24VDC coils with a power consumption of less than 1 watt. The cables have a total shielding layer of 1.5 mm2, and they run through instrument raceways to reach the MCC. It has been running for 10 years with no reports of malfunctions.