Regarding the issue of DI signal isolation for electrical signals
Thread Content
In the design of DCS systems, relay isolation is typically used for dry contact DI signals coming from high-voltage electrical areas; we do the same here. However, there is one issue that has puzzled us: even with relay isolation, the DI circuits coming from electrical systems still require power supply, and this power comes from the DCS itself. As a result, complete isolation is not achieved. We would like to know how other companies handle this issue. Thank you.
That’s indeed the case. If the electrical signals are connected incorrectly, for example if a dry contact signal is connected to a high-voltage signal, it can lead to a conflict between the high-voltage signal and the power supply of the DCS system; this may even result in serious accidents such as damage to components due to issues with the DCS system’s power supply! It is recommended that the power supply for the DI system be separate and equipped with isolation, so as not to affect the DCS system.
Place the isolation intermediate relay in the electrical distribution cabinet; the power for the coil of this intermediate relay is supplied by the power source in the electrical distribution cabinet. To connect to the DCS, use the NC or NO dry contacts of the intermediate relay to interface with the DI card of the DCS
On the 3rd floor, if this is done, the dry contact signals and electrical signals may be connected incorrectly, resulting in a short circuit between power sources. Even with circuit breakers for protection, it could still cause harm to people; therefore, I still recommend using a DCS power supply. In the worst case, it can be replaced if it breaks down, which is better than putting people at risk.
You’re referring to the power supply required for things like current transmitters, right? Here, the electrical cabinet obtains the drive power directly.