In instrumentation and control system circumstance, the terms wet contact relay and dry contact relay are two that every instrument engineer should understand clearly. If an instrument engineer can’t figure them out then it must be a mistake! Here are explanations of wet and dry contact relay and some cases for make you easly figuring them out in designing control system.
Wet Contact Relay
In other word, wet contact relay is a contact of a relay that has volt ready. The voltage value is depend on the device which provide it. It could be 5V, 12V,24V, 48V DC or even 120/220V AC. By knowing the contact is wet then you will know what should be done to the rest.
In the real practice, if you buying any ready stock device from vendor, this wet contact relay is rare but you can request when you ordering any custome made device. Below is wet contact relay wiring.
Dry Contact Relay
Absolutely, this is the opposite of above. Dry contact relay is volt free contact or no current.
All of these explanations will be help you to figure out your case while you interface/interconnect some devices/systems. However, if you just preparing for single system and not interconnecting to other system then you will not know why understanding wet and dry contact relay is so important. Let you review the case samples below for making you images.
There is a project that construct a gas turbine generator (GTG) power plant to supply offshore platform electricity demand. To work, the company will buy a GTG from Vendor A. The company should prepare a fuel gas line complete with the safety system e.g. Shutdown Valve (SDV), Blowdown Valve (BDV) and or Pressure Transmitter (PT). The company also want to have the Supervisory Control and Data Acquisition (SCADA) System.
In this case, there are three area need to be interfaced. i.e., the GTG, the fuel gas line and the SCADA system. To make it simple, We will limit the case as follow:
Area 1: The GTG has a controller that includes wet contact relay for its on/off status.
Area 2: The fuel gas line has Pressure Transmitter for sensing the high pressure for safety.
Area 3: The SCADA consists of Programmable Logic Controller (PLC) and Human Machine Interface (HMI).
Instrument and Control Design
We are going to displaying the on/off status of GTG to the SCADA HMI and also the high pressure alarm from fuel gas line system to the SCADA HMI too.
The step one
We connect the relay in GTG Panel to the SCADA PLC’s Digital Input (DI), so that the HMI will read the status from the PLC. The contact relay is “Open” when the GTG is OFF and “Close” when the GTG is ON.
Because of the contact of the GTG’s relay is “WET”, from the figure 2. it has 220 VAC of Voltage, then it can’t be directly connected to the SCADA PLC DI, because the PLC DI has its own voltage rating. Then you need to prepare another relay to interpose (the GTG’s relay will energize the interposing relay to close the contact), as shown in figure 3.
Now by providing the interposing relay, we can work at PLC’s DI voltage rating. In other case may be you ask, why there is 220 VAC connected to the GTG’s relay 1? Yes, you have a good question! If the case changes like that then the relay becomes Dry Contact Relay guys. Moreover, the already made device usually use plug and play slot to terminate the output rather than TB 1 (Terminal Block), sure you don’t want to change and rewiring the circuit because it needs more time and the manufacture documents such as schematic diagram and manual are not fit anymore to the actual device’s wiring. Even, if you push to change the wiring then you have no documents that stand for future maintenance tracing ability.
So, let the device supplied by vendor stay as what it is and our job is to prepare interface of them to our system (in this case the SCADA system).