Dry Contact and Wet Contact


Thread Starter

Irfan Choudary


please let me know what is difference between dry and wet contact in relay system with practical examples.
1)A dry contact is a No Volt contact where the contact is usually used to indicate an event - movement, closure, alarm, etc.

2) A wet contact is contact with power applied, usually used to energize something - solenoid, light, etc.
The C-form set of contacts depicted below is voltage-free, or "dry." They can be associated with a relay, a temperature switch, a limit switch, a pressure switch, a pushbutton, etc. The concept is that at the terminals (represented by the "0" character, there is NO voltage present.<pre>
-----| |-----0</pre>
The set of C-form contacts below is also voltage-free, or "dry." Although someone has connected a red indicating lamp to the terminals there is no voltage/current source to illuminate the lamp filament (or LED) when the NO (Normally Open) contact closes.
| \ /
|------------0-------- R ------
| / \ |
-----| |-----0-----------------</pre>
The set of C-form contacts below is also voltage-free, or "dry." However, in this case someone has provided a source of voltage current, connected to the COM (COMmon) terminal, connected one side of the red indicating lamp to the NO terminal, and connected the other side of the red indicating lamp to a Neutral, so that when the NO contact closes the red indicating lamp will illuminate. The voltage source (and Neutral) is <b><i>external</b></i> to the set of contacts--hence, the contacts are still "dry" (voltage-free).
|------------0---- 220 VAC
| \ /
-----| |-----0--------------- R -------- Neutral
/ \</pre>
In the C-form set of contacts below, an "internal" source of voltage/current is connected to the COM leg of the contact set. This is usually done internal to the control system and so the contacts have a source of power that can be used for many (but not all) applications. THIS is considered to be a "wet" set of contacts--one which does not require an external source of voltage/current for the load. One simply connects an appropriate load to the terminals and the contacts will switch an internal voltage/current source.<pre>
220 VAC------|------------0
-----| |-----0

In the drawing below, a red indicating lamp is connected to the "wet" set of C-form contacts and when the NO contact closes the lamp filament (or LED) will be illuminated. No external source of voltage/current is required--it's available at the contact terminals.<pre>
220 VAC------|------------0
| \ /
-----| |-----0----- R -----
/ \ |
The voltage/current source can be 12 VDC, 5 VDC, 24 VDC, 24 VAC, 110 VAC, 125 VDC, etc.--it all depends on the type device/output being used. The caveat is that one can't simply connect any load to a wet contact--the ability of the "contact" to supply current has to be taken into account. In other words, you can't connect a load that requires 3.2 Amps to a set of wet contacts that can only supply 1.0 Amps; well you can, but you can't expect the load to work for very long and you can expect to have to replace the contacts or the fuse (if included) in the source.

This concludes our wet-/dry contact tutorial for the remainder of all time.


the term wetted-contact originated when extremely small relay contacts were used to switch minute electrical currents. They were coated with Mercury.

Don't confuse them with the Mercury-pool contacts, that are contained in glass-vials. The mercury-pool has the mass to absorb the heat caused by large transitory currents, e.g., current-inrush.


> do you have any practical example(s) in GE gas turbine for dry contact?

Can you tell us what version of Speedtronic turbine control panel you are working with?

Dry contacts of discrete outputs of Speedtronic turbine control panels are used for starting/stopping motors; for permissives for generator control (synchronizing; voltage regulator raise/lower signals; etc.). The power sources for these circuits originate OUTSIDE the Speedtronic turbine control panel.

Wet contacts of discrete output of Speedtronic turbine control panels are used for solenoids (20CB-1; 20FD-1; 20VG-1; 20PG-1; etc.). The Speedtronic provides the 125 VDC for powering the solenoids being energized/de-energized by opening/closing the contacts of the discrete outputs. The power sources for these circuits originate INSIDE the Speedtronic turbine control panel.

Ignitor power supplies are powered by 120 or 220 VAC switched by a dry contact of the Speedtronic turbine control panel.

Mr. Corso is correct in his example of "wet" contacts being contacts which used mercury as the conducting medium. But switches using mercury are so rare in industrial settings these days that the usage of the term "wet" contacts has shifted over the decades since mercury switches were common for many applications in industrial settings. (Yes, Phil Corso, they are still commonly used in residential and commercial thermostats, but those are not technically industrial applications.)
thanks for your reply. I've worked on GE gas turbine mostly with mark5 control system but I don't have enough experience. My background is mostly in field and I asked that question because that was odd for me to think is there any contact (NC or NO) without any current in its terminals?!

However I learned that when the source of power is inside the ST that's a wet contact and when the power source is outside the ST we have a dry contact.

So can we say that all of pressure switches and thermoswitches in G heavy duty gas turbine are wet contact?

and another question: did u mean spark plug from ignitor?

Please don't think that any power that comes from the Mark V (or any Speedtronic) makes a set of contacts wet. If you disconnect any pressure switch or temperature switch from the Mark V's contact input terminal board and connect a load to one of the contacts of the switch nothing will happen. The contacts are dry--until power is applied from some external source.

Now, on the other hand look at Q_QD1_CO05, on <QD1> DTBC. (Refer to the Signal Flow Diagrams in Appendix D of the Mark V Application Manual, GEH-6195.) It has a pair of jumpers on the DTBC (P5 and M5), that when installed, will apply +65 VDC to the COM terminal of the C-form contact output and -65 VDC to the SOL terminal of the C-form contact output, making it "wet" because the voltage/current comes from the Mark V. It is a contact output <b>OF</b> the Mark V, that has power available at the terminals. So, if you connected one side of a red indicating light to terminal 17 (the SOL terminal) and the other side of the red indicating light to terminal 18 (the NO terminal) and then energized the logic signal that picks up contact output 05 the NO contacts will close and DC will flow through the red indicating light. This is a "wet" contact output of the Mark V.

The pressure switch, or temperature switch, is an external device connected to the Mark V. Power from the Mark V is applied to the terminals of the device--but just because the power comes from the Mark V DOES NOT make the contacts "wet", it simply means that when the contacts close current will flow through the contacts and the Mark V will see this as a closed contact. The contacts are NOT associated with the Mark V--they are part of the pressure switch or temperature switch--neither of which have their own power source. They are simply devices for switching an external source of power (in this case, from the Mark V) at some calibrated setpoint to provide an indication to the Mark V. They are still "dry" contacts--even if power from the Mark V is applied to them. In this case, the Mark V is the load and the load is supplying the power.

In the case of a "wet" Mark V contact output the Mark V is supplying the current to power an external load through the contacts the load is connected to. So, in addition to switching the power to the load, the Mark V is also providing the power to the load--from the same terminals doing the switching. (Okay; one additional terminal--the SOL terminal--but only because we're dealing with an ungrounded DC voltage in this case.)

You are correct--any contact in the field that's connected to the Mark V needs to be able to flow current through it, or to interrupt the flow of current though it, but that doesn't make the contact "wet." The concept of a "wet" contact output is that a load is powered from a set of contacts in the Mark V--without the need to supply an external source of power. In the case of temperature switches and pressure switches and limits switches--the load IS the Mark V, and it's supplying it's own power. In the case of a solenoid, like 20CB-1, it's separate from the Mark V, and the "wet" contact outputs of the Mark V are both providing the power to the load (the solenoid's coil) and switching the power (through NO contacts) to the load. That's what makes it a "wet" contact output.

The voltage/current used on contact inputs (pressure switches; temperature switches; limit switches; etc.) is sometimes called the "interrogation voltage" or the "wetting" voltage--but it's not the same as a contact output. It's just the power used to sense current flowing--or not flowing--through an external set of contacts. The contacts are still dry, but they are "wetted" by the interrogation voltage of the contact input circuit. It's an external voltage to the pressure switch or the temperature switch or the limit swith--even though it comes from the Mark V. Again, the Mark V is the load on the switch--sensing if there's current flowing through the contacts, or not. If you took a pressure switch off a shelf from the stores warehouse and connect a red indicating light to it and change the state of the NO contact to be closed--nothing is going to happen, unless you apply an external source of voltage/current to the circuit connected to the switch. The pressure switch has no internal source of voltage/current--it is only a switch.

When the switch also can provide voltage/current, it is considered to be "wet."

Ignitors is another term for spark plugs. The Mark V must have an external source of AC voltage connected to it to power the ignition transformer(s) used to make the ignitors (spark plugs) arc and spark. The contact output used for the ignition transformer(s) is a dedicated output, but one that requires an external source of AC to operate. Yes, the AC is internally connected to the contact output, but without that external AC source the contacts will not be capable of powering the ignitor transformer(s) (spark plugs; ignitors).

The Mark V requires a source of 125 VDC output to operate--and for heavy duty gas turbine applications it also uses the same 125 VDC source as the power source for its "wet" contact outputs. About half of the 30 contact outputs of both the DTBC and DTBD can be configured to be "wet" contact outputs--usually called "solenoid outputs." This is done by putting the Mx and Px jumpers associated with those dual outputs IN.

The other half of the contact outputs can only be "dry" contact outputs. And, they are used for starting/stopping motors, controlling the generator excitation and synchronization circuits, etc. They are just switches--the power comes from outside the Mark V. In this case, the "dry" contact outputs of the Mark V are like pressure switches or temperature switches--they have no internal source of power with which to power loads. The power has to be provided by an external source--like the control power transformer of a motor starter, or by the exciter regulator (the "AVR"), etc.

This concept of "wet" and "dry" contacts can be very confusing. Actually, it's the English language that makes it so confusing--because as a technical language it really leaves a lot to be desired. There are lots of terms for the same meaning--and that confuses a lot of non-native English speakers.

Some electricians will call a contact "wet" if it has voltage/current on the contacts--regardless of where the voltage comes from. And they will call a contact "dry" if it has no voltage/current on the contacts. This is a perversion of the terms, and, really calling energized contacts "wet" implies liquid--which as Mr. Corso has mentioned came into the vernacular in relation to contacts which had liquid mercury to carry the current between the contacts. The liquid could still be there even if there wasn't any power/current, and technically they would still be "wet" if there was no power on the switch. We're talking here mostly about slang usages of terms, and I guarantee you that if one electrician used these terms in relation to a set of contacts of a pressure switch being tested in the field he would have to explain it to another electrician or engineer or technician because most people have not encountered this particular usage.

In all my years of working on GE-design heavy duty gas turbines, and with DCS vendors and other control system manufacturers, a "wet" contact output meant a contact output which included an internal source of voltage/current to power a load. And, a "dry" contact output meant a contact output that could be used to switch an external source of power to power a load. Contacts of field devices and instruments to be connected to control systems were rarely, if ever, referred to as "wet" or "dry" because they were almost ALWAYS "dry" and had to have an external source of voltage/current connected to them. They just switched the power--which came from an external source.

About the only instance of "wet" contacts associated with a field device I can recall would be the outputs of some motor-operated valve mechanisms. They had "wet" contact outputs that could be used to drive indicating lamps, but they also had "dry" contact outputs that could be connected to the Mark V or the DCS or the PLC to provide indication of valve position (fully open; fully closed; etc.) depending on how the limit switches were adjusted. I've never encountered a pressure switch which had its own source of voltage/current, or a temperature switch, for that matter. Again--some limit switches (such as on a motor operator for a valve), but they were rarely used by the Speedtronic or the DCS or the PLC.

Hope this helps!