# Dry Contact to Wet Contact Conversion



## just the cowboy (Sep 4, 2013)

If it is a safety door you will need to install a separate switch and relay IMO. If you tie into the safety circuit and something goes wrong you are on the hook. Just by a standard prox or limit switch and run it to a contactor. Let the contactor run the pump, you don't say what size motor.
Wet and dry are not the way it is normally described for relays because relays can be wired for either. Proximity switch's that are built with an output off of line power is what is most commonly called wet or some types of other controls have wet contacts but most of us use dry. 
Here is a book on basic controls.


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## LGLS (Nov 10, 2007)

just the cowboy said:


> If it is a safety door you will need to install a separate switch and relay IMO. If you tie into the safety circuit and something goes wrong you are on the hook. Just by a standard prox or limit switch and run it to a contactor. Let the contactor run the pump, you don't say what size motor.
> Wet and dry are not the way it is normally described for relays because relays can be wired for either. Proximity switch's that are built with an output off of line power is what is most commonly called wet or some types of other controls have wet contacts but most of us use dry.
> Here is a book on basic controls.


WTF is a “wet contact?”


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## 460 Delta (May 9, 2018)

LGLS said:


> WTF is a “wet contact?”


Mercury switch?


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## CMP (Oct 30, 2019)

Mecury switch or contactor is what came to my mind when I heard wet contact. But it sounds as though the original poster doesn’t know what it means either. Tied into the safety circuit kind of confirms that thought.


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## MikeFL (Apr 16, 2016)

jpratik92 said:


> Hello,
> 
> I have a safety door switch with a NC contact output. I want to control the pump using this contact. But since the contact is dry how can convert it into a wet 24VDC output ? if yes what kind of relay i need to buy (please give a name).
> 
> ...


If you want 24VDC out of it you need to put 24VDC into it.

If the relay is not rated for the pump load, you're going to have to have that relay control a (bigger) relay which is rated for the pump load. It's done all the time.

In a dry contact arrangement the coil voltage and the load going through the contacts are not the same. By wet I just assume he wants the coil voltage and load voltage to be the same.


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## SWDweller (Dec 9, 2020)

"wet contact" has voltage on it.

mercury switches are pretty much outlawed where I am.

I remember in HS chemisty class my buddy Bill dropped a bottle of mercury and the 3rd floor was off limits for 2 weeks while it was cleaned up. We used to put it our hands and roll it around. Oh the good ole days.


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## splatz (May 23, 2015)

SWDweller said:


> "wet contact" has voltage on it.


👍 A device with a wet contact supplies power, a dry contact doesn't. 

I assume the pump has a starter with a 24V coil, and the OP probably wants to put the door switch in series with the coil power so that if the door open, the door switch opens, and the pump is stopped - which may work if the door switch is rated for the load. There's no details in the OP so who knows.


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## yankeejoe1141 (Jul 26, 2013)

LGLS said:


> WTF is a “wet contact?”


An excited one??


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## just the cowboy (Sep 4, 2013)

For those that want to know wet vs dry contact.
Good example it a proximity switch.
A 2 wire prox switch is a dry contact, you can put them in series just like a standard contact.
A 3 wire prox switch is a wet contact it gets power applied to it then it send voltage out when activated. These can not be hooked up in series ( They can but uses creative connection). Mostly they go into a PLC or fire there own relay.


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## macmikeman (Jan 23, 2007)

I don't use wet contacts because water and electricity don't mix.............


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## Almost Retired (Sep 14, 2021)

macmikeman said:


> I don't use wet contacts because water and electricity don't mix.............


sure it will , you just have to contain/control the water while the electricity is in it LOL


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## John Valdes (May 17, 2007)

Its a contact. NO/NC nothing more. I don't get the wet vs dry thing. You are basically just closing a switch with another source doing the closing.


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## CMP (Oct 30, 2019)

I was thinking, maybe a wet contact should have a drain wire to prevent electrons from leaking?


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## oldsparky52 (Feb 25, 2020)

Is this guy an electrician?


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## CMP (Oct 30, 2019)

A 24VDC three wire normally closed proximity switch is too long and hard to spell, so lets just call it a wet contact, good enough to get a reliable answer from the inter webs right?

Oh, and I need a relay to go with it, so I need a name and part number to go with it, it doesn’t matter what the load size or voltage is, any old relay will work for me.

Thanks In Advance….


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## frankendodge (Oct 25, 2019)

Wet contacts


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## LGLS (Nov 10, 2007)

OK it is 9:50 PM, I’ve had one natty daddy, and two bushes…

I’m getting closer to understanding with a wet contact is…

Or not caring.


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## paulengr (Oct 8, 2017)

LGLS said:


> WTF is a “wet contact?”


I think all you are trying to say is you have an NC contact but want to switch a load when the contact is off. That’s very easy to do. Just hook up the contact in series with a relay. The door switch energizes the relay when closed, de-energizes when open. On the output if you get one with Form C contacts (double throw) and wire up the load through the relay to the NC contacts it does the opposite of whatever the coil does. You will want to do this anyway. A door switch is a low power device, usually limited to say 0.5 A or less. You may need a surge suppressor on the relay as well or buy an “interface relay” which is designed for PLC outputs.

Be warned though…with “inverted” logic losing power on the control side means suddenly loads turn on.

If there is ALREADY a safety relay, these are not meant to “tap into” them There are two options. The first is that most safety relays have “monitoring” outputs. These are non-safety outputs you can use for status lights or PLC inputs which can drive a relay. You need to check the safety relay manuals to do this. Don’t try tapping into the safety circuit itself…it might work or it might not.

The second option is the door switch itself. Most are available with both NO and NC outputs. These are used two ways. Some safety relays require both types of inputs (NO and NC) and use this to defeat any attempts to “jumper” out a relay. But if the NO isn’t used you can still tap into it. Again see comments above.this is low power and you probably need a relay anyway if you are drawing significant power.

If you are just buying a door switch, simply buy the dual contact (NO and NC) version.


As to “wet” contacts these are pretty rare but here goes.



https://mercuryrelays.com/



This is a wet contact. The solenoid operates a bellows that submerges the contacts in mercury. There was at one time also a type of relay called “mercury wetted” where the contacts weren’t submerged, just had a light coating of mercury. The greenies think that a cheap door switch is going to cause the Earth to be destroyed so they have been taken off the market.

You would never use those on 24 VDC though.

Outside of some specialty contacts like the above AC contacts all oxidize. The contacts are designed in a way where they actually slightly slide as they make contact. The arc hears the surface up and the sliding action wipes the oxides off leaving fresh surface and good, low current contact. Over time the contacts are used up…hence the advantage of mercury relays when you have a huge amount of make/break going on.

With 24 VDC there are two problems. First it’s not AC and wiping just doesn’t work. Second there isn’t nearly enough power to arc and clean contacts. So for 24 VDC we need a totally different contact. All 24 VDC contacts should be gold. It doesn’t oxidize so no wiping needed.

There are “universal” contacts. These are AC contacts with a light coating of gold. So if you use them on AC the gold plating is vaporized the first couple times it gets used. You can switch DC to AC but not vice versa. This is why if you “convert” to drives or PLCs and try to reuse contactors or aux contacts you will find yourself replacing them about a year later when the contacts “fail” as they get too oxidized to work.

If you do controls of any kind it’s best to keep two octal relays with DPDT contacts with bases. Keep both 120 and 24 V coils. Get 10 A ratings with 250 VAC/DC (universal) contacts. These are cheap and interchangeable between brands. The octal relays are a bit big and clunky but easier to work with and more universal than the square socket ice cube relays or the various “micro” relays. And you can use several specialty relays from Macromatic who also private labels for almost everybody that fit in the same sockets like timers, alternating relays, voltage and current monitor relays.

For you youngsters round sockets may look really odd for relays and they are. The “octal” sockets are actually vacuum tube sockets. The design just carried over into relays. They are keyed so the relay only plugs in one way and the pins are huge so they can’t be bent or broken easily. There are 8, 11, and 14 pin sockets. All SPDT, DPDT, 3PDT, and 4PDT relays use the exact same pin out that is easy to memorize.


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