# Wet VS Dry Contact



## rrolleston

Wet contact is like a mercury switch dry contact is like a relay.


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## BBQ

rrolleston said:


> Wet contact is like a mercury switch dry contact is like a relay.


I agree, here is some back up


http://www.switches.machinedesign.com/guiEdits/Content/bdeee4/bdeee4_13.aspx


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## Jlarson

It gets used for either. More people seem to use wet and dry for the voltage/no voltage thing. 

IMO if you have a contact with mercury you should call it mercury-wetted like in BBQ's link.


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## BBQ

Jlarson said:


> It gets used for either. More people seem to use wet and dry for the voltage/no voltage thing.


I know people do use it for that but I don't think that is technically correct. 

I think people assumed that based on 'dry contact'


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## Jlarson

I don't think there really is a technically right answer. I think if you go look at the IEEE definitions they have yet another set of definitions entirely. 

Ever engineer has their own definitions too :laughing:


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## rrolleston

Thank you. That is a great read. I will be sure to read it well. Can do some amazing stuff with relays.


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## bobelectric

I believe relays were invented to transmit Morse Code signals.


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## MechanicalDVR

In general terms for what we come in contact with (alarm relays, hvac controls, relays, demux baords, and control systems) the term dry contacts implies that there is no voltage present on the contacts.


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## BBQ

MechanicalDVR said:


> In general terms for what we come in contact with (alarm relays, hvac controls, relays, demux baords, and control systems) the term dry contacts implies that there is no voltage present on the contacts.


And I think that is common trade slang and not what 'wet contact' really means.


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## MechanicalDVR

BBQ said:


> And I think that is common trade slang and not what 'wet contact' really means.


 
And that's why I only referred to "DRY". But you will see in the spec sheets from the manufacturers they will note "dry" contacts.


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## MHElectric

Good info, I always thought that a wet contact was one that had voltage running across it and a dry contact didnt have voltage on it until engaged. :thumbsup:


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## BBQ

MechanicalDVR said:


> And that's why I only referred to "DRY". But you will see in the spec sheets from the manufacturers they will note "dry" contacts.


What I am saying I think people have just assumed a dry contact is dead and a wet contact has voltage. I think that is incorrect.

In my opinion both wet and dry contacts are dead until you bring power to them.


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## MechanicalDVR

BBQ said:


> What I am saying I think people have just assumed a dry contact is dead and a wet contact has voltage. I think that is incorrect.
> 
> In my opinion both wet and dry contacts are dead until you bring power to them.


 
Can you give an example of a "dry" contact that has power before control voltage is applied?


Or a "wet" contact that doesn't have voltage at anytime?


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## Semi-Ret Electrician

MechanicalDVR said:


> Can you give an example of a "dry" contact that has power before control voltage is applied?
> 
> 
> Or a "wet" contact that doesn't have voltage at anytime?


A wet contact is usually an electronic switch like a Triac. If you measure the resistance from the input terminals (signal to the device) to the load terminals you will read resistance. Manufacturers can save a lot of money by using wet contact relays in their systems because they know all about their "relay loads". An electronic switch module (Triac), probably has "wet contacts"

A dry contact is totally isolated from the signal or coil so a lot less thought has to go into using such a relay. An ordinary sp toggle switch has "dry contacts".


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## jmsmith

From a controls standpoint, when a device calls for a "dry" contact, the control voltage is obtained from an outside source to provide isolation between devices. A "wet" contact device provides the power to the device you want to control. It is NOT isolated in this case. That is why you see a lot of specs calling for dry contacts. There are quite a few manufacturers that don't want voltage in their equipment from foreign sources. Hope this helps...


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## MechanicalDVR

Semi-Ret Electrician said:


> A wet contact is usually an electronic switch like a Triac. If you measure the resistance from the input terminals (signal to the device) to the load terminals you will read resistance. Manufacturers can save a lot of money by using wet contact relays in their systems because they know all about their "relay loads". An electronic switch module (Triac), probably has "wet contacts"
> 
> A dry contact is totally isolated from the signal or coil so a lot less thought has to go into using such a relay. An ordinary sp toggle switch has "dry contacts".


 
Good answers, too bad they don't fit the quoted questions criteria.


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## Speedskater

From my 1970 Electronics Dictionary

*dry contacts:
*Contacts which neither break nor make a circuit.

*wet contact:
*A contact through which direct current flows.


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## MechanicalDVR

Speedskater said:


> From my 1970 Electronics Dictionary
> 
> *dry contacts:*
> Contacts which neither break nor make a circuit.
> 
> *wet contact:*
> A contact through which direct current flows.


 
What could contacts be used for if not to make or break a circuit? That had to be a typo. Does that same book say that a contact is the current carrying part of a relay or switch that engages or disengages to open or close an electrical circuit?


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## jmsmith

MechanicalDVR said:


> What could contacts be used for if not to make or break a circuit? That had to be a typo. Does that same book say that a contact is the current carrying part of a relay or switch that engages or disengages to open or close an electrical circuit?


Current flows through either set DVR, but in the case of a wet contact, the source that makes the contact sources through that contact to the device you want to control. With a dry contact, the source originates from the controlled device, goes through the contact, back to the device. An example of this would be an aux contact on a motor contactor. You would source a voltage from a PLC or alarm panel, through the aux (dry) contact, back to PLC or alarm panel for a run status. Now, if you wanted to use that same aux as a wet contact, you could jumper one side of the aux to the control power in the bucket, then send the other wire to another device (let's say a remote mounted pilot lamp). Whenever the starter is pulled in, it sends power out to the lamp, indicating that the contactor has pulled in. You are right... a contact is a contact. Wet or dry just has to do with whether you are sourcing the power to another device, or just allowing another source of power to pass through it back to the device that sources the power. Hope this helps.
- Jim


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## MechanicalDVR

jmsmith said:


> Current flows through either set DVR, but in the case of a wet contact, the source that makes the contact sources through that contact to the device you want to control. With a dry contact, the source originates from the controlled device, goes through the contact, back to the device. An example of this would be an aux contact on a motor contactor. You would source a voltage from a PLC or alarm panel, through the aux (dry) contact, back to PLC or alarm panel for a run status. Now, if you wanted to use that same aux as a wet contact, you could jumper one side of the aux to the control power in the bucket, then send the other wire to another device (let's say a remote mounted pilot lamp). Whenever the starter is pulled in, it sends power out to the lamp, indicating that the contactor has pulled in. You are right... a contact is a contact. Wet or dry just has to do with whether you are sourcing the power to another device, or just allowing another source of power to pass through it back to the device that sources the power. Hope this helps.
> - Jim


 
I am very comforatable in my understanding of what wet and dry contacts are but some of these responces are killing me.


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## jmsmith

MechanicalDVR said:


> I am very comforatable in my understanding of what wet and dry contacts are but some of these responces are killing me.


Got ya... Have a good day.


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## MechanicalDVR

jmsmith said:


> Got ya... Have a good day.


 
You too. 

I'd like an example from anyone of a contact that doesn't make or break a circuit.


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## dronai

I always thought of a dry contact in having no moving parts (using silicon electronic control). A wet contact would have a solonoid type plunger.


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## Semi-Ret Electrician

MechanicalDVR said:


> In general terms for what we come in contact with (alarm relays, hvac controls, relays, demux baords, and control systems) the term dry contacts implies that there is no voltage present on the contacts.


Why have a contacts with no voltage on either of them?


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## jmsmith

Semi-Ret Electrician said:


> Why have a contacts with no voltage on either of them?


:huh: :lol:


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## Big John

Where we often use them is when describing ladder logic. Where it's possible to have totally de-energized contacts until something upstream changes state, e.g.:

_"Those contacts are dry until the 4X relay closes."_

-John


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## oldtimer

jmsmith said:


> :huh: :lol:


 I M O they are just a switch. 

There is no power on them, power has to be supplied from another source!

A p.c. board may have a set of contacts, which are activated by the board, to control another piece of equipment that has it's own power source.



I have always considered this to be a set of dry contacts.

I may not be explaining this well!


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## Semi-Ret Electrician

Big John said:


> Where we often use them is when describing ladder logic. Where it's possible to have totally de-energized contacts until something upstream changes state, e.g.:
> 
> _"Those contacts are dry until the 4X relay closes."_
> 
> -John


John, within the PLC or DCS is there not some voltage present on one of the discrete outputs which will be passed to the load when the contacts change state? This is no diferent than a cube relay with a 120V coil and several sets of "dry" contacts.

Some relay output output cards (DO) have either "dry" totally isolated terminals that are usually supplied voltage from the load requesting the response, others are supplied 120V power to the common of the PLC, depending on the configuration. 

or

Triac electronic output cards (DO) which will have a "keep alive" voltage when the output is "off" or a lower voltage when it is considered "on"


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## MechanicalDVR

dronai said:


> I always thought of a dry contact in having no moving parts (using silicon electronic control). A wet contact would have a solonoid type plunger.


 
In a word, NOPE.


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## MechanicalDVR

Semi-Ret Electrician said:


> Why have a contacts with no voltage on either of them?


 
Meaning the contacts only have the voltage on them from the circuit they are in series with.


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## MechanicalDVR

oldtimer said:


> I M O they are just a switch.
> 
> There is no power on them, power has to be supplied from another source!
> 
> A p.c. board may have a set of contacts, which are activated by the board, to control another piece of equipment that has it's own power source.
> 
> 
> 
> I have always considered this to be a set of dry contacts.
> 
> I may not be explaining this well!


 
That's exactly how I have always understood it to be.


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## Big John

_Semi-Ret_: That's usually the case. I can think of some schemes where a set of contacts would be totally 100% isolated during certain conditions, but it's true that's not as common.

I think we can make this thread rival the one on Mike Holt's about why residential is called "single phase" :lol: If you really want to know what "dry contact" means, ask the guy saying it.

-John


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## yankeewired

jmsmith said:


> From a controls standpoint, when a device calls for a "dry" contact, the control voltage is obtained from an outside source to provide isolation between devices. A "wet" contact device provides the power to the device you want to control. It is NOT isolated in this case. That is why you see a lot of specs calling for dry contacts. There are quite a few manufacturers that don't want voltage in their equipment from foreign sources. Hope this helps...


I would agree . Dry contacts have no voltage present and will operate from another source and is isolated ,wet contacts will have voltage present from the same source.


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## Semi-Ret Electrician

yankeewired said:


> I would agree . Dry contacts have no voltage present and will operate from another source and is isolated ,wet contacts will have voltage present from the same source.


As jm pointed out aux contacts are "dry" but could be fed control voltage from the same motor starter to light an indicator when a motor has started.

IMO "dry" contacts offer a designer maximum flexability, higher current capacity and easier troubleshooting than "wet". But, they cost more and usually take up twice the room on a digital output card.


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## Semi-Ret Electrician

Big John said:


> _Semi-Ret_: That's usually the case. I can think of some schemes where a set of contacts would be totally 100% isolated during certain conditions, but it's true that's not as common.
> 
> I think we can make this thread rival the one on Mike Holt's about why residential is called "single phase" :lol: If you really want to know what "dry contact" means, ask the guy saying it.
> 
> -John


You got that right John. We had about a dozen "dry" contacts in series on one application. They would all "make" for a second or two then drop out.
Turned out we had a relay race that failed about every 100 times. Fun to trouble-shoot.


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