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Old January 16th, 2003, 12:43 PM   #1
Terry Woods
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"WET" Contact / "DRY" Contact

Consider the following...
(this text added by PHIL to fix formatting of main page.this text added by PHIL to fix formatting of main page.
this text added by PHIL to fix formatting of main page. Sorry Terry it was a board glitch)






On the Left, Machine-A (from the OEM) is providing power to drive a relay.
In the upper section, the relay is resident in the Customer’s Machine (Machine-B).
In the lower section, the relay is resident in the OEM’s Machine (Machine-A).

On the Right, Machine-B (from the Customer) is providing power to drive a relay.
In the upper section, the relay is resident in the OEM’s Machine (Machine-A).
In the lower section, the relay is resident in the Customer’s Machine (Machine-B).

A design spec might call for the OEM to "Provide a set of WET Contacts", or "Provide a set of DRY Contacts" for the Customer. Or, the spec might call for the Customer to “Provide a set of WET Contacts", or "Provide a set of DRY Contacts" for the OEM.

Looking back to my military days, the phrases "Feet DRY" and "Feet WET" had some significance.

To say "Feet DRY" meant that an aircraft had reached land-fall. That is, it was no longer over the "WET" part of the trip and was now over the "DRY" part.

"Feet WET" was used, on the return flight, to indicate that the aircraft was no longer over “DRY” land and was now over the "WET" part of the return trip.

The same type of terminology is used for control systems, whether PLC type or plain-old ordinary relay type. There is a difference though... the difference is in the "Relative" usage of the terms.

If we assume that the space between Machine-A and Machine-B is an ocean (or, at least, extremely wet, as I have seen in some of the worse places I've worked) then the phrases make sense.

If, as the drawing indicates, Machine-A is from an OEM and Machine-B is an existing machine at the customer site, then, we have four physical possibilities.

OEM to provide “DRY” Contact for the Customer:
The OEM provides wiring and power to drive a relay in the Customer’s Machine. The OEM gets "Feet-WET" when his wiring leaves his control cabinet. The Customer has access to a set of contacts, controlled by the OEM, while keeping his “Feet-DRY”. The customer wiring does not leave the control cabinet. The OEM might or might not actually provide the relay.

OEM to provide “WET” Contact for the Customer:
The OEM provides a relay and a set of contacts, for the Customer, in the OEM’s Machine. The Customer gets "Feet-WET" when he, the Customer, provides wiring to the set of contacts residing in the OEM’s Machine. In this case, the OEM typically provides the relay. The Customer’s wiring goes “Feet-WET” when it leaves his control cabinet.


Customer to provide “DRY” Contact for the OEM:
The Customer provides wiring and power to drive a relay in the OEM’s Machine. The Customer gets "Feet-WET" when his wiring leaves his control cabinet. The OEM has access to a set of contacts, controlled by the Customer, while keeping his “Feet-DRY”. The OEM wiring does not leave the control cabinet. The OEM typically provides the relay.

Customer to provide “WET” Contact for the OEM:
The Customer provides a relay and a set of contacts, for the OEM, in the Customer’s Machine. The OEM gets "Feet-WET" when he, the OEM, provides wiring to the set of contacts residing in the Customer’s Machine. In this case, the Customer typically provides the relay. The OEM’s wiring goes “Feet-WET” when it leaves his control cabinet.

Last edited by Phil Melore; January 16th, 2003 at 09:18 PM.
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Old January 16th, 2003, 01:47 PM   #2
Eric Nelson
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OK, Now I'm REALLY Confused...

Terry,

I agree completely with your basis for describing wet/dry contacts, but disagree to the fact that this is "standard" terminology used in the industry.

Or were you just trying to establish a standard that could be followed?...

As an OEM, I've NEVER has a spec that called for me to provide a "wet contact" for a customer. I regularly provide the connection shown in the lower section of your left drawing, but that's usually referred to as a "set of dry contacts" for customer's use. It might be to signal a box indexing conveyor to move, or enable upstream equipment, etc., etc.... I also commonly use the connection shown in the lower section of your right drawing. If the customer's machine is providing a voltage signal, I'll add the relay in the CUSTOMER'S cabinet.

I can't think of a time when I've provided a voltage signal OUT to auxiliary equipment, but even then, I don't think I'd consider this a "wet" contact. Heck, that's not even really a "contact"...

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Old January 16th, 2003, 03:30 PM   #3
Rick Densing
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The europeans like to call a "dry" contact a "potential free" contact.
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Old January 16th, 2003, 03:38 PM   #4
Terry Woods
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Eric,

As an OEM, I was called on to provide all four types at various times. I have been asked to install as many as three of the four types in the same system. (I don't recall ever doing all four types in the same system, but I have done all four types.)

All four types were identified on our "Design Specification Form". All of our machines were installed in Mid-Process positions. So there was a lot of "signaling" going on at the Input and Output ends.

In the "Design Review Meeting", when the questions got around to interfacing, we would ask, "Do you want us to provide the contact?" The customer-rep would usually say, "Sure, why not." Then we'd ask, "Wet? or Dry?" Then we would usually get the blank stare. We would hand over the form and show the different types. The blank stare would change to a frown. At that point it was useless to discuss the rest of the interface signals. "I'll have to get back to you on this. I'm gonna have to talk to my people. Can I have a copy of that to take with me?"

Sometimes it was simply a matter of preference on the part of either the OEM or the Customer. Other times, the method used was dictated by the available "real-estate". The Maintenance people also had their preferences.

Whether or not a relay actually needed to be installed, in one place or the other, was determined by the required signal, the existing (or non-existing) hardware, and, of course, the real-estate.

In any case, somebody's voltage went into the other guys cabinet. Which, of course, required the "WARNING !! - Dual Voltage Source" sign.

I can't claim it's an industry "STANDARD", but then, none of the replies to the original post identified all of the possibilities, nor did they provide a satisfactory explanation of the difference between Dry and Wet. At least, this is plausable! Regardless of which type anyone might use most often, all four cases are reasonable, reliable, and they will perform as expected.

And, having used it, I can tell you it works; there were no surprises (or, at least, damned few!).

BTW, before anyone asks, we created the "Design Spec Form" ourselves. It was created specifically for the particular machine. We had several unique forms; one for each of the different machines we built. Modifications to the forms were developed through experience. And they all had the Wet/Dry drawing.
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Old January 16th, 2003, 05:06 PM   #5
Tom Jenkins
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I think you are making this way too complicated, Terry.

First, I have never heard the feet wet and feet dry terminology, and I've kicked around a little. And the term "wet contact" is used so rarely as to hardly count - I've mostly heard it from rookies, and then only in juxtapostion to the commonly used dry contact terminology.

I would specify your four instances as follows:

Upper Left: "Machine A shall provide a 120 VAC status signal to Machine B" (Use the appropriate voltage to match the coil in Machine B - this is the coordination issue I talked about, and is why most people prefer getting a dry contact.)

Lower Left: "Machine A shall provide a dry contact status signal to Machine B"

Upper Right: "Machine B shall provide a 120 VAC status signal to Machine A"

Lower Right: "Machine B shall provide a dry contact status signal to Machine A"


This is simple, un-ambiguous, and suits the common terminology.
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Old January 16th, 2003, 06:18 PM   #6
Eric Nelson
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What piqued my interest was Terry's definition of "dry contact"...

OEM to provide “DRY” Contact for the Customer:
The OEM provides wiring and power to drive a relay in the Customer’s Machine


This is directly opposite of how I've seen it defined elsewhere...

I guess as long as I don't need to sell Terry a machine, I'll be just fine...

beerchug

-Eric
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Old January 16th, 2003, 06:35 PM   #7
Tom Jenkins
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I'm with you, Eric - and my definition is in agreement with Rick's "dry = potential free" usage.
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Old January 16th, 2003, 07:23 PM   #8
DickDV
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As far as I'm concerned, a dry contact is a contact with nothing wired to it. It is available for my system to supply whatever voltage, current, and ground reference I choose provided it is within the design capacity of the contact set. On that basis, all four contacts in the original post are dry contacts. They're just located in various different locations.

If a contact is not "dry" (I've never heard "wet" used), it means to me that it is already connected to some source of power and is likely arranged to send that power to my equipment. It becomes necessary to identify just what voltage and ground reference this power has and, further, to consider whether my equipment can handle that power.

From a design standpoint, it is almost always easier to deal with "dry" contacts then the other type, whatever they are called!
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Old January 16th, 2003, 07:44 PM   #9
rsdoran
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I guess that answers the posters question.

DRY= no voltage supplied

WET= voltage supplied

Thats what it looks like to me.

As I stated earlier I never heard the use of the term "wet contacts" but have known for years of the term I stated before of "wetting current" that is needed to keep contacts free from deposit buidup. Following that line of thought, wet must mean (at least seems to mean) "voltage/current applied across contacts".

This aint a plc specific question as much as electrical and terminology differs depending on location, background and the instructor in the course being taken. I wouldnt worry about not being familiar with any terminology that may be spoken in reference to any subject in the industrial/electrical field.
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Old January 16th, 2003, 08:00 PM   #10
Eric Nelson
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Sorry Terry, I think you're "all wet" here...

I was bored, so I searched for both "wet contact" and "dry contact" in Google.

For "dry contact", I got lots of hits referencing the "non-Terry" description, as I expected...

The search for "wet contact" turned up quite a few different definitions. HERE it's described as a NC contact, yet HERE I haven't a clue what they're talking about.

Interesting topic Terry!

beerchug

-Eric
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Old January 16th, 2003, 08:27 PM   #11
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Actually this is part of a topic I will be posting soon.

This is a chapter from a book (I didnt write it and the author will be credited) that will soon be available.
http://www.patchn.com/DIGI_4.html

I wasnt ready to do this yet but in Terry's defense (like he really needs any) the term "wet" or "wetting" is not uncommon. There are references I have heard and others I am reading.

Let me know what you think of the link.
BTW be sure to look at the section on or just after the snubbers.

NOTE: That my research has also shown a reference to mercury switches (and other liguid types) as "wetting" or "wet".
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Last edited by rsdoran; January 16th, 2003 at 08:41 PM.
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Old January 16th, 2003, 11:33 PM   #12
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I always tought the reference to dry and wet came from those mercury filled bulbs

Dry was for N.O.
Wet was for N.C.

Terry's stuff did get me tincking tough.
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Old January 17th, 2003, 07:20 AM   #13
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As I re-read Terry's original post, I think that he is backwards of how I and it appears others, interpret the term dry contact.

Here is how I would draw "dry" contact interfacing between machines.
Attached Images
File Type: jpg dry1.jpg (39.1 KB, 482 views)

Last edited by Rick Densing; January 17th, 2003 at 07:47 AM.
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Old January 17th, 2003, 09:04 AM   #14
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One more pick at the nit!

You are correct, Rick, by my lights. However, you only cover two of Terry's four conditions. And I have, like him, encountered all four conditions over the years.

There are two factors to consider when defining the status signal. The first is the device controlling the status change, and the second is the source of the applied voltage used to signal that status change.

In both of Terry's left hand examples the status change is controlled by Machine "A". In both of his left hand examples the status change is controlled by Machine "B".

In both of Terry's upper examples, the source of the voltage signaling the status change is the machine controlling the status change. In both of the lower examples the source of the voltage is the machine receiving the status change information, and the controlling machine is signalling the status change by a change of contact.

Returning to my preferred terminology:

Upper Left: "Machine A shall provide a 120 VAC status signal to Machine B" Machine A controls status, Machine A provides voltage.

Lower Left: "Machine A shall provide a dry contact status signal to Machine B" Machine A controls status, Machine B provides voltage.

Upper Right: "Machine B shall provide a 120 VAC status signal to Machine A" Machine B controls status, Machine B provides voltage.

Lower Right: "Machine B shall provide a dry contact status signal to Machine A" Machine B controls status, Machine A provides voltage.
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Old January 17th, 2003, 10:00 AM   #15
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Just to add another observation to the lot.

It seems to me that in Terry's diagram all of the contacts are dry, they have nothing connected to them yet.

Really is what it seems to come down to is who's cabinet is the Relay in, the OEM's or the customers and who is controlling the Relay, the OEM or the customer. That would all depend on what information needs to be passed.
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