Explaining Dry contacts, and normal contacts

[brother]

Ok, I thought I had this 'simple' concept down, but obviously I dont. I was trying to explain what 'dry and wet contacts' are as related to regular contacts of a relay, normally open and normally closed.

I think I got the basic relay down, but a little help in explaining the 'dry and wet' would help. I know some wet ones use mecury I believe to make the contact. correct me if im wrong please.

 

[StephenSDH]

Think of a dry contact as a light switch. A device with a dry contact gives you access to both sides of the contact. Systems that need to interface to the device might want 277v,120v,24AC,24VDC signal. Since you have access to both sides of the contact you supply the one side with the voltage you want to receive back from the other side. Same light switch you can use with either 120 or 277v.

As far as wetted goes, I'm not familar with that term.

 

[Smart $]

Ok, I thought I had this 'simple' concept down, but obviously I dont. I was trying to explain what 'dry and wet contacts' are as related to regular contacts of a relay, normally open and normally closed.

I think I got the basic relay down, but a little help in explaining the 'dry and wet' would help. I know some wet ones use mecury I believe to make the contact. correct me if im wrong please.

A dry contact is one which has no power supplied to it by the controlling equipment. In usage, the power (voltage and current) comes from another source. A wet contact is the opposite.

 

[John Valdes]

A contact is a contact. NO & NC. No more. Regardless of voltage present or not, it still does the same thing. "Dry contact" is trade/slang term, that means nothing.

 

[dbuckley]

The term 'wet' comes from the telecom folks. A dry pair is a copper pair that isn't connected to anything, a wet pair has voltage on it, which pretty much always comes from the phone exchange.

I've never seen the term "wet contacts" used, though dry contacts (or sometimes "volt free contacts") is used all the time to mean you have access to (usually) relay contacts that have nothing connected to them, so they're all yours to do with as you please.

 

[StephenSDH]

The term 'wet' comes from the telecom folks. A dry pair is a copper pair that isn't connected to anything, a wet pair has voltage on it, which pretty much always comes from the phone exchange.

I've never seen the term "wet contacts" used, though dry contacts (or sometimes "volt free contacts") is used all the time to mean you have access to (usually) relay contacts that have nothing connected to them, so they're all yours to do with as you please.

I thought it came from the telephone industry. Thanks.

 

[richxtlc]

We used the term wet and dry contacts in describing energizing a timer circuit (when testing relays) as to whether the circuit has a potential on it (wet) or we use a dry contact, no potential. So circuit require a that voltage be present to initiate a circuit, while others require just a contact closure to initiate.

 

[brentp]

What Smart $ said around here.

Brent

 

[LarryFine]

A contact is a contact. NO & NC. No more. Regardless of voltage present or not, it still does the same thing. "Dry contact" is trade/slang term, that means nothing.

No, as the others have said, a dry contact means it's only the switching done by the contacts, and does not provide a source of power. In other words, it's what you started by saying.

 

[LarryFine]

Wetted contacts have a blob of mercury on them, so they effectively open faster and suffer less pitting, because the mercury bridges the gap as they open, then they open suddenly as the surface tension is broken.

 

[ELA]

Mercury Wetted contacts are also used for the fact that they exhibit much less contact bounce than unwetted contacts. We used to use them when performing current inrush testing for that reason (prior to solid state switches).

We also used them as main contactors in traffic signal equipment. It was fun to go out in the really cold weather mornings of winter and witness the contacts fail as they "stuck on". You could hit the side of the cabinet with your hand (called -using the Fonz touch) and the contactor would then drop put as it should.

 

[GeorgeB]

I think I got the basic relay down, but a little help in explaining the 'dry and wet' would help. I know some wet ones use mercury I believe to make the contact. correct me if i'm wrong please.

You got some very good answers, and the wet vs wetted I'm accustomed to seeing with mercury used was new to me. That wet may have originally been from telephone circuits suggests their "wet cells"???

Dry, in the electronic controls side of things also has 2 meanings. In addition to the "have access to all connections, no power present", it is also used as a term for suitability for extremely low levels (current). A relay is considered "dry circuit rated" if it is suitable for switching analog voltage (commonly +/-10VDC) into high impedance (10k+) loads. These will usually have "crossbar" or knife-edge contacts and often be of palladium construction.

 

[dbuckley]

A mercury wetted relay is a very different animal; to a wet circuit.

I've not even seen a mercury wetted relay in decades...

 

[LJSMITH1]

When I was in the biz of manufacturing audio pushbutton switches for military audio headsets, we had various MIL-SPEC's to conform to. One of them was MIL-S-8805. In this specification, aside from current handling tests (i.e. 125V @ 2A) there were other electrical requirements. Our product had to be able to switch a lamp load, a straight resistive load, and an inductive load. These tests were also known as "wet tests".

However, the most difficult tests to pass were the "dry contact" tests. They typically were run with 10mA of load at 5VDC. The product was required to have no greater than a 2.1VDC voltage drop across the contacts during the cycling test. The reason why this was difficult is that the contacts and actuator would wear and become dirty after 200K or 300K cycles. If they became too dirty or didn't wipe themselves clean, they would not pass the dry contact test. Typically, if there was a dry contact test requirement, we would gold plate the contacts to reduce oxidation and improve conductivity.

If the product failed the dry contact test, it would typically still pass the wet tests without a problem. Why? Because the higher current and voltages were enough to 'push through' any minor contact resistance issues.

 

[LJSMITH1]

A mercury wetted relay is a very different animal; to a wet circuit.

I've not even seen a mercury wetted relay in decades...

You mean you haven't seen a DuraKool Mercury relay?

http://aecsensors.com/html/mercury_relays___contactors.html

These things las forever. I had put one in my Carrier AC condenser and it has been going for at least 8+ years. I used to have to replace the standard contactor every 2-3 years.

 

[iwire]

If the product failed the dry contact test, it would typically still pass the wet tests without a problem. Why? Because the higher current and voltages were enough to 'push through' any minor contact resistance issues.

Thank you, your post was the best explanation of dry contact that I have seen and it makes good sense.:smile: If it the product has states it has dry contacts I should be able to count on it to reliably switch circuits running at low voltages. A lot of fire alarm modules operate at 4 VDC :smile:

 

[ELA]

BoB,
Just because a product states that it provides "dry contacts" does not guarantee that it will will reliably pass low currents.

You always need to check the specification on a relay for "minimum load current" to be sure . As mentioned gold plated and often bifurcated contacts are used on low current relays.

Ignoring the minimum current requirement on relays is one of the most common design errors I have encountered over the years. Often times a manufacturer will not specify a minimum current so you have to check carefully.

The big problem is that a miss applied relay in a low current circuit may work for a long time before exhibiting the higher resistance that results in loss of proper conduction. Then everybody wonders why it failed. They may then just replace it and the new one also works for "a while". The design issue may never be noticed.

 

[iwire]

BoB,
Just because a product states that it provides "dry contacts" does not guarantee that it will will reliably pass low currents.

It looks to me that there are standards that apply to dry contacts and that if the contacts do not meet that requirement they are just contacts not dry contacts.

At least that is what I get from LJSMITH1's post. :smile:

 

[ELA]

I cannot give any definitive answer on any universally accepted standards involved. If some one can then I would be very happy to learn something new.

What I read from LJSMITh was that he ran what his company called dry "contact tests". I did not read where these particular tests or test levels were tied to a particular standard? If they were then the text from that standard would be greatly appreciated. It may have been specific to the audio industry loads. Note that he mentioned "typical test" values.

It would seem to me to be easy to state that the contact has no galvanic connection to a voltage supply (dry contact). It seems very difficult to assign a particular voltage level and current level that would apply in any and all "dry contact" applications.


A standard could of course be in effect for a particular industry but could never apply globally to all equipments with greatly varying load impedances.

In the case of the relay - you will eventually need to compare the dry contact specifications for voltage and current to your load to be sure you have a proper match.

 

[wptski]

You mean you haven't seen a DuraKool Mercury relay?

http://aecsensors.com/html/mercury_relays___contactors.html

These things las forever. I had put one in my Carrier AC condenser and it has been going for at least 8+ years. I used to have to replace the standard contactor every 2-3 years.

My Lennox AC compressor installed in 1981 had the original contactor till around 2006!