Explaining Dry contacts, and normal contacts

[quogueelectric]

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.

This thread brings flashbacks of me explaining to EES "I am begging you to give me 5 minutes to reverse the logic on your contacts to make them work" When you have fixed the logic in 5 minutes instead of 6 men 2 days ripping out all controls and rewiring everything. Somehow he was still mad at me when I saved him thousands of dollars.

 

[LarryFine]

And yours reminds me of the time, when I was but a wee helper, and we were wiring a medical building. We were putting a thermostatic switch on the far end of the main hot water line for controlling a circulating pump. There was a return pipe back to the water heater.

The mechanic I was under connected the control wires to the common and NO contact instead of the NC contact. I tried to explain that the water was supposed to circulate when the water was NOT hot, and should be running now, since the water heater wasn't on yet.

He refused to listen, left it his way, and we had to go back after the building opened to change the connection. He just couldn't let his helper be right, but the boss caught on that I was right, and gave me the 'tricky' stuff after that.

 

[quogueelectric]

And yours reminds me of the time, when I was but a wee helper, and we were wiring a medical building. We were putting a thermostatic switch on the far end of the main hot water line for controlling a circulating pump. There was a return pipe back to the water heater.

The mechanic I was under connected the control wires to the common and NO contact instead of the NC contact. I tried to explain that the water was supposed to circulate when the water was NOT hot, and should be running now, since the water heater wasn't on yet.

He refused to listen, left it his way, and we had to go back after the building opened to change the connection. He just couldn't let his helper be right, but the boss caught on that I was right, and gave me the 'tricky' stuff after that.

They walk away muttering if it is in the box off the shelf yadda yadda yadda. They dont understand that one simple actuator reverses the logic. On off on off I feel your pain!!

 

[hillbilly]

Some Info..

http://www.control.com/1026171624/index_html

steve

 

[dbuckley]

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:

Although LJSMITH1 may have that definition is his industrial world, it is far from universal; I've never seen those definitions and I've been around the block with stuff with wires on.

However the low current and voltage handling is really important if you are switching those kinds of loads. Check out the K&N cam switches; they have switches that are rated for many amps, and others designed for low voltage and/or low current use. Handily, they look the same from the outside so the panel doesn't have mis-matched switches on them...

To re-iterate - dry contacts are just contacts that you can do what you like with, and the term is interchangable with volt-free contacts; there is no "wet" opposite.

Many fire control panels have both dry contacts that you can connect to whatever you like, and other outputs that have specific purposes like sounder or strobes. Ever seen that latter group referred to as wet outputs in the documentation? I didnt think so...

 

[LJSMITH1]

To address some of the questions raised, here are some applicable excerpts from MIL-S-8805. This specification is the "gold standard" for any snap action switch. Of course, most commercial versions from folks like MicroSwitch, Cherry, or Honeywell, have an industrialized spec., but this is usually a watered down version of the MIL-Spec. Hopefully, it will answer most of your questions.

6.1.2 Control of electronic logic circuits. Contact bounce and transfer time of most sensitive, snap action basic switches is of concern when used to control devices having very fast response time. In the
case of a double throw, break-before-make, snap action switch, there is no circuit through either contact during the transfer (snap over) time. Thus, there will be a definite interval between the time one circuit opens and the other closes. Associated with circuit closure is contact bounce. When the moving contact strikes the stationary contact, the kinetic energy is converted to heat and potential energy in the form of deformation of the contacts. As a result of elastic deformation, the moving contact rebounds from the stationary contact, the contact pair being reclosed by the contact force. This can occur one or more times until bouncing ceases and the contact system reaches static equilibrium. In general, the combined transfer time and bounce time will usually not exceed five milliseconds and cause no problems on slow responding devices. On fast responding electronic logic circuits, each contact bounce may be erroneously interpreted by the circuit as a separate signal causing a false output. To compensate for the contact bounce of the switch, a ten millisecond buffering circuit is usually adequate to
prevent false outputs of the logic circuit.

The environmental conditions in which a switch must control a logic load level or low level electrical load may directly affect its performance in the application. Under these electrical load conditions, there is no arc present to remove contamination from the contact interfaces which prevent proper contact closure.

The probability of proper contact closure under nonarcing conditions is inversely proportional to the contamination in the environment. The detrimental effects of environmental contamination can be significantly reduced by one or more of the following:

a) Enclosure: By enclosing the contacts in a sealed enclosure, the contacts are protected from the environment.

b) Contact material: The use of gold at the contact interface will prevent the formation of sulfides and oxides.

c) Contact configuration: There are two basic contact concepts specifically suited for nonarcing electrical loads:

(1) Multiple-point contact designs to provide redundancy such as bifurcated contacts.

(2) Point-to-plane or point-to-point contact designs such as wedge-shaped contacts, crossed-cylinder contacts, or crossed-prism contacts.

Here is the Actual test for Low Level circuits also known as "Dry Contact":

4.7.33 Low level circuit (when specified, see 3.1 and 6.2) (see 3.37). Switches shall be tested for the number of cycles specified (see 3.1 and 6.2), as follows:

a) Contact load: Each switch contact shall make, carry, and break a resistive load of 10 milliamperes maximum at an open circuit voltage of 30 millivolts maximum dc or peak ac. Both normally open and normally closed contacts shall be loaded. Contacts shall be connected to individual loads.

b) Operate cycles: Rate not to exceed 60 cycles per minute with nominally equal "on" and "off" times.

c) Monitoring circuit: The monitoring equipment shall provide a record of the number of cycles and shall record failures or discontinue the test if a failure occurs. During each closure, the contact potential shall be monitored for at least 50 percent of the time the contacts are closed.

d) Operating temperatures:
(1) Twenty-five percent of the test cycles at the minimum ambient air temperature as identified by the temperature characteristic (see 3.1 and 6.2).

(2) Twenty-five percent of the test cycles at room ambient air temperature.

(3) Fifty percent of the cycles at the maximum ambient air temperature as identified by the temperature characteristic (see 3.1 and 6.2)

Electrical logic circuit (5.0 V dc, .010 ampere):

Applicable when specified (see table II): 25,000 cycles at an actuation rate of 120 cycles per minute maximum with no "stick", or "misses", allowed when tested in accordance with EIA RS448, method 17 as follows:

Test condition:
Each switch contact shall be tested using a 5.0 +/-0.5 V dc, 10 +/-1 mA resistive load. During each closure of the contacts, the voltage drop across the switch terminals shall be monitored for a duration of no less than 50 percent of each contact static closure. The switch contacts need not be monitored until 10 milliseconds after the initial contact closure to exclude any contact bounce. During each opening of the contacts, the voltage drop across the switch terminals shall be monitored for a duration of no less thn 50percent of each contact opening.

A voltage of 2.1 volts or greater across the switch terminals shall constitute a contact "miss" (failure to properly close the circuit). A voltage drop of less than 90 percent of the open-circuit voltage shall constitute a contact "stick" (failure to properly open the circuit).

The monitoring device shall either record the number of contact closures at which "sticks" and/or "misses" occur, or discontinue the test when "sticks" and/or "misses" occur.

For us, the Dry Contact test was the most difficult test to pass. Our actuator design created a bit more dust and contamination than other designs, which made it tough for the contacts to keep clean after 200K+ cycles..

 

[LJSMITH1]

Although LJSMITH1 may have that definition is his industrial world, it is far from universal; I've never seen those definitions and I've been around the block with stuff with wires on.

However the low current and voltage handling is really important if you are switching those kinds of loads. Check out the K&N cam switches; they have switches that are rated for many amps, and others designed for low voltage and/or low current use. Handily, they look the same from the outside so the panel doesn't have mis-matched switches on them...

To re-iterate - dry contacts are just contacts that you can do what you like with, and the term is interchangeable with volt-free contacts; there is no "wet" opposite.

Many fire control panels have both dry contacts that you can connect to whatever you like, and other outputs that have specific purposes like sounder or strobes. Ever seen that latter group referred to as wet outputs in the documentation? I didn't think so...

FYI...Contacts which are rated for low level circuits are also designed to handle the standard current loading specs of the switch. However, if you operate a low level switch with standard loads, and then change to a low level switching application, they will most likely not be reliable. Why? Because the gold or other high conductivity plating will disappear instantly when the first small arc forms under standard loading. This will expose the silver, copper, or palladium under contact material which will not work well with low level (dry) contact applications.

 

[LJSMITH1]

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

My Carrier scroll compressor had a bit higher start load Amps than I think the standard contactor could handle reliably. The contacts were always getting burned to a point where they would sometimes drop a phase into the compressor. After replacing with the same brand relay a few times, I decided to try a Durakool. It worked well, right up to 2007 when my compressor siezed.