Dry contacts

Wild Bill

Despite my numerous years in the business, I am embarrassed to say that I am not clear on the meaning of "dry contactS", would someone please enlighten me...

Thank you,
Bill H.

Maine doug_2

When you

try to take them off your eyeballs at the end of the day and you rip out your eyes, you will know the meaning.

Einsiedler_2

dry contact.

a set of contacts that close w/o power flowing thru them.

for instantance;
a SPDT relay if powered, it will close its normally open contacts & open its normally close contacts.
if you DON NOT run any power thru these contacts they are considered to be dry contacts. (you get continuity, but no voltage)

hope this helps.

Ken_8

Huh?

It does not help, since your definition is wrong.

Dry (a/k/a "cold")contacts are "spare" contacts that are not directly part of the load switching aspect(s) of a specific purpose relay or contactor. Dry contacts are "slaved" to the actual "working" contacts and are typically used to make or break ancillary circuits that are not directly involved in the primary relay purpose - but used to switch another device simultaneously.

The Honeywell 832 relay would be a case in point. One set of contacts is powered to turn on and off a load. The 'X' contacts are dry (cold) and are in and of themselves unpowered from within - but require power from another source to make and break the ancillary load.

They are NOT unpowered! They ARE powered. Just not from the same source.

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Mike T., Swampeast MO

Think of dry contacts as an ordinary two-way wall switch. (Remember that both the load and the switch each count as a "way").

The power comes from OUTSIDE, e.g. the breaker box. When you make the contact, you can take some action like turning on a light.

The key is that dry contacts can only "make or break" a connection between an outside source of power and an outside load. If DC, you generally switch the positive; if A/C you generally switch the "hot".

Note that like all contacts, they will have load ratings--generally quite low. If you need to switch more current you use the dry contacts to energize the coil of a relay with more highly rated contacts.

Einsiedler_2

tell me then

why does a tekmar have boiler contacts that are considered dry and only close a switch (like the UNPOWERED leg of a relay).

A light switch is considered dry contact.. unless you run power thru it.

I could hook up a light switch to the TT terms on the boiler and when i flick the switch, the boiler will fire. And I didn't send power to it.

so dry contacts can be used to send power, but are unpowered as per sai.

Mike T., Swampeast MO

WATCH OUT!!!

What I described in the previous message is ONLY when the device is providing dry contacts to operate OUTSIDE devices. This allows them to control things of unknown load, voltage, etc. without being harmed if that device should have a problem. (Well you can always burn out the contacts if you try to pass too much current.)

Things that commonly provide this sort of contact are outdoor reset controls, alarm systems, telephone systems, etc. The DEVICE WITH THE CONTACTS IS PERFORMING THE CONTROL OF THOSE CONTACTS!!!

There is another case.

If the device with the contacts is BEING CONTROLLED FROM SOME OUTSIDE SOURCE the case is different!!!!!

I (and others) are guilty of calling these "dry contacts" when in fact these are contacts that want a "DRY CONTACT CLOSURE"!!! In other words, to control the device with the contacts you merely provide a path across the contacts--usually via the contacts of a relay. DO NOT APPLY POWER TO SUCH CONTACTS, ONLY CONTINUITY!!!!!

An example is the Vitodens' "External Call For Heat". The contacts provided should merely be connected together via DRY contacts of a relay, thermostat, etc. DO NOT APPLY EXTERNAL POWER!!!!

Again, if the device with the "dry contacts" is CONTROLLING something external, you feed EXTERNAL power through those contacts. If the device with the contacts is BEING CONTROLLED from an external source, you only need to provide continuity--NEVER power!

While I can't think of anything right now, I'm certain that I've used devices that take control action based on EXTERNAL power. In this instance, the "type" of power will be clearly stated in the instructions! If you see "dry" in the instruction you know what to do; otherwise you will be told whether it's AC or DC, what voltage and the current draw.

Robert O'Connor_7

GreaT GOoGlA MOoglA.....Dry Contacts..,

Try "RESTASIS" There really is no cure...Robert O'Connor/NJ

bob_25

Dry

I think I pissed myself. LOL bob

Alex Giacomuzzi

Dry Contacts

Ken has captured and defined a textbook definition for "dry contacts". Nice job. Mike has a good handle on this also and I just wanted to add a few points.

These contacts are termed dry (because they have no voltage attached to them initially) and as a result they can be used in any voltage circuit for which they are rated for be it only a 5 VDC or up to 460 VAC or higher. They can carry milliamps or thousands of amps depending on their rating (both voltage & current).

A question was asked about a Tekmar with dry contacts. These Tekmar dry contacts are used to either switch a 24 VAC boiler start circuit or a 120 VAC circulator pump circuit. A dry control contact switching an outside voltage. In both cases, an outside voltage get applied to the dry contacts to provide a desired control or power function.

Regarding the question of a "light switch hooked up to TT terms on the boiler. When I flick the switch, boiler turns on and I did't send power to it." In fact the boiler will turn on, BUT the switch did in fact switch power and allow current to flow to a relay within the the boiler circuitry. The switch by itself represents a manually switched (versus an automated one like a thermostat) dry contact that was wired into a circuit (24VAC probably) which picked up a relay closing another contact to provide a boiler start.

The real purpose of "dry contacts" is normally to either isolate two voltages and their control functions 24VAC (burner circuitry) and 120 VAC (circulator pump) or to amplify the ability to switch higher currents with a smaller current. An interposing relay for example.

In all cases ---- some voltage always gets applied across those "dry contacts" which are use in a circuit sometimes from an external voltage source, sometimes from a voltage source contrained within the controls themselves. The confusion comes around understanding what constitutes external voltage.

In the case of dry contacts, everything is an external voltage regardless of where it comes from. Dry refers to no voltage connected to the contact before it is wired into its desired purpose or function. You can use these dry contacts safely in any voltage circuitry for which they are rated.

soot_seeker_2

Dry Contacts

Hmmm,

the term 'dry contacts' originally simply meant: NOT a 'mercury based wet contact' [like the indestructible & far superior glass hydrogen encapsulated switch used inside your honeywell T87 round thermostat.] 'wet contact' or mercury based switches handle a lot more power than a 'dry contact' switch and can last longer than 3 generations of your family.

'dry contacts' (now-a-days) may also imply that no power exists on the contacts - that they solely provide a hard closure to an external circuit. the same would be true (no power) if the contacts were mercury based or 'wet contacts' in this application - but the term 'wet contacts' seems to throw everybody off outside of the guys that work in the switch & relay industry.

since there is no mercury 'blob' or 'wetted' contacts in a 'dry' switch, the term is supposed to indicate that there are no orientation requirements for the switch to work properly. -(mercury based switches & relays have to be oriented in a certain fashion - a honeywell T87 has to be mounted horizontally on the wall to function & be accurate -although that may seem obvious - you can't tilt it more than 10-15 degrees either way.)

the term 'dry contact' is not in any way defined by the electrical application in which it is used.

does any of THAT help??

soot_seeker

Ken_8

All switches are

unpowered until two things happen:

1) they close

2) a load is applied.

It is assumed that any switch being used in some circuit has power potential available. Otherwise, it would not be a switch (whether it be an operating switch or a dry (or cold) switch.

The closest meaning, and simplest word that most accurately defines "dry" contacts (a/k/a "cold") is "ancilliary," i.e., "spare."

"why does a tekmar have boiler contacts that are considered dry and only close a switch (like the UNPOWERED leg of a relay)." Because the switch is "ancilliary" and does not derive the power diectly from the Tekmar unit.

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don_58

meaning has changed...

over time, yes it refers to "mercury wetted contacts",

"wetting voltage/current", etc.

tech writers have bastardized the historical reference.

the guys have given good explanations.

today, the reference is to more or less powered or

unpowered contacts.

here's the example from what Ken was saying;

R832A terminal 3 and 4 would be reffered to as ( potential )

"wetted" contacts, the X terminals would be "dry".

R845 terminals 3, 4, 5, and 6 would be "dry".

how they actually got "mercury wetted" and "potential

(voltage) wetted" confused over the years is something

lost to history. ( there still is a difference )

here's an example of what Mike was talking about,
logic wise.

hope this helps

Ron Schroeder_2

Over the years, the meaning has changed but today "dry" contacts (almost) always means just the contacts.....power or voltage for the contacts comes from outside of the relay or switch.

Usually, but not always, "dry" contacts also means low power, i.e. load no biger than a relay coil or indicator lamp.

Ron

jerry scharf_2

My $.02

As noted, there are a number of meanings to "dry contact." It originated with air gap mechanical contactors. It now generally means an electrical switch that has no applied voltage or current.

It might be helpful to look at a couple things that are not dry contacts to give some perspective. If there was some coupling between the coil and one of the contacts of a relay, that would not qualify. If someone decided that one side of the coil was "ground" and tied one of the relay contacts to that same ground, then you have a problem.

A case I run into more is the use of CMOS switches at low voltage. The impedance spread and current capacity of these is not the same as the contacts of a relay and are not dry contacts. I would assume that any solid state relay could be interpreted to fail someone's dry contact test, but it's a more subtle thing related to the exact test.

hope that helps,
jerry