Types Of Relay Contacts 2

[mikemcc]

What are the differences between a (Form A), (Form B), (Form C) relay contacts?

Is there such a thing as a "wet contact" as opposed to a "dry contact"? If so, what is a "wet contact"?

 

[peebee]

a = normally open (NO)
b = normally closed (NC)
c = 1 NO & 1 NC with 1 common lead (3-wires)

dry contact = just a plain old contact, any configuration. Some other power source is required to use a dry contact. In contrast, some devices switch an internal power source and can drive a load directly with nothing more than a pair of wires required. You need to supply a separate power source with a dry contact.

wet contact -- I've never heard that term used, although it would seem to naturally follow from "dry contact".

 

[busbar]

The terms “wetting voltage” and “wetting current” are used for signaling circuits in telecomm.

 

[wbd]

i believe that the wet contact refers mercury wetted contact. These are used in applications where there are many operations in a short period of time - such as in telecommunications.

 

[dpc]

I think in this usage a dry contact would be an isolated contact with no applied voltage to it. A dry contact is specified when the voltage will be supplied remotely.

A "wet" contact would be a contact that is energized from a local voltage source. This is sometimes done when providing a contact input to a remote device that is designed to detect a voltage difference but does not provide a source of power for the contact input.

This is sometimes an issue with PLC and DCS inputs. Some types want only dry contacts and supply the interrogating voltage internally, while others need a remote voltage source.

Hope that helps.

 

[jbartos]

Suggestion: The opposite of the dry contact would be galvanically conducting contact, e.g. SCR switch, transistor switch, solid state relay, etc.
The C-form contact is sometimes referred to as the three-way switch or the transfer switch.

 

[ve7brz]

A wetted contact indeed refers to a (liquid) mercury wetted contact; primarily but not necessarily used in telecommunications. A dry contact also frequently refers to a contact in which current flow is measures in nano-amperes. So, you see, everyone is correct.

 

[ve7brz]

Just an addendum to the previous post. In classical usage, 'dry contact' refers to a reed relay with a contact rating of less than 1 mA.

 

[Marke]

Just to add a little more useless information, wetting current is the minimum current to keep the contact continuous over a long period of time. Usually specified at a particular voltage and of primary interest to electronic and signal interfaces. If a signal is passed through a closed contact with a currrent of less than the wetting current of that contact, then you can expect over a period of time, that the contact will go dry and become noisy or even go open circuit. This is due to the long term oxidation of the surfaces of the contact.
Contacts that are operating frequently usually survive due to a wiping action on the contact surfaces as they open and close.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[nickbo]

Yes, there is such thing as a "wet contact" relay. It is technically called "Mercury wetted" relay. Suggest you visit SRC devices web site for more info.

 

[PUMPDESIGNER]

I will throw in my two cents now.

A dry contact probably does not have any "official" assigned and agreed upon definition. However there is by my experience a rather simple and reliable definition that most people in my industry agree upon.

When two control systems are present, each having its own voltage and signals that must never touch each other, a dry contact allows one system to signal the other system without any direct wire connection between the two. This signal is achieved by a relay. The coil of the relay is operated by one control system, while the contacts are read by the second control system. There is no direct connection or circuit between the two systems.

If others disagree then please tell me because I have never seen anyone disagree with this definition.

Richard Neff

Richard Neff
Irrigation Craft

 

[PUMPDESIGNER]

Oops! I forgot to say why the word "dry" is used.

Dry refers to the fact that the control system operating the coil of the relay has no wires involved with the contacts of that relay, therefore the contacts are "dead" or "dry", they have no voltage on them. That way the second control system can safely connect to any of the contacts of that relay without any danger of receiving voltage from the first control system.

Richard Neff Richard Neff
Irrigation Craft

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 100-2000 "Dictionary.."
Dry contact is one through which no direct current flows

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 100-2000 "Dictionary.."
Wet contact (telephone switching system) is one through which direct current flows. Note: Term has significance because of the healing action of direct current flowing through contacts.

 

[PUMPDESIGNER]

I have something to learn here, but what?

1. Could someone explain the IEEE references to me? I do not have that document or easy access to it.

2. I am sure that jbartos knows something I do not, but could jbartos elaborate more? For instance, has jbartos
seen the concept used as I explained it? Would I be advised to stop using the term in the manner I described.

3. Also, what does jbartos and the IEEE mean by "Dry contact is one through which no direct current flows"? I can think of a few misinterpretations such as:
Does this mean low current or direct current as in the opposite of alternating current?

I hated to ask all those questions but I am determined to get this right and if need be correct my understanding.

Thank you,
Richard Neff

Richard Neff
Irrigation Craft