from the Automation List department...
Volt-free contact, dry contact, wet contact, proximity contact!!??
Application Questions and Problems topic
Posted by Dennis Koay on 23 April, 2003 - 11:39 am
Can someone help me to explain the meaning of volt-free contact, dry contact, wet contact and proximity contact?

What are the differences between all of them?

What do the symbols look like in the ISA wiring diagram?

Posted by BobB on 24 April, 2003 - 11:28 am
A volt free contact is a contact without any voltage on it. It is usually supplied in a system for other parties to use and they then supply their own voltage into their own system eg: to a BMS system.
A dry contact is just that - dry - as opposed to a wet contact that is usually something like a mercury switch or it can also have a mercury wetted contact.
A proximity contact is one I have never heard of but would assume it is meant to be a proximity switch of somw type, probably with a transistor output of the open collector variety.

Posted by MCS on 25 April, 2003 - 10:33 am
Correction - a dry contact is the synonym of volt free - means it is not "wetted" by a voltage source. Dry vs. mercury wetted is a different story (not in this context)

Meir Saggie

Posted by Arnold Dillon on 24 April, 2003 - 11:40 am
Volt-free and dry contact mean the same thing. If a control system supplier offers a dry contact for you to read as a status bit, then he is offering to close a contact (relay or contact output) that is nothing more that a stand-alone set of contacts with no voltage, current, or anything else impressed across the contact set. It becomes the user's responsibility to determine how to sense that contact closure. Usually you do this by putting a voltage on one side and sensing the voltage on a return line from the other side of the contact when it closes. Once you apply voltage to the contacts, it becomes a wet contact. You "wet" the contact with a sensible voltage level.

A proximity contact is a contact set that makes without any kind of direct coil or physical actuation of the contact set. Proximity contacts or switches are usually either optical devices that switch because a light beam is broken or completed, or in some cases magnetic switches are called proximity contacts. The contact set will open or close when a magnet gets close enough to cause the switch action like a door switch on a burglar alarm.

Posted by Peter Whalley on 27 April, 2003 - 11:30 pm
Hi All,

From memory, wetting a contact refers to the passing of a small "wetting" current through the contact that has the effect of lowering the contact resistance. It does this by micro welding the contact together. Without the wetting current the contact resistance can be quite high even though the contact is closed. This was particularly a problem in telephone exchange equipment where the relay contact only needed handled high impedance AC signals so a small DC wetting current had to be deliberately added.

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*
delete *no-spam* before sending

Posted by J.Carlisle on 24 April, 2003 - 11:28 am
Volt-free contact should be the same as a dry contact. It means a set of contacts that is mechanically operated, but has no power of their own. You must provide a power source to 'read' their condition into your down-stream device. Wet contacts have a power source included with the equipment so that you do not have to provide it. Proximity contacts are the mechanical closures from proximity switches, which use lights, magnets, or small levers to register the presence of objects that pass near to the switches. Hope this helps....JCC

Posted by Gordon John Hinds on 30 November, 2012 - 11:53 pm
Capacitive proximity switches have not been mentioned. they are true proximity switches which detect changes in the capacitance of the surrounding area.

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