Dry Form C Contact

[eeee]

I am searching for a definition of what a dry form c contact is. I knew it at one time. I searched through the electrical inspector's handbook and an electrical wiring book on switches and searched the internet under dry form c contact and received no information.

I believe it is a definition based on the way the switch makes it's contact.

 

[petersonra]

Re: Dry Form C Contact

a "dry" contact is one that is a hard contact such as a relay or switch (as opposed to a triac or transistor).

a form c contact is also called spdt.

form a = no
form b = nc
form c = spdt

form x = no with both terminals disconnected when off

form y = nc with both terminals disconnected when off

form z = spdt but without a common

[ September 26, 2005, 11:26 AM: Message edited by: petersonra ]

 

[jim dungar]

Re: Dry Form C Contact

A dry contact is one that is not connected to any voltage source. Another term would be an isolated contact.

A standard unwired wall switch is a single pole dry contact. But if it is connected to a "hot" wire only, it is still single pole but now it is "wetted".

 

[charlie b]

Re: Dry Form C Contact

I wonder if I have missed out on something here, as this discussion differs from my (admittedly limited) experience.

I have never seen the notation of "a, b, and c contacts" used in connection with manual switches. For switches that are part of a process sensor, such as a pressure switch, or level switch, or temperature switch, I usually see the contacts described in terms like "closes on high temperature" or "opens on low pressure," and not as "a, b, or c contacts."

My experience with the "a, b, c contact" notation has been limited to relays. In that world, an "a contact" is open if the relay is de-energized, and a "b contact" is closed if the relay is de-energized. The only relay I have ever used that had "c contacts" was one built by the manufacturer to be capable of being used as an "a" or a "b," but that required the installer to physically unscrew, flip, and re-tighten the screw, in order to change it from an "a" to a "b." In other words, that particular relay did not have an SPDT contact, but rather a "wire it as you wish" contact.

 

[eeee]

Re: Dry Form C Contact

I am concerned about the terminology unwired.

The switch will not work based on my interpretation of "dry" being unwired. It must be wired I would think in order to work when the switch is manually activated by the operator?

 

[charlie b]

Re: Dry Form C Contact

Originally posted by eeee: I am concerned about the terminology unwired. The switch will not work based on my interpretation of "dry" being unwired.

You misinterpreted what Jim said. When you buy a switch at the store, and while it is still in the box, you can accurately call its contacts "dry." When you attach wires, depending on what else is on the circuit, the switch's contacts might still be called "dry." But in the specific case Jim cited, with one of the contacts wired to a live source, that switch contact is no longer called "dry," but rather is called "wetted." It's a slang term, and Jim was just giving a counter-example for the term "dry."

 

[jim dungar]

Re: Dry Form C Contact

Charlie,

Form a, b, and c contact designations are common with circuit breaker and cell interlocks. I think the only places I have seen these terms is with the mechanical "micro-switches" that are built into something else.

 

[petersonra]

Re: Dry Form C Contact

http://www.naisweb.com/e/relaye/mech_eng/mech_eng_rti/idazc4w.html

here is a picture of form a, b, and c contacts

http://www.allegromicro.com/techpub2/phoenix/relay3.htm

here is another

form x, y , and z are basically the same thing except that both terminals are disconencted when you open the contact in the case of a form x and y contact.

http://relays.tycoelectronics.com/datasheets/9400_Series.pdf

shows a form y and z contact at the bottom of page 2 of this pdf.

 

[rbalex]

Re: Dry Form C Contact

The various "forms" are defined in NEMA ICS5, Industrial Control and Systems Control-Circuit and Pilot Devices, a free copy may be downloaded from their website here

The terms are general and apply to a broad range of applications.