I've been working with some pid controllers and electronic timers and was wondering if there is any standard to the state of relay contacts with the device powered on versus a non power condition.

More specifically I have one controller where the Form C, NO contacts and the NC contacts are in that specified position when the device has no power. When powered on those contacts change position and the NO is now closed and the NC is open. Then when setting an alarm condition, and that condition is met, they reverse back to their documented state(which in my view is contrary to their stated status).

Adding to my confusion is that I have another controller with only a Form A relay which is NO at both powered and non powered state. It only changes to a closed position when a set condition is met(alarm type condition).

Does one being a Form A versus Form C relay make a difference? Short of testing every component can you ever know if the contacts are open or closed at power on?

 

I'm not certain you are talking about system power on and off or weather the relay coil is "energized" or not. In any case here are the details of Form A and Form C.

http://www.ni.com/product-documentation/4782/en/

 

I understand Form A, Form C, etc. relay contacts. My question is when the relay contacts are labeled NO and NC as in the case of a Form C relay, why does there not appear to be a standard as to whether this state is in the powered off or powered on status.

I can understand a simple device like a tube relay changing the contact state when powered on, but with an intelligent device like a pid controller, these contacts are used for specific preset conditions and a simple power on of the device doesn't meet those conditions.

 

I've always believed that for a Form C relay that..
C-NC continuity is "good" state (no alarms or whatever)
C-NO continuity is "bad" state (alarm state)

Just like a circuit.. A closed (normally closed) circuit is "good" and functioning..
An open(normally open) circuit is "bad" and not functioning..

BUT.. just because a relays coil is energized doesn't mean thats the "bad" or "good" state.. Just that its energized.
Some alarm circuits may indicate C-NO continuity when the relay is energized
Some may indicate C-NO continuity when the relay is de-energized (sometimes called a "fail safe" setup in that if power is lost or a circuit problem that the relay would "fail" into the de-energized C-NO alarm state.).
I use both in products I design.. BUT I always use C-NC as good and C-NO as bad as far as the customer/end user connection goes.. They have no idea if my relays are energized or not.

 

Thank you for the response. I guess in my usage it's not a question about designing a relay into a circuit, but using an intelligent device with a built in relay that can be programmed or set for various conditions. These pid controllers are being used to manage to a desired temperature and the internal alarm relay output function is used to monitor for an over temperature condition.

So in my thinking, which maybe is contrary to normal standalone relay process, is that an internal relay being used to activate an alarm should be in the NO position(no continuity), and only change to a closed position to activate an alarm when an error condition is met. The confusion on my part is that different model controllers and some timers I've worked with handle this process completely opposite from one to the next.

 

A relay can have multiple sets of contacts. Contacts come in two flavors, NO and NC. The normal condition is when the coil is not energized. When the coils is energized the NO contacts close and the NC contacts open. Are you telling me you have relays whose contacts do the opposite?