Form C Relay wiring

[PaulD]

Help me settle an argument I am having with myself. I am using Form C relays to switch 24VAC power to a load using the C (common) and NO terminals on the relay. Trying to determine is there is any good reason to hook the 24VAC to the NO terminal and the load to the C...or vise versa. I know either way will work but I am curious if there is any standard practice or good reason to do it one way vs the other. The main reason I can think of to apply the 24VAC power to the NO terminal is that if I apply the power to the C terminal, then the NC terminal is also hot when the relay is not activated.

 

[dutchyn]

I think of AC as having a hot and a neutral. I would connect hot to the common, just like electricians do with HV switches. Keep the power to as short a run as possible; when the relay is open, there's no power that can get accidentally grounded out at the load.

 

[Sandpiper]

Help me settle an argument I am having with myself. .....I know either way will work but I am curious if there is any standard practice or good reason to do it one way vs the other......

Your argument with yourself is sound. If you are just switching something on and off it makes absolutely no difference which wire goes on C versus NO. If you are switching two different loads, one on when the relay is energized, and the other on when de-energized, then it would make a difference, as you pointed out to yourself.

Sorry for the intrusion, now you can go back to arguing with yourself again.

 

[gatchel]

I also agree with the hot and neutral approach. I switch the hot and land it on the common terminal just because. With 24vac there is probably less risk due to the fact that the 24vac is usually isolated from the line voltage because it comes from a transformer winding, usually...not to say that something can't happen though.

 

[PaulD]

Thanks for feedback. Only using to switch power to a load (irrigation valves) thus the NC terminal is not connected to anything.

I also think of AC as hot & neutral but with VAC wall warts, there is no hot/neutral designation which has always confused me a bit. However, I am now using a 24VAC power supply to feed my system and it does have a hot/common designation so it lines up better with my view of AC.

I ended up with load to C and power to NO. Avoids any confusion on wether or not the NO terminal is ever hot.

 

[Sandpiper]

I also think of AC as hot & neutral but with VAC wall warts, there is no hot/neutral designation which has always confused me a bit.

Allow me to offer a quick lesson on this which may eliminate some of the confusion.

When a secondary voltage source is derived from a transformer, both leads are isolated from ground unless intentionally grounded. In other words, if you measure the voltage from either of these leads to ground, the voltage reading you get will be unreliable and variable. That is what it means to be isolated.

Once you ground one leg, as is done with 120VAC household power transformers, you can get a reliable voltage measurement from either leg to ground. One leg will be zero volts to ground, the neutral leg, and the other leg will read 120 volts to ground, the hot leg.

If you ground one leg of your 24VAC transformer, you will create hot and neutral legs in the same way. But since these are normally not grounded, there is no hot and neutral designation, both are just AC legs.

 

[PaulD]

Allow me to offer a quick lesson on this which may eliminate some of the confusion.

When a secondary voltage source is derived from a transformer, both leads are isolated from ground unless intentionally grounded. In other words, if you measure the voltage from either of these leads to ground, the voltage reading you get will be unreliable and variable. That is what it means to be isolated.

Once you ground one leg, as is done with 120VAC household power transformers, you can get a reliable voltage measurement from either leg to ground. One leg will be zero volts to ground, the neutral leg, and the other leg will read 120 volts to ground, the hot leg.

If you ground one leg of your 24VAC transformer, you will create hot and neutral legs in the same way. But since these are normally not grounded, there is no hot and neutral designation, both are just AC legs.

Thanks...it helped me get my mind wraped around one aspect of AC that has always escaped me.