# Relay or PLC



## nolabama (Oct 3, 2007)

Which one would you use to engage a contactor in the absence of power. The contactor control power will be different from the power I am monitoring. My brain quite working a couple days ago an was wondering what you guys would use. It's gonna monitor three circuits and in lose of power engage three different contactors depending on what circuit loses power.


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## Calebxx12 (Oct 11, 2010)

Use a relay, power it with the source you are monitoring and then when it goes off it will close a N.C contact and power through one side of the contact to get your signal from the relay to whatever you are using to see if it lost power or not?


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## nolabama (Oct 3, 2007)

Ok. Let me ask one more question. This device should keep that coil energized possibly years. Is this a problem. I worry about welding the contacts shut over time.


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## Calebxx12 (Oct 11, 2010)

That I am not aware of. If it us a bug concern you could add a fail safe relay, meaning 2 hooked to the same power source and the the 2 NC contacts from the 2 separate relays in series? That way if one fails then hopefully the other does the job.


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## Calebxx12 (Oct 11, 2010)

I meant in parallel. The NC contacts.


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## just the cowboy (Sep 4, 2013)

No I don't think you need to worry about welding contacts the load is only the contactor coil.


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## JRaef (Mar 23, 2009)

just the cowboy said:


> No I don't think you need to worry about welding contacts the load is only the contactor coil.


I agree, welding happens by opening and closing contacts because of the arc it creates, not by them being closed for a long time.

But keeping a coil energized for years runs a potential risk of failure due to heat in the coil causing the winding insulation to eventually break down and possibly some of the plastic parts getting warped and jamming. That's why lighting contactors are often speced out as magnetically latched, which requires two coils, one to close it, another to open it. Something like that might be a better idea on your main power contactors.


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## nolabama (Oct 3, 2007)

A latching relay?


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## gnuuser (Jan 13, 2013)

nolabama said:


> A latching relay?


here are a couple schematics of latching relays

you can also use solid state relays (one to energize the other and latch it)


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## nolabama (Oct 3, 2007)

Then no to latching relay. Normal relay. Thank you.


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## Jlarson (Jun 28, 2009)

What's the application, more detail?


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## vinister (Apr 11, 2012)

this is such a coincidence. I am dealing with this exact same issue right now. 

Mine is a safety interlock, but it is very similar to yours in function. I have 3 inputs that are constantly energized until there is a problem. If any of them drop out, then I shunt trip the corresponding breaker. 

I had it drawn up with relays and even ordered the parts. But I just changed my mind and will be using a solo PLC, which is really just a programmable relay. 

Cost is a little more for the PLC method but I feel better about it. It's all solid state outputs, so no moving parts to fail. Plus we can very easily program in a delay, and we can expand the system with more inputs/outputs very easily. We added a power indicator and shunt indicators.


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## ScooterMcGavin (Jan 24, 2011)

We have probably close to a hundred normally energized permissive relays that are energized 24/7. They are mostly Westinghouse (now cutler hamer ) AR relays. We never have any problems with them. Depending on how critical the safety functions are I would hesitate to use a plc for that application. We have had plcs fail and freeze up on us before.


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