# Current Relay



## Wirenuting (Sep 12, 2010)

We have a lot of them that are tied back to the building control panel.. Siemens, and they stink, IMO. 
They look for proof of state and are adjustable. It's the building controls that kill a relay and shut the equipment down. 
I've always liked using an aux contact on the starter as proof. 
They way they are done with us, it's a cheap part that costs a lot to do very little when used as run proof only. 
Now they can be used to tell if an air handler has a belt that broke. They can be adjusted to show a minimum current. But that also means a separate program point and an extra wiring point , i.e., Run proof and low current.


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## dronai (Apr 11, 2011)

This one was stand alone. 
In your second example, if the belt broke, and the unloaded motor amps went up, I can see the OL's wouldn't trip, and this circuit would be needed.


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## McClary’s Electrical (Feb 21, 2009)

dronai said:


> This one was stand alone.
> In your second example, if the belt broke, and the unloaded motor amps went up, I can see the OL's wouldn't trip, and this circuit would be needed.


Amps would go down if the belt broke.


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## xpertpc (Oct 11, 2012)

10 hp single phase is a bit unique and conventional overloads don't always work well with them, a CT may be the most effective protection rather than serpentining thru an overload block and hope for the best of luck.

Or I may have misunderstood what you were saying -


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## hardworkingstiff (Jan 22, 2007)

The only time I've seen something like that was in a card control system for gas pumps, the current relay (that's what they called it, but it was just a CT) would send a signal to the controller to let it know when the pump was running and when it was not.


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## dronai (Apr 11, 2011)

xpertpc said:


> 10 hp single phase is a bit unique and conventional overloads don't always work well with them, a CT may be the most effective protection rather than serpentining thru an overload block and hope for the best of luck.
> 
> Or I may have misunderstood what you were saying -


Typical starters with Ol's I don't see why 2 poles going through those is any different ? This one forward and reverse​


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## xpertpc (Oct 11, 2012)

They are snaking the wiring through a pair of overloads, the schematic is not complete as what I am looking for is missing.


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## micromind (Aug 11, 2007)

I've seen this type of current switch used when the operation of a motor is critical to a process. 

Usually, the operation of a motor is determined by an auxiliary contact on the starter, but occasionally a motor will fail even though it is properly protected. Or a belt can break, drive coupling fail, etc. 

If a current switch is installed, and anything happens to the motor or the driven equipment, the current will change and the contacts will change state, even with the starter still pulled in. 

I've also seen current switches (usually called 'potential relays') used to de-energize the start winding of a single phase motor. Just about every submersible pump and hermetically sealed refrigeration compressor works this way. 

Never seen a separately installed one used as an overload though. Most starters size 5 and up use CTs on the O/Ls, but they're factory installed.


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## Big John (May 23, 2010)

I've put in current relays in addition to overloads where damage would occur to equipment if the motor output was overloaded. It wasn't an issue of motor protection, it was drive-train protection because the motor could exert more force than a mechanical link could handle.

I can't speak to "standard practice" but it is definitely best to design any protective circuit as normally-closed failed-open.


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## JRaef (Mar 23, 2009)

Big John said:


> I've put in current relays in addition to overloads where damage would occur to equipment if the motor output was overloaded. It wasn't an issue of motor protection, it was drive-train protection because the motor could exert more force than a mechanical link could handle.
> 
> I can't speak to "standard practice" but it is definitely best to design any protective circuit as normally-closed failed-open.


I agree, this is a very common use for them, sometimes referred to as an "electronic shear pin". A shear pin in the mechanical world is a piece of softer metal in a drive train that is easy to replace, designed to break first, before more critical and expensive or harder to replace components break when there is a jam or something. These devices are the electronic equivalent, designed to act much faster than an overload relay. For example, an OL relay with Class 20 heaters in it is designed to trip in 20 seconds at 600% current, which is what you get if the rotor is locked. In critical machines, 20 seconds is an eternity and the high dollar parts may have already broken when the current got to 400%. So as an example the current trip relay is set to trip at 300% in 2 seconds, long before an OL relay would even begin to react. 

The other big use is to detect a broken belt or shaft, where the current drops because the load went away and if that happens, you need to turn off other things associated with that machine. 

For the most part, all of these features can be found in newer solid state overload relays, so you only need the one device to do it all, plus more. But before those existed, this was how you did it.


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## Handasee (Dec 18, 2009)

I have seen current relays like this in old metal shredders to prevent mechanical damage to the drive train. Most were Allen Bradley. Their function and description is described here:
http://literature.rockwellautomation.com/idc/groups/literature/documents/br/809-br001_-en-p.pdf


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## dronai (Apr 11, 2011)

Handasee said:


> I have seen current relays like this in old metal shredders to prevent mechanical damage to the drive train. Most were Allen Bradley. Their function and description is described here:
> http://literature.rockwellautomation.com/idc/groups/literature/documents/br/809-br001_-en-p.pdf


 
That's exactly what it was !! Had a gear reducer


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## newbi (Dec 17, 2011)

mcclary's electrical said:


> Amps would go down if the belt broke.


Can you please explain why this is so ?


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## dronai (Apr 11, 2011)

newbi said:


> Can you please explain why this is so ?


 Unloaded motor


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