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Latching relay or non-latching with circuit breaker for fault handling?
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calzap:
The application is fault/trouble sensor-handling for a pump controller.  Example sensors are temperature sensors on the motor/pump  and water sensors under the pump and associated plumbing.  Sensors will close one set of contacts while abnormal condition exists.  Sensor contacts will automatically re-open if abnormal condition goes away.  All the control circuitry runs on 24 VAC; pump runs on 240 VAC.

What I would like is that when an abnormal condition occurs, a mechanical switch in the pump controller moves, has a visual indication that it has moved, and requires a manual reset.  The purpose of the switch will be to interrupt the 24 VAC  supply to the motor contactor coil and secondarily to illuminate a trouble LED and/or alarm.

The two ways I can think of to accomplish this is with a latching relay with magnetic hold or a circuit breaker in series with the coil of a regular relay, and the sensor in parallel with the relay coil.  When the sensor contacts close, the breaker will trip.  There will be a ballast resistor in series with the sensor to limit current, but still allow enough to trip the breaker.

The manual reset feature is very important because it will indicate to personnel that the system should be inspected before restoring it.  A pair of regular relays in latching configuration and latching electronic modules won't work because a power outage will cause them to reset.  During the outage, the abnormal condition may have resolved, and personnel would never know that it had existed.

I'm favoring the breaker plus regular relay over the latching relay because of cost.  But I'm open to ideas including alternatives to the two presented above.

Mike in California

rstofer:
Shorting something out to trip a breaker doesn't sound good.  There are circuit breakers with shunt trip and under-voltage release.  In the case of shunt trip, you apply a voltage to the control coil to trip the breaker.  In the case of under-voltage release, you open the control circuit to trip the breaker.  Of course, under-voltage release also drops out on momentary interruptions in power.

I would probably use a standard start-stop circuit but I could be talked into using shunt trip.  Start-stop will drop out on power interruptions, shunt trip will not.  Since you want a visible indication of the fault, perhaps the shunt trip is best.


calzap:
Thanks for the reply.  I'll look into shunt trip breakers.   However, the design I was contemplating would not have resulted in a short because of the ballast resistor ... over-current to cause a trip, yes ... short, no.

Mike in Calif.
duak:
How important is it to know that a fault occured and prevent potentially damaging operation?  I've worked a very little bit with industrial controls and one thing I've found is that if someone can work around an interlock, they will.  They often will also not tell anyone responsible about the fault.  If so, I would consider a latching relay that remembers the fault and changes the start or run operation to require something special, like a second start switch, pehaps even key locked.  In essence, an "are you sure?" step.  The fault relay would be reset by a keyswitch in the custody of a techical or other responsible party.

The fault relay could just prevent continuous or automatic operation but still allow an operator to test or clear the blockage by jogging the motor.
calzap:
This will be for a small operation with about 4 persons involved.  Pumps are used to move irrigation water.  Some faults, like a slow drip from pipes that sets off a water sensor, are no emergency.  Still needs fixing fairly quickly though.  If the water sensors are bypassed, then there will be no protection from a blowout  ... and, yes, blowouts have happened.  Others, like a pump motor overheating, need to be fixed ASAP.  There are redundant systems, including pumps and water sources.  If something goes wrong with one system, and the trouble isn't immediately obvious or fixable, we switch to an alternate system while we scratch our heads.

I'll think about using key-lockable reset switches though.

Mike in California
 
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