Latching relay or non-latching with circuit breaker for fault handling? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: February 21, 2020, 04:36:37 pm
content: 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

author: rstofer
author_rank: Super Contributor
timestamp: February 21, 2020, 06:04:24 pm
content: 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.

author: calzap
author_rank: Frequent Contributor
timestamp: February 21, 2020, 07:12:03 pm
content: 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.

author: duak
author_rank: Super Contributor
timestamp: February 21, 2020, 07:28:38 pm
content: 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.

author: calzap
author_rank: Frequent Contributor
timestamp: February 21, 2020, 10:54:01 pm
content: 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

author: rstofer
author_rank: Super Contributor
timestamp: February 22, 2020, 01:17:32 am
content: Quote from: calzap on February 21, 2020, 07:12:03 pmThen you need to look at breaker trip curves. At 3 times nameplate, it might take 25 seconds to trip. To get an instantaneous trip, you might need 5 times nameplate. These curves vary depending on the breaker and the application. I just picked one out of thin air. Don't be surprised to see much longer overcurrent trip times and much higher instantaneous pickup points. 10x is not out of the question for an instantaneous trip.https://electrical-engineering-portal.com/time-current-curvesI don't think tripping the breaker is a good idea unless it is a very small breaker and, even then, it might be hard on the breaker. Any time trip includes heating the breaker and any instantaneous trip relies on a magnetic solenoid action.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 22, 2020, 03:49:00 am
content: If a shunt trip breaker is hard to find and/or expensive, an alternative is to use a RCD/GFCI along with a small relay switching a resistor to connect one of the load side hots to line side neutral, effectively simulating a ground fault. An added bonus is that it will also trip on actual ground faults.

author: SeanB
author_rank: Super Contributor
timestamp: February 22, 2020, 08:39:37 am
content: You could actually get the RCD mechanism from Hy-Mag as a unit, with the case only containing the contacts and trip coil, with no current transformer installed. they were used for prepayment meters, to provide a low cost yet certified manual reset relay. Only drawback was that they still had the test button hole in the case, and only a label over it, so the more unscrupulous people did jam the meter by simply placing a pin through the plastic label, jamming the latch closed so they would not trip.

author: calzap
author_rank: Frequent Contributor
timestamp: February 22, 2020, 07:52:40 pm
content: I've tested Potter W28-XQ1A-0.25 thermal breakers. These are inexpensive, compact and have a 0.25 A trip. They have a spec'd resistance of 14 ohms (I measured it at 11.2 ohms). At 24 VAC and a relatively low resistance ballast resistor, the breaker resistance has a significant effect on current draw.For testing, I used a nominal 24 VAC transformer, but controlled its output to 24+/- 0.1 V with a Variac for all measurements.I used resistors of 20, 25, and 33 ohms which, when used alone, produced currents of 1.17, 0.93 and 0.72 amps, respectively ... all obeying Ohm's law within error limits. If the circuit breaker was added in series, the currents were 0.78, 0.64 and 0.53, respectively ... again following Ohm's law if a little over 11 ohms is added to each resistor's value.The trip times were 12, 16 and 31 seconds, respectively. Even 31 seconds should be OK for the types of faults that may occur and their consequences. However, I would opt for using 25 ohms. In actual use, the times would be slightly shorter because a relay coil would be connected in parallel with the resistor/sensor combo.This type of fault-handling would be used only for certain types of faults like motor too warm or pipes dripping water. Having a short persistence-time for these faults might even be desirable. Some faults, like loss of prime or motor overload, will be handled differently and will cause an instant shutdown. On inspection after such a shutdown, the cause is readily apparent.The cost of the resistor, circuit breaker, and regular relay would be about 1/3 the cost of a latching relay and reset switch (US$20-25 vs US$55-80). If price weren't a factor, the latcher would be first choice ... and I still may go that route. I may need to increase the power of the 24 VAC transformer from the original plan to accommodate the circuit breakers and ballast resistors, which would add to the cost. I need to do an energy budget for the contactor, relays, ballast resistors, sensors, breakers, etc. Good news is there are already some larger transformers sitting on the shelf.I'm not worried about wearing-out the breakers. Faults are uncommon.Thanks for all the replies. The RCD/GFCI idea is interesting. But are these available for 24 VAC?Mike in California