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