Hi,
I got the impression that they have various ratings.
What you will have to do is see if there is a correlation between how the units fail and what the customers use as load.
If the units fail open then it sounds more like an overcurrent situation, but if they fail short then it sounds more like a over voltage situation, but unfortunately sometimes if they fail short then they will open anyway so it might still be hard to tell what is going on.
But in any case if the units are made of quality parts then they should not fail too soon under normal operating conditions. If the customers use a load that is too high in current or in voltage then that's going to blow the unit out. So it is one or the other.
If there is inductance involved then it could be that something else can also switch the load on and off and that could cause high spike voltages which could blow the unit. It is also necessary to use a SCR or Triac type switch if inductance is involved, not a transistor type. If there is capacitance involved then there is a turn on surge current that could be blowing the units.
So what might be nice is if you made a list of all the units that had blown in the past, and what the customers had been using for their load at the time. This would be a place to start anyway. For example, the list might look like this:
1. Unit 1 Westinghouse A342B rated 10 amps 120vac, customer load is 1/2 HP sump pump.
2. Unit 2 GE 755G rated 5 amps 120vac, customer load is 120vac 100 watt incandescent lamp fixture.
3. Unit 3 GE 755G rated 5 amps 120vac, customer load is 120vac 100 watt fluorescent lamp fixture.
4. Unit 4 GE 755G rated 5 amps 120vac, customer load is 120vac 100 watt LED lamp fixture.
Note that the customer load is detailed, and so is the make and model of the switch unit although the model numbers here are fictitious just for the purpose of illustration on the level of detail that should be incorporated. If you can get the model number of the loads used too that would be a good idea. You should also include the customer geographical location.
So the units are blowing and what we do here is see if we can find a systematic reason for this, then look into why that happens.