You can claim all kinds of things in a warranty statement, but as a practical matter it is going to be hard to make some exclusions stick in court or with a customer.
Are you going to exclude everything from warranty because the customer's electrician opened up an MCC to replace a fuse?
Replacing a fuse is not working on the equipment. Trying to nit pick the verbage of the posters who are in the "not my problem" crowd, soed not reflect what the point I think most are trying to make.
I am with the "not my problem" crowd here. If Simplex installed the fire alarm system and then Joe Blow came along, purchased some duct detectors off of Ebay and installed them, do you think Simplex is going to honor any warranty?
Regarding court issues, I would expect that once someone else touches my stuff, the court would place the burden on them to prove that the equipment itself or my work was the direct cause of the issue. Normally the burden would be on me to prove that the issue was caused by circumstances beyond my control to void the warranty. So, yes, I think the court would back me up. To expect that the warranty is void just because we say so is not what I think others here meant either. I have made that statement. That said if the customer called me out because the B phase lug was burnt up because it was not tightened enough, I am probably going to fix it, because I would believe that it was bad craftmenship by us. If the bus burned up because a breaker was not properly tightened and the panel was factory assembled, then they can get the new electrician out there to fix it, or pay me up front and deal with the factory. It is their equipment and they had someone else work on it. I no klonger feel obligated to spend the time and deal with the factory for the repair.
Lets face it, if we use this argument it is protectionism. I don't want someone else to go in to a building I am working on just becase they were $3 cheaper. So I will use any legal means I have to prevent it. I think you would be a fool not to.
I have used this argument many times. It has never failed me yet. I have not had to go to court over it, but the customers and GC's I have dealt with accepted the premise.