They have been around for more than 20 years, maybe 30.
Id feel safe sticking to 20... But the problem is if it trips a few tims the HO decides a 30A fuse is better - often at the advice of some fool... IMO S type rejectors are better, and once they are in - you're not getting it out to use this fuse/CB... Or at least without some difficulty...
I saw either this or something similar in the mid-70's, before I started doing electrical work. After that I never saw them again until around 5 years ago. I bought some to keep on truck, knew I'd wished I'd had them many times, knew I'd use them a lot while troubleshooting. I haven't needed one since.
Actually, I prefer using a medium-wattage bulb, either in the fuse-holder, or in a rubber socket in line with the fuse or breaker.
I've done that a few times too. I don't seem to be running across as many fuse boxes lately.
It works just as well with breakers. The idea is that it's easy for the eyes to detect a change in brightness, and makes looking for an intermittent short faster.
No, the bulb works as a current limiter. The breaker will never trip since the bulb's current is the max that can flow, and that's with a direct short downstream.
Anything, really. A 100w is more than enough to expose both shorts across the circuit and series intermittents, the latter requiring a load on the circuit, of course.
