Today I replaced the RCD in my distribution panel, as it has grown very touchy, and would trip from even the lightest mechanical shock,like closing a cupboard door too hard. So I went and bought a replacement unit, and then took the old one apart to see why they fail after around 3 years of operation.
It has an interesting construction with the main board powered by a capacitive supply, and this probably is where the failure is, as the capacitor is only rated for 300VAC, and likely has self healed somewhat, as it is the first component and is subjected to all the spikes that come down the supply, and this will have a very low impedance. It does have some series resistance, but from the looks of the board in this area there has been a lot of heat generated in this small area over time, the 4 SMD diodes are black.
The test circuit is interesting in that it is a long actuator and a silicone rubber and carbon pad press button, and this shorts part of the power supply to ground via a separate earth pin. Interesting is the warning on the earth lead that this must be connected for the test button to work.
The main detector ( Not available on a cursory search, anybody want to find a good HML235 datasheet?) uses the outgoing mains for power ( from 2 wire springs that touch the output contacts) and will energise a trip relay to defeat the magnetic field from an embedded neodymium magnet at the rear of the coil, which holds the trip mechanism in the closed position after it has been reset. The magnet though is a very poor unit, the coating of nickel which is supposed to provide protection from oxidisation peeled off completely as I took the core off, and the magnet is corroding into magnetic dust.
However these units have no overcurrent protection, but have a very good breaking capacity of 6kA, which is fine for residential use, and the contact separation is large (>4mm), which allows them to be used as both a breaker and as a mains disconnect.
I had a choice on the new unit, buy the same CBI unit at $60, or buy a compatible Schenker at $25, that is a perfect drop in replacement. As I have bought a few of the Schenker units in the last year to replace CBI, that failed all the same way after a few years, I spent the $25. Older CBI ( or the even older Hymag and before that Heinemann and Fuchs) units seem to run on forever, the new stuff definitely has been designed with failure ( safely) in mind, as it does not last too long before you have to replace it.
There was a variant of this breaker, that is no longer on the market, which had built in MOV overvoltage protection, 275VAC on the line side and 130VAC on the neutral side, which were specially designed for providing protection if there was a line neutral reversal in the supply, or if there was a lost earth or open neutral. These would trip on this condition, and were very good, though the suppliers had a lot of complaints about them tripping in operation. Thus they were discontinued, so that they would not trip on overvoltage, but would still allow system killing voltages through.
