Originally Posted by Omatako
That's an interesting observation.
The most common thing that will cause the diodes to pop in your alternator/regulator is if you open-circuit them. In other words, disconnect the charge line from the battery bank while the alt is spinning. This will only happen if the switch turns one circuit off before the other is turned on. The four way switches commonly fitted to boats don't do that. They always make the second circuit before breaking the first.
If the switches you get in the US are different to that then they must be custom-made for the American market just to piss you guys off. I can't recall in thirty years of boating having a four-way switch that breaks without making first and all the switches I have had have always been made in the USA and the only time I've ever blown diodes is when SWMBO has turned the switch from One past Off to get to Two.
Or else I've had 30 years of charmed boating :-)
I have seen some boats that have a bank of separate switches to switch this on and that off and then if you follow the wrong sequence you'll have problems. But a four-way? Not in my experience.
My guess is that you have such good luck because you have an isolator in the systems you've worked with. As this article states...without one, damage is a real possibility:
An optional component for installation is an automatic charging relay or battery isolator, two names for similar, solid-state devices that allow all batteries to be charged by the alternator, regardless of how the battery switch is set. Without one, only the batteries that are currently selected at the switch will be charged. If the battery selector switch is in the ALL, BOTH or 1+2 position (depending on how the switch is marked), both batteries will be charged, but if only battery ONE is selected, battery TWO wonâ€™t be charged. An ACR or battery isolator also protects the boatâ€™s alternator from possible damage if the battery switch is operated with the engine running.
Boating World: Hands-On
Off, 1, 2 Both Battery Selector Switch
The next step up is to install a “Off, 1, 2, Both” switch for the batteries. It is then up to the user to select either Off when leaving the boat, 1 for starting the engine, or for charging the engine start battery, 2 for running the domestic side, lights, instruments etc., or both to charge both batteries. Unfortunately, “both” can also be used to ensure that both sets of batteries are discharged together, thus ensuring that you cannot start the engine, a source of comfort when being blown onto a lee shore! An added “bonus” of these switches is that when the engine is running and the switch is changed from position 1 to 2, through the off position, the alternator is disconnected from the battery, thereby burning out the diodes in the alternator.
From:Sailing Boat Battery Charging
Can you read the small print under the off position on that switch? It is there for a good reason!