Comment by eru
10 hours ago
Alternatively, I guess you could also use really thin cables to carry the low voltage paths; and that act as fuses, if ever a lot of current at high voltage was flowing across them? But probably not very reliable both in regular operation and as fuses.
We have a Miele washing machine and a Miele dryer. Solid machines all around even after years of use.
Fusible traces aren’t uncommon - but they would effectively destroy the device if current limits are exceeded, and they certainly would be if the power supply is non-isolated, so it wouldn’t actually be a solution to the firmware update problem.
The usual solution here is an optical coupling of some kind - optocouplers (a box with a LED, photodiode, and air gap between them) are very common for this purpose, and are an inexpensive and effective option for digital signaling across an isolation boundary.
In this case they’ve basically built a custom optocoupler out of discrete components, which is a bit unusual, but makes sense when you consider the risk of corrosion due to water ingress at the diagnostic ports, and the need to access it while - for example - a dishwasher cycle is running.