Can anyone suggest what caused my MOSFETs to fail?

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Zurn

Joined Mar 4, 2019
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Some users may recognize this circuit from this thread: three microcontrollers on separate power supplies each switching their own respective MOSFETs in order to provide redundancy when driving a single load.

At some point it stopped working - the load (just driving an LED at this point) was always on instead being being switched on by the MOSFETs. Naturally, I've been trying to figure out why.

First I tested all the MOSFETs, and only one of them passed. A MOSFET short is the best explanation, as it would provide my load with a direct path to ground. If this is the case, can anyone suggest some possible causes for the short and how to prevent it in the future?

And if not what else could it be?

Here's my schematic. The sea water/anchor/burn wire section is the load (sea water acts as a resistor).

Burn-Wire-Circuit.png
 
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