Electrical subteam on TMU's Baja SAE car. I specced the master power cutoff, wrote the design report behind it, and took a subsystem from schematic to PCB layout in KiCad.
The cutoff
Baja rules require a master power cutoff that fully de-energizes the 12V system in an emergency. There were two reasonable ways to build it: a single high-side contactor breaking the feed at the battery, or two contactors behind a hardware interlock. The redundant version fails safer but costs weight, space and money, and all three were tight on this car. I went with the single contactor and specced a 12V SPDT part.
Sizing on inrush
The usual approach is to size a contactor on continuous current, but inrush is what actually kills them. When everything powers on at once the current spikes well above steady state for a few milliseconds, and that transient can weld the contacts shut, which leaves you with a cutoff that looks fine and does nothing. I did the load analysis, sized on inrush margin, and checked the result against the manufacturer datasheet before putting a part number forward. Then I wrote a design report with the analysis, the reasoning for that part over the alternatives, and the tradeoffs in fault tolerance, packaging and cost, and presented it to the team.
Other work on the car
I took one subsystem into KiCad and did the schematic and layout, which was my first board on this team. The rest of my time went into sensor wiring and harness crimping during assembly, on a system that was already half built before I joined.
What I would change
The calculations were the straightforward part. I underestimated packaging and serviceability, and next time I would factor those in from the start instead of working around them at the end.
