Failed 18V TVS overvoltage protection.

[GintsK]

I had bad case with 18V TVS diode in the center of board. It failed with shortcut between two consecutive track runs. Competition runs not training . Engine was switched off. Not stalled. After that was not started again.

Board use PTC fuse as all new boards now. Is it possible to kill somehow this diode with fuse inline?

Gints

[GintsK]

We found the reason. It was wrongly wired main safety switch. Germans in famous race factory it wired this way: it disconnected battery and ignition just from coils. But alternator gives all kind of surge to connected ECU and other electronics until engine stops...

Bad thing - other electronics survived. VEMS - not 

[z0tya]

Use 6 pin cut off switch. It has a resistor which shunt the alternator;saving diodes and ecus

[GintsK]

Yes it is clear.
But in this case it would be much better if VEMS survives. Because another electronics lived for years  in this race car. So it means they use higher protection level. e.g. STACK.

It is not clear how TVS fails in this case. If it draws too much current for cutting surge spikes why fuse didn't do their job?

[mattias]

The fuse is for amperage protection, not voltage surge protection.

[GintsK]

I suppose when TVS working hard ECU current rises rapidly. Or no?
TVS diode is even rated for 1.5kW. 1,5kW @ 18v means 83A. OK - for short time. How laggy is fuse?

Anyway how to improve protection circuit to prevent this in future?