Originally Posted by SECS

Ok here goes....

The power input positive was protected by a series diode to prevent
power reverse polairty which in its own right works fine.
A bridge rectifier could NOT be used as this would raise the ground voltage
level with respect to the vehicle earth about 0.5 volts higher and this
would affect comms signal quality and switching reference levels up.

The problem comes when the lead connecting the tester to the car
diagnostic socket has its own ground in the lead.

So, when the tester is plugged in to the car its own earth is correct.
Then when you connect the GROUND battery lead accidently to positive,
this created a short circuit across the battery - BANG.

The unit was protected with a fusable resistor in the comms lead ground
connection which is designed to blow in such circumstances to protect
from it from fire.

Hope that explains it.