Surprising Philosophy
I'm a little surprised two people have mentioned switching/control on the ground circuit on the starter relay. I had assumed (incorrectly?) the other way was better.
For bus isolation, relay (contact side) control on the ground makes sense; no related circuit protection, very short/no wire runs.
- FMEA open circuit -> no difference/advantage between hot side control.
- FMEA short circuit -> bus isolation lost but it is still fail-op/ bus isn't (uncommanded) taken off-line; typically a lessor effects multiplier of the matrix.
For engine starter control, the aforementioned makes less sense to me.
- FMEA open circuit -> same as above
- FMEA short circuit -> depending on where short is: loss of switch safety to inadvertent starter engagement.
Having an uncommanded starter engage can be dangerous if the engine is not running. Having it engage while it is running is expensive at the least.
From this, contact control should be via the hot side. Am I seeing it wrong or not the whole picture?
If this is an old dead horse topic, my apologies. Intelligent responses/reasoning would be appreciated. "xxx hours so far. No problems" responses won't be helpful. This is one of those "fine until it suddenly isn't scenarios."
Anyone?