Grounding and shielding between isolated devices

Question

I have a battery operated micro controller with a hall sensor, mounted on a motorbike. The hall sensor has a shielded cable to counteract spark plug induced EMI. By default, the entire setup is isolated from the bike.

I am not sure what to do with battery negative / motorbike chassis / cable shield - please, give me advice regarding the following setup options:

• option 1: cable shield connected to battery negative
• option 2: cable shield connected to motorbike chassis
• option 3: cable shield connected on both, the battery and the motorbike

...which further rises the following questions:

• is connecting the battery negative to the motorbike chassis good or bad?
• presumably, when the cable shield is connected to the battery negative, I do not need a separate negative carrying wire inside the cable as the shield can be used for that purpose - is it true?

Thanks!

Answer 1

Ideally you'd like the shield to be:
1) The same voltage throughout the shield, unchanging with time
2) No current flowing on the shield.

This means that in most cases, tying the shield to two points will usually induce a current to flow on the shield, which can couple to the senor wire inside through mutual inductance. In short, usually tying shields in two places induces currents, don't do it.

is connecting the battery negative to the motorbike chassis good or bad?

As long as the shield doesn't touch anything else, this would be a good point to tie it to.

presumably, when the cable shield is connected to the battery negative, I do not need a separate negative carrying wire inside the cable as the shield can be used for that purpose - is it true?

Usually not a good idea if there are sensitive signals on the inside because current should not flow on the shield. Best to use the negative wire on the inside to carry the current.