On 12.07.18 15:33, John Dammeyer wrote:
> So what happens when the equipment with the 24V supply is 30m long in
> multiple steel frames? There would be a bonding wire from frame to
> frame since you wouldn't want to bond one end to one AC ground outlet
> and the other end to a different AC ground outlet.
Rather than chain a whole lot of modules on one long ground wire, where
each can interfere with the others by imposing HF noise on the common
ground impedance, thereby coupling it into the others, I'd wire them
individually back to the power supply. ("Star Earth", as Gene so rightly
points out.)
If there is internal DC connection to the equipment case, I'd provide
isolated mounts to defeat the earth loops which would most likely
otherwise cause problems.
If there's RF EMI into the equipment from a hostile environment, then
I'd be tempted to connect the metallic case to its internal equipment's
earth via a good RF bypass capacitor, to put the circuitry's Faraday
cage at earth potential.
...
> What about if you have a vehicle instead. Might have equipment
> mounted on frames that need to be bonded together. If they run an
> independent battery pack and/or genset then the DC ground doesn't need
> to touch the frame. But what if the vehicle 12V battery which does
> have negative connected to the frame also provides some sort of
> vehicle connection. Say a radio that has a modem that connects to a
> PC.
One connection, as at the battery would be fine. It's earth loops which
best radiate EMI, proportional to the area of the loop antenna.
> Logic would dictate you want the DC ground of everything connected to
> the frame with some bond wires even if just for lightning protection.
> But now you run risk of ground loops on the 12V circuits interfering
> with the system battery pack.
I'd put gaseous arrestors on external lines as primary protection,
followed by MOVs or transorbs further inboard, with some impedance
between, to further clamp the surge not entirely swallowed by the
arrestors. But I'd try to read up on that bit first. Grounded shielding
on external wiring has to help too.
Erik
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