On 21/07/2019 23:39, Theo wrote:
Suppose I have a variety of equipment which is variously powered from one
of the phases of a three phase supply. I'm concered about the potential
difference between two boxes on different phases. The boxes have low
voltage electrical connections between them (similar to USB ports).
Each box has a PC-style switching PSU. Leakage from the mains side into the
DC side could cause a high potential difference between boxes on different
phases, which would be enough to zap sensitive inputs on the
interconnections.

I would have thought any decent SMPSU would have good isolation between
its mains and ELV sides anyway.

Suppose I connect the ground of the low voltage side of the PSU to that
box's chassis,

Depends a bit on what you mean the "low voltage side" of the PSU? Its
casework etc I would already expect to be connected to the chassis
electrically. The "ground" / "-ve" etc of the output would not
necessarily be connected directly to mains earth.

and then bond all the chassis together and join that to
earth. Will that be sufficient to prevent any potential difference between
phases arising that could cause trouble with sensitive inputs?
Are there any issues this might cause (such as EMI, or tripping the RCD)?

It might also be sufficient to introduce lots of mains (or local
earthing system) bourne interference into your signal ground. So I don't
think I would want to join them.

In multi phase environments with Class I devices, then its important to
make sure they do have a working earth connection right up to the
appliance. Generally the earth connection in the supply cable for an
appliance is also considered an adequate EQ bonding conductor.

Is there a common way to detect automatically that this earth bond has come
adrift (ie not by waitng for the annual PAT test)? I suppose checking for a

Not the kind of thing a PAT would find...

voltage between ground and earth, or putting a voltage across them and
checking the current hits some limit. But do people ever do this sort of
thing?

Thanks
Theo



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/