On 22/07/2019 14:10, Theo wrote:
John Rumm wrote:
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.

I would hope that there would be very little current flow. However it's
voltage that kills electronics, and a capacitive connection might be enough
to zap. I've felt leakage through supposedly good quality Apple PSUs when
using a non-earthed mains cable (to a two-pin non-UK wall scoket).

That's down to the mains input filters typically rather than the PSU
design itself. As you say, on a properly earthed supply the problem goes
away.

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.

I meant connecting the case of the equipment to GND on the PSU's output.

Yup, I don't think I would be keen to do that personally.

Do you mean the metal case of the equipment would be grounded with respect to
the LV PSU output, but not necessarily mains earthed? I'd have thought it
should be earthed to prevent live-to-case faults?

The case of the PSU will be connected to the case of the equipment,
thence mains earth. However the DC 0V line out of the PSU might not be -
and I can't see any advantage to adding one if its not there already.

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.

I'm confused. Bond the case to the mains earth, but not to the LV ground?
In which case, would you have a separate low impedance ground connection
between equipment on different phases? Would that avoid mains interference?

(Bearing in mind that there will be a connection anyway via data cabling,
although not necessarily low impedance).

Data cabling either has data ground and a separate and non connected
shield. Or just a data ground in many cases (depending on what kind of
connection it is)

If a shield is present that will likely couple to the casework on metal
clad machines, and hence mains earth.

--
Cheers,

John.

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