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.

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

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
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

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.

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

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).

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.

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?

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).

Theo

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.

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

John Rumm wrote:
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)

Indeed, although such grounds are not always low impedance (think the teensy
thin wires on USB cables).

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

So do you reckon it's better to have a low impedance ground bond (connect
all the GND lines of different boxes together, not relying on data grounds)
and separately earth bond the chassis together?

Theo

On 21 Jul 2019 23:39:57 +0100 (BST)
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.

Sounds like a job for opto-isolators.

On 24/07/2019 22:11, Theo wrote:
John Rumm wrote:
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)

Indeed, although such grounds are not always low impedance (think the teensy
thin wires on USB cables).

They are there to facilitate comms, not provide equipotential bonding...
i.e. they are functional earths and not protective ones.

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

So do you reckon it's better to have a low impedance ground bond (connect
all the GND lines of different boxes together, not relying on data grounds)
and separately earth bond the chassis together?

In the general case, I think you might be over thinking this a bit.

In a multi phase environment then its important that any Class I kit has
a proper mains earth connection. That way any chassis can not be dragged
to half line volts by its filter caps (and where those line volts may
not all be the same to due to different phases being used on different
circuits). The earth connection in the mains lead should also provide an
adequate EQ bond in the vast majority of cases.

Data connections may have a shield - which will hence bridge one chassis
to the next. The data ground however may or may not be tied to mains
ground. If it is, you don't need to worry since they are all properly
earthed, ans so should be seeing pretty much the same potential. If they
are not, the they are floating wrt to "true" ground, and hence again
nothing to worry about.

(the exceptions here would be when discussing interconnected systems
where lightening strikes are more likely).



--
Cheers,

John.

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