Metal enclosure grounding

[Mr Harry]

I'm building a device which comprises a circuit board in a metal
enclosure along with a mains smps.

The circuit board is totally isolated from the metal enclosure. The
enclosure is connected to mains ground.

Should I connect 0v on the circuit board to the grounded metal
enclosure or leave the circuit board floating wrt gnd? If connection
*is* recommended should I connect it directly or via a resistor?

Part of me thinks it'd be a good idea to ground the 0v and part of me
thinks maybe I should leave it floating. Any suggestions?

Mr Harry

 

[John Woodgate]

I read in sci.electronics.design that Mr Harry <[email protected]>

I'm building a device which comprises a circuit board in a metal
enclosure along with a mains smps.

The circuit board is totally isolated from the metal enclosure. The
enclosure is connected to mains ground.

Should I connect 0v on the circuit board to the grounded metal
enclosure or leave the circuit board floating wrt gnd? If connection
*is* recommended should I connect it directly or via a resistor?

Part of me thinks it'd be a good idea to ground the 0v and part of me
thinks maybe I should leave it floating. Any suggestions?

It depends very much on what the circuit board does. If it's a digital
circuit you can probably leave it floating, but if it's a sensitive AF
or RF amplifier, you probably need to connect its 0 V line directly to
the enclosure.

 

[Mac]

I'm building a device which comprises a circuit board in a metal
enclosure along with a mains smps.

The circuit board is totally isolated from the metal enclosure. The
enclosure is connected to mains ground.

Should I connect 0v on the circuit board to the grounded metal
enclosure or leave the circuit board floating wrt gnd? If connection
*is* recommended should I connect it directly or via a resistor?

Part of me thinks it'd be a good idea to ground the 0v and part of me
thinks maybe I should leave it floating. Any suggestions?

Mr Harry

I would definitely ground it, directly, particularly if it has any outputs
that go off board. It would be so sad if the SMPS failed in such a way
that a wire going off board became energized at mains voltage. With
circuit ground at earth potential, such a scenario is much more likely to
blow a fuse or give rise to a tell-tale puff of smoke.

By the way, does this design have to get any regulatory agency approvals?
I'm guessing not, but if it does, make sure you look into what they entail.

Mac

 

[Ken Smith]

The circuit board is totally isolated from the metal enclosure. The
enclosure is connected to mains ground.

Should I connect 0v on the circuit board to the grounded metal
enclosure or leave the circuit board floating wrt gnd? If connection
*is* recommended should I connect it directly or via a resistor?

You don't say how the power supply circuits are arranged. If your circuit
ground is connected to one side of the mains, you absolutely must not
connect it to the box.

That said:

If the circuits in the box have no electrical connections to the outside
world other than the power cord, you can connect the PCB's ground to the
power cord's ground very near the point where the power cord comes into
the box. It is usually best to make this the only connection between the
ground and the box. This way AC currents from your circuits will not flow
through the metal of the box and thus radiate.

If this device has a cable other than the power cord leaving it, in
general you want to connect the PCB, the box and the shield of this cable
together at a point near where it leaves the box.

 

[R.Legg]

I'm building a device which comprises a circuit board in a metal
enclosure along with a mains smps.

The circuit board is totally isolated from the metal enclosure. The
enclosure is connected to mains ground.

Should I connect 0v on the circuit board to the grounded metal
enclosure or leave the circuit board floating wrt gnd? If connection
*is* recommended should I connect it directly or via a resistor?

Part of me thinks it'd be a good idea to ground the 0v and part of me
thinks maybe I should leave it floating. Any suggestions?

Providing a return path for Common-Mode conducted output current from
the mains SMPS using (at least) an output capacitor to the
safety-earth-grounded chassis is a good idea. If this is a commercial
SMPS assy, this connection may already be present, as well as
conventional filter connections on the AC input.

The best location for your system (load) to connect directly to the
safety earth will depend on what else is present in the system, the
noise immunity performance expected, and any effects demonstrated by
the unavoidable pre-existing connections (ie other converters or
modules with input line filter, signal line filter and chassis
connections).

RL