Does a metal/conductive enclosure need to be grounded in order to shield?

[454Casull]

If safety earth is not brought in at all, ground loops cease to exist - but does the enclosure still act to shield from electric and EM interference?

How is this different from a cable with shielding disconnected at both ends?

[AndrewT]

Ground is generally applied to all return voltages that are referenced to zero volts.

The Safety Earth is referenced to PE.
If PE does not exist, that still leaves all the other grounds and ground loops among those conductors most certainly can exist.

[NicMac]

I would say that a shield needs a drain to work, although common sense tells me a metal box thick enough will keep EMI out without grounding....
I have always been taught to keep cable shields connected at one end....
In any case it is perfectly possible to ground the chassis without creating any loops loops.
Cheers,
Nic

[454Casull]

Originally Posted by AndrewT

Ground is generally applied to all return voltages that are referenced to zero volts.

The Safety Earth is referenced to PE.
If PE does not exist, that still leaves all the other grounds and ground loops among those conductors most certainly can exist.

What I'm thinking of is an aluminum box, with insulated connections, with power delivery via two-conductor wall-wart. The box should not be part of any circuit...?

[auplater]

Faraday cage - Wikipedia, the free encyclopedia

[jcx]

the box will be a capacitive antenna to the outside world, and capacitively couple to your circuit

how effective it is for magnetic fields depends on material, thickness, frequency

at high enough frequencies you will get reflection, absorbtion of incident EM waves

but because of the capacitive coupling of lower frequency E fields the enclosure, or an inner circuit shield should be grounded to signal ground

some interpret electrical safety codes to require PE for any exposed metal

double insulated, "reinforced" insulation with 4 kV Hipot pri-sec mains transformer meeting creepage, clearance requirements, internal mains wiring insulation, construction standards should be safe without PE

this requires securing all mains side wiring, adding supplemental insulation such that any single failure will still meet insulation, spacing requirements

[NicMac]

[454Casull]

Originally Posted by jcx

the box will be a capacitive antenna to the outside world, and capacitively couple to your circuit

how effective it is for magnetic fields depends on material, thickness, frequency

at high enough frequencies you will get reflection, absorbtion of incident EM waves

but because of the capacitive coupling of lower frequency E fields the enclosure, or an inner circuit shield should be grounded to signal ground

some interpret electrical safety codes to require PE for any exposed metal

double insulated, "reinforced" insulation with 4 kV Hipot pri-sec mains transformer meeting creepage, clearance requirements, internal mains wiring insulation, construction standards should be safe without PE

this requires securing all mains side wiring, adding supplemental insulation such that any single failure will still meet insulation, spacing requirements

Speaking only to the last point, does all low-level voltage need to be Class 2 insulated if PE is not present? I mean we're talking less than 3V signal and a Class 2 wall wart, here.

[auplater]

Originally Posted by jcx

the box will be a capacitive antenna to the outside world, and capacitively couple to your circuit

insulation, construction standards should be safe without PE

this requires securing all mains side wiring, adding supplemental insulation such that any single failure will still meet insulation, spacing requirements

a bit presumptive about a seemingly open ended question, eh???

Without more information as to application, it most certainly could be a Faraday cage

[454Casull]

I guess this is a little academic anyway. It is not a problem for me to short the enclosure to another enclosure which is connected to PE via safety earth.