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?
How is this different from a cable with shielding disconnected at both ends?
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.
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.
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
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
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...?
its not a Faraday Cage with holes, wires to the outside potential
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
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
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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.
the Class 2 wall wart is the double insulated barrier to the mains
if the output is SELV I don't think there are further requirements on insulation for consumer/non-medical use
practically I have seen AC leakage currents from some wall warts that is annoyingly high
this can be a source of ground loop current (< 1 mA if it meets consumer standards, but still tingles)
the OP mentions specifically signal inputs insulated from enclosure in post #4 - violating condition for effective Faraday shield unless he's talking optical fiber
or shielded xmfr i/o
if the output is SELV I don't think there are further requirements on insulation for consumer/non-medical use
practically I have seen AC leakage currents from some wall warts that is annoyingly high
this can be a source of ground loop current (< 1 mA if it meets consumer standards, but still tingles)
the OP mentions specifically signal inputs insulated from enclosure in post #4 - violating condition for effective Faraday shield unless he's talking optical fiber
or shielded xmfr i/o
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Henry W. Ott who is probably the leading authority on subjects like this, writes:
home page
Grounding of Shields
Question: We have designed a battery powered handheld amplifier circuit. To shield the amplifier circuit, we have covered the amplifier box with aluminum foil. The shield is connected to the amplifier circuit common at a single point. In all the shielding topic discussions that I have read, it is emphasised to ground the shield. Since the equipment is portable, how can it be grounded? Will the shielding be effective without the ground connection?
Answer: A shield does not have to be grounded or have its potential controlled in any way to act as an effective shield. Many shields
are grounded for other reasons, such as electrical safety.
home page
Grounding of Shields
Question: We have designed a battery powered handheld amplifier circuit. To shield the amplifier circuit, we have covered the amplifier box with aluminum foil. The shield is connected to the amplifier circuit common at a single point. In all the shielding topic discussions that I have read, it is emphasised to ground the shield. Since the equipment is portable, how can it be grounded? Will the shielding be effective without the ground connection?
Answer: A shield does not have to be grounded or have its potential controlled in any way to act as an effective shield. Many shields
are grounded for other reasons, such as electrical safety.
did you read all of the book?
the inside of a conductive box is an iso-potential for frequencies with wavelengths much larger than the dimensions
but the inside of the conductive box isn't somehow magically gnd - a ESD pulse to the outside of the floating conductive box will raise the potential relative to a wire/circuit passing through an opening in the box which is externally gnd referenced - if the parts are too close you can get a arc discharge from the inside of the shield to the externally gnded wire/circuit
in general you make the assumption that a unknown generic signal source is grounded - you can construct scenarios of all Class 2 systems or iPod as your signal source - but you can't rely on it
you also get capacitive coupling as I explained - do people really not understand?
the inside of a conductive box is an iso-potential for frequencies with wavelengths much larger than the dimensions
but the inside of the conductive box isn't somehow magically gnd - a ESD pulse to the outside of the floating conductive box will raise the potential relative to a wire/circuit passing through an opening in the box which is externally gnd referenced - if the parts are too close you can get a arc discharge from the inside of the shield to the externally gnded wire/circuit
in general you make the assumption that a unknown generic signal source is grounded - you can construct scenarios of all Class 2 systems or iPod as your signal source - but you can't rely on it
you also get capacitive coupling as I explained - do people really not understand?
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Crap, I meant "connected to PE via loop breaker".
Sorry for the brain fart; I would have edited but alas, the 30 minute limit is up.
I don't mean to go on a tangent, but are there any negatives to adding additional grounds to a amplifier box if one believes that the 3rd slot (grounding pin) on an electrical socket is not making a great connection to ground?
To the OP, why would you not want to use a 3 pin electrical plug? It seems like it is much more difficult to build a properly-functioning double-insulated device.
To the OP, why would you not want to use a 3 pin electrical plug? It seems like it is much more difficult to build a properly-functioning double-insulated device.
The point is to avoid ground loops without having to use a loop breaker if the chassis is earthed... proper star grounding is assumed.
Anyway, the matter is largely academic as I said before; I can short the chassis to a loop-free earth.
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