There are 3 reasons for grounding:
1) for safety, as done in idolation class 1 devices. Here it need a low impedance link to PE
2) for charge build up or higher stray votlages. Here a high resistance like to ground is sufficient and sometimes wanted or even requited as with the anti ESD bands.
3) for EMI reasons, so for the high frequency range. Here a capacitor is sufficient and if there is no PE one can use N and L to equal parts for the EMI cap (usually class Y) found with SMPS. These caps to L and N are were the stray voltage problems come from.
Some instruments have a slightly odd combination with a resistor and 2 anti parallel beefy diodes (e.g. P600 or 1N540x) in parallel to get a reasonable low impedance link to ground to help with safety and not too strict a PE link, so they don't get much ground loop problems. In most aspsects it helps with safety, but it may not strictly meet the standards so the idolation in the instrument should be OK also without a safety-ground. It is kind of a compromise getting the instrument essentially grounded but still keep possible ground loop currents (ususally small votlage) small.
Case #1 is typical of earthing exposed metalwork in domestic white goods, dishwashers, kettles, toasters, washing machines and such like. Even so, should a full live to chassis framing fault occur, there's still likely to be as much as a 60% mains live voltage transient lasting several hundred microseconds before the safety fuse/circuit breaker/ELCB/GFCI protection is blown/or tripped.
Should someone happen to have the misfortune to be supporting themselves with one hand on a stainless steel sink whilst handling a dishwasher or washing machine at the time with their free hand, the worse they'll experience will be a briefly painful but non-lethal jolt which is a quite acceptable risk, considering the already low risk of such a framing fault occurring in the first place, let alone the several orders of magnitude reduction in the odds of it happening in such a circumstance. Simply the fact that the resulting brief 'jolt' in itself is virtually non-fatal is what makes the risk of such transient voltage spike events acceptable from a safety issue point of view.
Case #2 is an important guard against static voltage build up in system wiring that is intended to be unearthed and completely floating free of any ground contact (as far as the effective circuit operation is concerned). If no provision is made to limit static build up using a high value resistance leakage path to ground, there is the risk of damage to insulation via ESD which could trigger an arcing event of mains live (or other high voltage supply) to ground.
With small 10W rated class II smpsus (2.1A USB wallwarts, phone chargers and the smpsu boards used in the Feeltech AWGs, the main problem is the half live mains voltage that appears on the zero volt leg of the DC voltage outputs via the 1.6M ohm reactance of the 1nF Y cap used to shunt the interwinding capacitive coupling of the HV switching transients back to its source to reduce the amount of this common mode EMI that takes the scenic route via the safety earth loop wiring further adding to the HF switching pollution that now routinely plagues most domestic electrical wiring today.
In the case of the FY6600, a neat solution to eliminating the half live mains voltage ESD risk present on the BNC shield connections is simply to upgrade the figure of 8 C8 mains inlet socket with either a C6 or C14 3 pole socket to provide access to the safety ground by which to strap the BNC grounds via a 1 to 10K resistor to a trustworthy grounding point.
Even on a 240vac supply, a 10k resistor is sufficient to attenuate the Ycap leakage voltage down to less than 3vac, rendering it harmless to even the most static sensitive of DUTs whilst neatly providing a broadband attenuation of mains earth noise from DC to tens of MHz getting into the DUT or circuit under test. The very last thing anyone needs in this case is the introduction of a low impedance earth loop connection that had previously never existed to begin with, yet that is exactly the sin committed by Feeltech in addressing their customers' concerns over the half live ESD risk present in the FY6600 and previous AWG models with their rather hazardously bodged 'Final Solution'
.
Case #3 in the light of case #2, can be a definite no-no unless the device is entirely self contained in its own metal enclosure. The last thing you want to do is use a capacitor to "ground" out any HF interference since the typical safety earth connection is not so much an RF ground as an RF antenna which not only radiates such 'HF ground currents' but also picks up HF noise injected by other SMPSU powered devices and all the transients generated by appliance on/off switches, including thermostats and light switches, to name but two examples, into the mains earth wiring which is coupled quite tightly at HF to the Live and Neutral wires.
In the case of an AWG designed to output arbitrary waveforms from DC up to several MHz, using a classic PC styled SMPSU to generate intermediate level DC voltages, you face the complication of having to use DC-DC converters with fully isolated outputs to allow the AWG to generate fully floating output waveforms free of earth loop issues other than for a 10K 'static drain' connection to the safety earth.
You can use a filtered C14 socket with a class II smpsu as long as you only connect the PE tag to the zero volt rail via a 1 to 10K 'static drain resistor'. The problems of unwanted ground loops occur, regardless of a Faraday enclosure or not when the BNC shield connections are hard wired to this PE connection.
The two antiparallel connected heavy duty rectifier diodes connected in series with the safety earth 'trick' is one I used myself over four decades ago with a Thorens Turntable into which I'd added a phono preamp to eliminate the sheer and utter folly of trying to send millivolt level signals from a magnetic cartridge direct to the amplifier over several feet of a phono coax link.
I'd fitted this as a 'backstop' safety measure to make using the classic "Eliminate the earth loop issue by only using the phono cable shields to ground the turntable" solution that was the common practice at that time to make this practice just little more palatable from a safety point of view.
Of course, these days one can add an A/D and opto-isolator to the turntable pre-amp and another opto-isolator with D/A at the amplifier by way of an upgrade to an existing 70s vintage vinyl Hi-Fi setup. I daresay you can buy a ready made phono pre-amp with optical out to connect your turntable directly to a more modern Hi-Fi amplifier to make the whole business of earth loop avoidance moot.
Anyone looking to this rectifier diode "Earth Loop Breaker" technique would likely best be served with a low voltage 50A rated bridge rectifier (+ & - terminals strapped together with the ~ terminals connected in series with the earth connection) and a 1K resistor in parallel to stop the diode bridge conducting during voltage peaks induced by stray leakage currents in order to avoid injecting unwanted HF harmonics into the mains wiring via the earth wire through the resulting diode clipping effect.
Whilst it's possible to generate KA transient fault currents via a live to earth fault in a 13A ring main socked just a few feet away from the consumer unit (30A fuse or 32A circuit breaker or ELCB), it's unlikely to exceed 3 or 4 hundred amps in the case of a live to ground fault within the equipment itself so a 50A bridge rectifier should be good for a 500A 20ms surge which should take out even a 13A fuse swiftly enough to 'save the day' without the drama of conflagration.
However, you'd be well advised to install a separate 13A fused feed to a dedicated (set of) 13A socket(s) (UK case) and test this using a DSO in one shot mode to measure the actual surge current peak (current probe or a 0.1 ohm shunt) rather than just take my word for the safety of using a 50A rated low voltage bridge rectifier in this fashion, trusting only to the reliability of the manufacturer's data sheet specifications.
John
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