Protective bonding doesn't sound like a practicable solution here.
SELV circuit part includes external USB and BNC connection, you need to guarantee that none of the external connected signals can carry dangerous voltage. It's not sufficient to have a low impedance connection of ground net.
Electrical products of protection class II have a reinforced or double housing insulation. The housing does have electrically conductive components – but these cannot be live due to the design. Such products are therefore electrically safe to touch by virtue of their construction. They therefore do not require a protective conductor. Thus, no protective earth resistance test is possible or necessary.
REF: https://www.schleich.com/en/the-protective-earth-test/
PPG (below looks like overkill) but if using a meter on a CAT III environment (high power) Arc Flash Protective Gear is necessary, just in case meter exceeds contamination level and arcs. Although TV repair guys used to routinely test 25kV safely with 100:1 to 500:1 probes, it was relatively low power, and would not cause spontaneous human combustion (SHC) as might a CAT III arc flash. Ref https://enesproppe.com/
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PPE & PPG
REF https://www.ecmweb.com/maintenance-...6/the-basics-of-personal-protective-grounding
Reference Specs https://www.fluke-direct.com/files/fluke-multimeters-detecting-intermittents-u.pdf
Fluke 190 III ratings
Altitude Operating up to 2000 m (6 600 ft)
All terminals to earth: CAT IV 600 V, CAT III 1000 V
BNC input shield to center pin CAT IV 300 V
Operating above 2000 m and up to 4000 m (13 000 ft)
All terminals to earth.: CAT IV 300 V, CAT III 600 V, CAT II 1000 V
BNC input shield to center pin CAT IV 150 V, CAT III 300 V
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What are Multimeter CAT (Category) Safety Ratings?
Discover the breakdown of Multimeter Category (CAT) Safety Ratings and the testing standards.www.digikey.ca
My comment: The 2 ohm source is the source R on the impulse on CAT III HV test raises the Q if tested on a long cable say 0.5uH/m with meter input capacitance can cause high Q ringing, so keep input capacitance as low as possible. I would design it for 8kV BIL. (basic impulse level)
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Safety info
Must be Grounded
This probe is grounded with the shell of BNC connector and an auxiliary grounding terminal, through the grounding conductor of the power cord of the measurement instrument. Before making connections to the input leads of this probe, ensure that the output BNC connector is attached to the BNC connector of the measurement instrument and the auxiliary grounding terminal is connected to a proper ground, while the measurement instrument is properly grounded.
Use Fused Test Prods if Necessary
If this probe is intended to use for measurements in circuits of INSTALLATION CATEGORY III, it should incorporate with fused test prods.
The ground terminal will be on the exposed metal parts not the plastic case. As I mentioned, the main case is plastic… but there are also exposed metal parts which I will mount onto a metal plate so that it can all be bonded together.Why would you have a ground socket to a plastic case in a double insulated instrument?
Yes this is exactly what I am thinking of doing in order to meet the requirements of CAT III for my designYou find similar isolation and ground concepts e.g. with differential osciloscope probes like SI9001.
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The operation instructions specify:
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I believe, that the grounding requirement has been primarly specified due to significant DC common mode current of several 100 uA caused by the resistive input divider. Although the input current is considerably below safety limits, it could charge a non-grounded instrument to possibly damaging common mode voltage.
Insufficient voltage strength for double insulation may be also an issue.
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The exposed metal on the instrument deserves to have a supplementary ground rated for 20A, the usual test.Why would you have a ground socket to a plastic case in a double insulated instrument?
Yes this is my thought tooThe exposed metal on the instrument deserves to have a supplementary ground rated for 20A, the usual test.
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