Safest way to ground a metal enclosure

Question

I am currently building a nixie clock with some high frequency components (GPS) and I am planning on putting the clock in a metal enclosure. The highest voltage used in the clock is approximately 510V (referenced to CLOCK GROUND). There are three different grounding configurations of the metal enclosure I am contemplating. I want to discuss the safety considerations of each and ultimately determine which is the safest / best practice option (or if the correct / safest option is something else and not one I have listed below!). I was unable to find a post which discussed the safety considerations of all of these options.

Below are a list of the 3 grounding options I am considering with accompanying schematic representations for each option. Each schematic has the connection in question highlighted in yellow.

EDITED: Images updated to explicitly show EARTH connection for clarity.

Option 1: Connect the metal enclosure to the CLOCK GROUND

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Since I will have some SMA connectors mounted to the metal enclosure, if I do not insulate these connectors from the metal enclosure of the clock, they will connect the metal enclosure to CLOCK GROUND. This option will allow the CLOCK TRANSFORMER to act as an isolation transformer and also will prevent large voltage differences from building up between the metal enclosure and the internal electronics of the clock. However, I am concerned that this does not follow the common practice of connecting the metal enclosure of mains powered equipment to EARTH.

Pros:

• CLOCK TRANSFORMER acts as an isolation transformer.
• Prevents large voltage differences from building up between the metal enclosure and the internal electronics of the clock.

Cons:

• Does not follow the common practice of connecting the metal enclosure of mains powered equipment to EARTH. The metal enclosure may build up a large potential difference with respect to EARTH, leading to a shock if someone touches the metal enclosure.

Option 2: Connect the metal enclosure to EARTH

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If I choose this option, I would insulate the SMA connectors (which are CLOCK GROUND) from the metal enclosure of the clock. This seems to be following the common practice of connecting the metal enclosure of mains powered equipment to EARTH. However, should I be concerned that the internal electronics of the clock may build up large potential differences with respect to the case?

Pros:

• Follows the common practice of connecting the metal enclosure of mains powered equipment to EARTH.
• CLOCK TRANSFORMER acts as an isolation transformer.

Cons:

• Large voltage differences may build up between the metal enclosure and the internal electronics of the clock.

Option 3: Connect the metal enclosure to both EARTH and CLOCK GROUND

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This option prevents the internals electronics of the clock from building up large voltages with respect to the case and, at the same time, follows the common practice of connecting the metal enclosure of mains powered equipment to EARTH. However, this option eliminates the safety of the CLOCK TRANSFORMER acting as an isolation transformer.

Pros:

• Follows the common practice of connecting the metal enclosure of mains powered equipment to EARTH.
• Eliminates the possibility of large voltage differences building up between the enclosure and the internal electronics of the clock.

Cons:

• CLOCK TRANSFORMER is no longer acting as an isolation transformer. Touching an internal electrical node in the clock (such as CLOCK VDD) could result in a serious electrical shock.

It appears that each option poses some advantages, but still has some safety flaws. I suppose working with mains power is inherently dangerous, however, is there one option that stands out as significantly safer than the other two?

Answer 1

Option 4: bond the enclosure to earth, and connect it to the clock ground with a high impedance, maybe 1 megohm.

This prevents large voltages building up between the internal circuit and the enclosure, and at the same time limits the damage that can be done by an electric shock from the internal circuit.

Answer 2

From a safety standpoint the enclosure should be connected to earth, not to the neutral leg of the power supply. You should have a three prong plug with the enclosure connected to the ground pin. Now anybody touching the enclosure is safe assuming the wall plug is properly grounded. Even if it isn't you need another fault to make the enclosure dangerous. You don't get to count on the neutral wire being at earth ground.

Answer 3

You can power this with a 9V and a boost regulator, both from the same low voltage low VA transformer over a wide range. All you need is a 12Vdc wall supply input with DC and a ground ESD wire to bleed off any charge on a metallised, plastic case or metal case.

There is no need for a big 1:1 transformer.
You do not need over 500V, only 190 Vdc for std Nixie tubes

The case can be double insulated or earth bonded with AC input or DC input with an ESD ground wire with 1 Mohm R and small ceramic C to shunt RF noise. The Resistor will bleed off static charge in a double insulated case that can be spray coated on a plastic box with static dissipative paint. But if you prefer a metal box , 3 wire earth bonding is must with fusing on power.