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I'm using a Matsusada HV Power supply up to 20 kV. The manual mentioned that the ground terminal of the chassis should be connected to earth. And I had a few questions

  1. What are the best, or at least acceptable, ways to make this connection. Would it be OK to connect it to the earth ground in the power outlet? If so, is there some sort of adapter or tool that makes it safe to access the earth ground from the power outlet? I'm wary of poking around outlets.
  2. Is there a reason why the user manual instructs the users to make the earth ground connection? Doesn't the instrument have access to earth ground through the power outlet via the AC power cord? The only reason I can think of is that they don't trust all buildings to be connected properly.

Thanks

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  • \$\begingroup\$ I use the same power supply daily. For a fixed installation or test setup, I run a 4 mm^2 earth wire from the chassis to a ground point a screw it down. For a mobile test in the feild I use the same setup whenever available otherwise a crocodile clip to the test object, nearby cable tray (grounded), radiator or what ground point I can find. \$\endgroup\$ – winny Dec 16 '16 at 6:52
  • \$\begingroup\$ @winny - Is there a specific reason you connect the supplemental earth conductor? \$\endgroup\$ – vofa Dec 16 '16 at 21:09
  • \$\begingroup\$ Yes, same as vofa expained below. If ayone trips over the power cord, unknowingly unplugs it, electrical fault, intelock in series or whatnot, you will loose the discharge path to ground and your circuit is left charged and floating. A potentially deadly situation. \$\endgroup\$ – winny Dec 16 '16 at 21:25
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I agree with @ThePhoton overall. The earth pin in the IEC connector should be an extremely low resistance path to the chassis ground stud. Any other arrangement would be irresponsible, dangerous, and probably illegal. How you connect the supplemental earth depends on your lab setup. A permanent, sealed mechanical bond (nut, bolt, and washers) between cleaned surfaces (buffed and cleaned with alcohol) is ideal. That level of rigor is probably not necessary in this application. The scenario @ThePhoton described will happen somewhere eventually, but it's still an unlikely event. If your power supply is sitting on a shelf and not moving, there is about zero probability that the IEC connector will fall out.

For short term use, the earth pin of a socket is safe to use because it connects to the dirt outside. (Unless your building is heinously, illegally, dangerously miswired) Just stay away from the live and neutral. Put some insulating tape over those sockets. The screws that hold outlet cover plates on typically screw into the junction box, which is earthed. I'm not convinced that's a highly reliable connection, but you could try using that screw to attach a ring terminal connector to earth via the junction box. If you're going to install the power supply permanently or semi-permanently, you need to know with confidence that that connection is robust.

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Consider this scenario: While the power supply is turned on and outputting a high voltage, somebody abruptly removes the power cord.

This would break the "protective bonding" provided through the power cord. However, capacitors inside the supply, or in whatever its output is connected to, could still hold a high voltage. It could take seconds or longer for such capacitors to discharge. If there is a leakage path from the high voltage to the case, the case could become hazardous during this time.

Having a second earth connection reduces the risk in this scenario.

I don't know which safety standard this supply is designed to, but I have access to IEC 61010, which is the safety standard for many kinds of test and measurement equipment, and other safety standards tend to have similar requirements. In section 6.5.2.2, 61010 requires,

Equipment using PROTECTIVE BONDING shall be provided with a TERMINAL that is suitable for connection to a protective conductor and meets the requirements of 6.5.2.3.

And in section 6.5.2.3,

b) The integral protective conductor connection of an appliance inlet shall be regarded as the PROTECTIVE CONDUCTOR TERMINAL.

c) For equipment provided with a rewirable flexible cord and for PERMANENTLY CONNECTED EQUIPMENT, the PROTECTIVE CONDUCTOR TERMINAL shall be located near the MAINS supply TERMINALS.

I'm not certain, but this distinction between an "integral" and a "rewirable" protective conductor or flexible cord may be why a device using a removable power cord might be required to have a separate protective conductor terminal.

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The Japanese have several (at least two) different mains power distribution systems -- one at 60 Hz and one at 50 Hz. This wiki page says the boundary between the two regions contains four back-to-back high-voltage direct-current (HVDC) substations to interconnect these two power grids.

These systems are grounded using a Terra-Terra grounding approach and often, but not always, with an entrance GFCI at each customer site.

I suspect this separate ground lug may be present to deal with power distribution systems similar to those found in Japan.

Why don't you contact Matsusada and just ask an appropriate engineer there? I'm pretty sure you will get a meaningful reply from them.


As a separate note, I think it's very unlikely the power inlet will be a danger when the plug is removed. But given the voltages involved, that's something else I'd test (even after properly attaching to the grounding lug, if appropriate.) I'd want to know if there is any possibility of accident due to pulling the plug out of the wall and touching any of the exposed, presented contacts. And it's simple to test. Worth doing, just in case.

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