safety: bench grounding conflicts

[Ivan Soleimanipour]

Working with HV in a lab environment requires satisfying two seemingly contradictory requirements:
1: Equipment chassis should be grounded; ostensibly to steer away any shorts.
2: All ground path opportunities for a human should be eliminated.

So we don't walk on concrete. throw away the grounding mat and wear crocs
while at the same time interacting with and coming close to grounded instruments,
the chassis of power strips and the grounding hook we keep around for discharging
stuff.

So how do you win, er, set a balance?

 

[dknguyen]

Isn't that why grounding straps have integrated resistors? To limit current flows in incidents but still allow a conductive path that the kilovolts of static electricity can discharge through.

 

[Nigel Goodwin]

Working with HV in a lab environment requires satisfying two seemingly contradictory requirements:
1: Equipment chassis should be grounded; ostensibly to steer away any shorts.
2: All ground path opportunities for a human should be eliminated.

You're making assumptions that may, or may not, be valid.

Earthing equipment only provides 'some' degree of safety, under 'some' circumstances - in other circumstances it makes it far more dangerous.

 

[Rich D.]

Ground straps have high resistances because it's needed for the static protection, not to protect us in case of electrocution. The resistance in the strap makes sure that when contact is made between us and something that might not be at a neutral potential, the discharge current is not so high as to cause the damage to the device we are trying to protect. If we are directly grounded when we touch a very sensitive device at a different potential, the combined charge of us and the ground (which can be considered infinite), may allow damaging currents to flow to discharge the built-up charges. Having a high resistance to ground ensures that only the charge in our bodies can discharge quickly to equalize with the charge in the device we touch, limiting currents. Of course having a resistance to ground certainly helps protect us from a high voltage relative to ground, so you could say the resistance is for both of our benefits. But a direct connection to ground isn't always going to protect a device that isn't at ground potential. That's why static resistance foam and packaging is used instead of just wrapping everything in aluminum foil.

As far as high voltage work, there is no substitute for simply avoiding making contact with any high voltage point - or in the case of extremely high voltages - keep a safe distance.

P.S. Not too many people realize the pink poly bags are designed to NOT GENERATE a large static charge from friction like normal plastic bags do. They DO NOT protect the contents from external static charges. To protect a device (such as when transporting it), you need the silver bags that have a STATIC SHIELD that prevents outside high charges from damaging the devices. If you want to, you can zap right through those pink bags, but not the silver ones.
When in doubt, use silver.

 

[dknguyen]

Working with HV in a lab environment requires satisfying two seemingly contradictory requirements:
1: Equipment chassis should be grounded; ostensibly to steer away any shorts.
2: All ground path opportunities for a human should be eliminated.

So we don't walk on concrete. throw away the grounding mat and wear crocs
while at the same time interacting with and coming close to grounded instruments,
the chassis of power strips and the grounding hook we keep around for discharging
stuff.

So how do you win, er, set a balance?

I should also point out that a human being in the middle of a ground path isn't quite the same thing as a human being in parallel with another ground path of much lower resistance (which is what you're referring to when you're talking about steering away shorts by grounding equipment chassis).