Re: Why An Open Neutral Kills 120V Devices
Here's the REALLY scary thing. Even if you had corner and tongue jack stands sitting on the dirt under your genny or production trailer, they offer such a high impedance path to earth that it's unlikely they'll have any grounding effect at all. A standard 8 ft ground rod buried in the earth can have an impedance as high as 25 ohms and still be within code. A great ground rod impedance for electronic assembly plants is maybe 5 ohms, and requires wet soil with salts added for conductivity on a regular basis. So 120 volts connected to a ground rod without benefit of a G-N bond will only draw perhaps 5 or 6 amps of current, (120 volts / 20 ohms = 6 amps), not enough to trip a 20 amp circuit breaker, but certainly enough to trip a GFCI if it was in the circuit.
I've not measured it in a while, but a few square inches of jack stand on the dirt will be in the hundreds (damp) or thousands (dry dirt) of ohms impedance. So a hot-to-chassis fault in a branch circuit somewhere will only send a few tens to hundreds of mA through the legs of the jack stands sitting on the dirt, which you'll never know about until you touch the generator panel while standing on wet grass. 8O~8-O~:shock:
And as you should be aware, the fault currents in a hot-to-chassis short should never go down the ground rod into the earth at all. These currents cycle back through the G-N bonding point in the service entrance, or at your generator panel, which is what actually trips a circuit breaker when you short a wire to the chassis. The earth itself has nothing to do with that shorting current. So without a proper Ground to Neutral bond with an earth connection from a ground rod, your stage’s “ground plane” can float around to whatever it likes, sometimes reaching 120 volts referenced to earth.
Of course, we humans are basically bags of salt water with a hand-to-hand impedance of around 1,000 ohms. You'll draw around 100 mA of current through your body if you stick one wet hand on a 120 volt electrified mic and the other on grounded guitar strings (or the other way around) which doesn't sound like a lot. But with just 10 mA alternating current through your arms it becomes difficult to let go of an electrified object because you're paralyzed by the current, and your hand grasping reflex is stronger than the opening muscles in your hand. That's why you get "stuck" when you've grabbed an electrified mic or instrument. And just 30 Volts AC which causes around 30 mA of current through your chest/heart is enough to put your heart into fibrillation. Without immediate CPR and an EAD (External Automated Defibrillator) you'll be brain dead in 5 minutes or so.
You should NEVER feel a shock on stage or standing at a mixer or lighting console. If you do, there's something seriously wrong with the grounding/bond somewhere in the power distro system, and it should be corrected immediately before somebody gets killed.
