[GreenKeys] Australian Power cords.

[Jones, Douglas W]

From: Dave Horsfall [dave at horsfall.org] -- Wednesday, June 8, 2022 10:54 PM

> I've often wondered about the weird US system, with no ground as such.

Huh?  I had to pass a test on the National Electrical Code here in the US before I could rewire my own house, and current code (as of at least 30 years ago) certainly requires a real ground.  Every 110-120V outlet has 3 pins, hot, neutral and ground.  Ground must be solidly wired to a ground terminal strip in the breaker box.  That strip is bonded to either a water pipe or a grounding bar pounded into the earth -- and in many cases, in my experience, both.  Sounds just like Australia, except for the 110-120V part instead of the 220-240V part.

For a typical residence (outside of New York City, where weird 3-phase wiring still persists), you're likely to have 3-phase 11kV to ground power distribution, with a transformer on the power pole stepping this down to 240V, with the center-tap connected to the neutral power line.  The neutral power line sits below the 11kV lines and above any other lines on the pole, and is grounded every so often.  Sometimes, in areas with a low population density, just one phase will go off along the back lot line of a block to serve that block, with a different phase serving the next block.

3 wires run to the house from the transformer, 2 insulated ones that are 110V to ground, and a neutral wire that serves as a mechanical support
for the others and that is grounded at the breaker box.  When no current is flowing to a branch circuit in the house, there should be no voltage between neutral and ground.  When current is flowing to some outlet on that branch, there will be a small voltage because of the resististive drop in the neutral line.

Any outlet in the vicinity of water (outdoor outlets, kitchen outlets, garage outlets, laundry outlets, bathroom outlets) must be equipped with a ground fault circuit interrupter, that is, a circuit breaker that will cut the power if the return current flow through neutral does not match the outgoing flow through hot.  The GFCI may be in the breaker box, or it may be at one of the outlets, serving other outlets on the same branch circuit.  GFCI outlets have a test and a reset button.  Where ground faults are likely, it's more convenient to reset the circuit from the outlet instead of having to go hunt through the breaker panel.

In old wiring in systems using conduit or metal-armored cable, it was permitted to use the conduit or armor for the ground connection, and in old buildings, I've found outlets where neutral was as much as 40 volts away from ground because the conduit was not well grounded -- probably bad bonding at joints or corrosion in or around the joints.

240V appliances (like air conditioners and electric clothes dryers) take balanced power, relative to neutral/ground.  3-pin 240V appliance plugs have just a ground pin.  Those appliances must be pure 240V appliances.  No auxiliary 110V circuits.  Before 1996, the rules were different, they allowed neutral and ground to be the same pin, with a separate grounding wire from that pin at the plug to the ground bar in the breaker box.  Nowdays, 240V appliances that need 120V circuits must have 4-pin plugs, with separate ground and neutral.

> Oh, our *big* lines (sitting just over the 415v) are 11kV...  Feel free to touch those, because you won't feel a thing :-)

We had a tree lean gently against one of those lines in our neighborhood.  The line arced to the tree (which was wet, so no fire persisted) until the line melted and the broken end fell to the pavement.  The arc, at the pavement, did not quite draw enough current to throw a breaker in the power distribution system.  It was bright enough that, in full sun, I could clearly see my shadow on the hedge ahead of me when I was about 75 meters from the arc walking away from it.  Later, after the repair was completed, there were puddles of pale green slag on the concrete surface where the arc had melted the concrete.  The power company had to replace a fair sized chunk of pavement that their line ate.

           Doug Jones
           jones at cs.uiowa.edu