Re: Potentially stupid question about AC

[david@uow.edu.au (David Wilson)]

supertimer@aol.com (Supertimer) writes:
>Rubywand <rubywand@swbell.net> wrote:
>>Supertimer writes ...
>>
>>S______(hot)_______________ typically Black (220V from hot to hot)*
>>S
>>S                110V
>>S____(neutral)__________    typically White*
>>S
>>S                110V
>>S______(hot)_______________ typically Red*
>>
>>
>>(110V and 220V are the traditional values. The actual voltages are going to be
>>closer to 115V and 230V.)

I have seen US gear marked for 110, 115, 117 and 120VAC. Japan uses 100VAC.

Here in Australia we have 3 phase power instead of the two phase shown above.
Most recently wired homes just have one of the 3 active wires and the neutral
coming into the switchboard. Our actives are 240VAC and between any two actives
we have 415VAC. Our color scheme for building wire is Red = Active, Black
= Neutral and Green or Green/Yellow striped for Ground. This can cause problems
with US/Canadian gear - we had an entire lab of Volker Craig terminals wired
with Black = Neutral, White = Active and Green = Ground (I do not think the
importer knew Black is active in USA).

If all three active phases are present they are Red, Yellow and Blue.

Appliance wiring tends to use the European standard of Brown = Active, Blue
= Neutral and Green/Yellow stripes for Ground.

Here we use the MEN system - Multiple Earthed Neutral.

		Fuse Box
		------------				+-------+ Metal case
Active ---------|----------|------- Active -------------|--/ -) | is earthed
		|          |				|     ( |
Neutral---------|--+-------|------- Neutral-------------|-----) |
	     	|  |       |				|	|
	     	|  +-------|------- Ground -------------+-------+
	    	---|--------
	    	   |
	copper stake
	into earth

Because normal current flow is via the active and neutral leads and they have
some resistance the voltage at the far end of the neutral lead is typically
non zero.

Many houses now have ELCB (Earth Leakage Circuit Breakers) which trip if the
current in the active line is not equal and opposite to that in the neutral
line. This indicates that power is getting to ground either via the ground wire
or through something or someone else.

Double insulated gear omits the ground wire and replaces the metal case
with two independent insulating layers.

Our plugs are Crowfoot so polarisation should always be maintained.

active	/	\  neutral


	    |  earth
>And why is it dangerous to defeat a polarized plug?

The device may only have a single pole power switch which breaks the active
line. The fuse should also be in the active lead.
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia