ATX Power Supplies:  An Overview

In late 1995, Intel replaced the "AT" standard and it's two separate "P8" and "P9" connectors with a single piece main connector from the power supply to the motherboard.  Enter ATX.

ATX 20-pin connector

ATX 24-pin connector

Your PSU is "LIVE", but not "On".

An ATX power supply is always "live", even when it's "off", when it's plugged in and it's rear switch (if applicable) is switched to the "on" position. .

The +5V standby is always coursing through your motherboard whether your PC is on or not.  This is how the CMOS keeps it's settings with or without it's CMOS battery and wake on LAN, wake on ring and keyboard power on is accomplished.

The power supply is instructed to energize the other power leads throughout the system when the "Power On" lead (the green wire on the main connector) is grounded.  Without grounding the green wire on the power supply, you will not be able to test ANY of the power leads on the power supply except for the +5V standby.

To test the power supply, you can do this: Unplug the PSU from the wall and unplug the power connector from the motherboard. Connect the green and black wires on the power connector together with a paper clip or a piece of wire.  It's ok... this is a signal lead and not a live lead. This will tell the PSU to energize once you plug it back in.  Now, using a multimeter, the 12V should read 12V, the 5V should read 5V and the 3.3V should read 3.3V (within +/- 5% per Intel specifications.)