Turning off the power supply's big red switch does not remove voltage from all components within the power supply.
With that information, some people may decide to also power off the 5170 at the wall socket.
Wattage - Breakdown
The IBM 5170's power supply is rated at approximately 192 Watts. Expressed that way, the wattage is misleading. For example, it does not mean that 192 Watts of power can be drawn from the +5V line if no power is drawn from the other lines. Each of the output voltage lines has its own power rating, that must not be exceeded. Click here to see those ratings.
The IBM 5170's power supply requires a sufficient load in order to start/operate.
In some cases of insufficient loading, the power supply starts (you see the fan turn) but the power supply stops after a second or so. In other cases, the fan won't even turn. Note however that both of those symptoms can also be produced by the power supply being overloaded by something.
The 5170 Technical Reference indicates that the minimum loading is 7 amps on the +5V line and 2.5 amps on the +12V line.
For 5170s without a hard drive, an IBM supplied load resistor of 5 ohms provides the +12V loading. More information about the load resistor is here.