Multiple +12 V Rails
As power supply capacity increased, the ATX power supply standard was amended (beginning with version 2.0[3]) to include:
3.2.4. Power Limit / Hazardous Energy Levels
Under normal or overload conditions, no output shall continuously provide more than 240 VA under any conditions of load including output short circuit, per the requirement of UL 1950/ CSA 950/ EN 60950/ IEC 950.
—ATX12V Power Supply Design Guide, version 2.2[4]
This is a safety limit on the amount of power that may pass, in case of a fault, through any one wire. That much power can significantly overheat a wire, and would be more likely to melt the insulation and possibly start a fire.
Ideally, there would be one current limit per wire, but that would be prohibitively expensive. Since the limit is far larger than the reasonable current draw through a single wire, manufacturers typically group several wires together and apply the current limit to the entire group. Obviously, if the group is limited to 240 VA, so is each wire in it. Typically, a power supply will guarantee at least 17 A at 12 V by having a current limit of 18.5 A, plus or minus 8%. Thus, it is guaranteed to supply at least 17 A, and guaranteed to cut off before 20 A.
These groups are the so-called "multiple power supply rails". They are not fully independent; they are all connected to a single high-current 12 V source inside the power supply, but have separate current limit circuitry. The current limit groups are documented so the user can avoid placing too many high-current loads in the same group.
This works in the same way, and for the same reason, as the many small circuit breakers in a circuit breaker panel as well as the main supply breaker. And just like typical domestic wiring, multiple outlets are connected to each circuit breaker for reasons of cost.
Originally, a power supply featuring "multiple +12 V rails" implied one able to deliver more than 20 A of +12 V power, and was seen as a good thing. However, people found the need to balance loads across many +12 V rails inconvenient. This problem was exacerbated by the fact that the assignment of connectors to rails is done at manufacturing time, and it is not always possible to move a given load to a different rail.
Rather than add more current limit circuits, many manufacturers have chosen to ignore the requirement and increase the current limits above 20 A per rail, or provide "single-rail" power supplies that omit the current limit circuitry. (In some cases, in violation of their own advertising claims to include it. For one example of many, see [5]) The requirement was deleted from version 2.3 (March 2007) of the ATX12V power supply specifications.[6]
[edit] Operation of overcurrent protection
When a power supply has multiple-rail overcurrent protection, if any rail reaches that limit, the entire power supply will shut down. This is not associated with any overheating or increase in ripple voltage by the power supply as a whole, as might be caused by an overall overload. The only reliability penalty from operating a rail close to its current limit comes from the risk of triggering the shutdown.
