Single-rail PSUs, Multiple-rail PSUs, and Safety: It's Serious Business

By Michael Valera, on April 18, 2011

Before I begin, I’ll define some terms: When a PSU is described as “single rail,” it means that all of the PSU’s power is available from a single source. Multiple-rail designs allocate the total available amperage across two or more “rails.” Single-rail PSUs can be much more convenient when setting up high-performance PCs, as they eliminate the need to balance the power load across multiple rails — all you need to worry about is whether your power supply meets the your system’s total power requirements.

But more importantly, there’s no effective difference in safety between single-rail and multi-rail PSUs. It’s been claimed that the higher amperage delivered across a single rail introduces risks not found with multiple rails with lower amperage. This is simply not the case, for reasons we’ll explain below.

Safety First

First, let’s get one thing out of the way. Corsair, like all reputable PSU vendors, takes safety very seriously. All of our power supplies are certified by UL and other regulatory agencies. In addition to UL/CUL certification in the US, our power supplies have CE (for Europe), CB (an international standard), TÜV (for the German market), CCC (for sales in China), and C-Tick, for Australia and New Zealand. And, of course, they pass all the relevant Intel certifications, as well. In short, they meet the highest worldwide standards for power supply safety. We wouldn’t have it any other way.

Here’s a brief look at the protective circuitry built in to any well-designed power supply:

Short-circuit protection: all modern power supplies have mechanisms in place to make equipment damage from short circuits all but unheard-of. Corsair PSUs have multiple redundant, isolated circuits that shut down the PSU in the presence of any condition that indicates a potential short circuit. When building a power supply, you simply cannot be too careful.

Over-voltage and under-voltage protection: these work in several ways. On the power input side, in the unlikely event of the electric company providing too little voltage, the power supply immediately shuts down. The PSU also protects your system in the event of a lightning power strike or other power spike. Well-designed power supplies are designed to accommodate for variations within a certain threshold to avoid unnecessary fuse replacements. The ATX design guide calls these “nuisance trips.”

On the output side, there’s also over-voltage and under-voltage protection on the power rail. If there’s any variation from the ATX specifications that can’t be corrected, the power supply immediately shuts down.

Over-current protection: this limits the amount of current that can be pulled from the output rail.

By the way, motherboards have their own mechanisms for protecting the CPU, memory, and add-in cards from damage in the event of non-standard power delivery. Motherboards do so by monitoring the “power good” signal sent by the PSU, and shutting down the CPU. The power good signal is +5V, but it’s provided by a dedicated monitoring circuit, and not simply the PSU’s regular +5V line.

Amperage: Is there such a thing as too much?

Let’s use an example. The Enthusiast Series TX750 V2 provides a single +12V rail, delivering a maximum current of 62 Amps and a maximum combined wattage of 720 Watts. A PSU from another vendor might have four +12V rails, each with a total amperage of 25 Amps (the higher total amperage on the +12V line in this example is made up for by providing less power on the other lines than the Corsair PSU — 25 Amps each to the +3.3V and +5V lines, vs. 30 Amps on the TX750 V2).

Is 62 Amps any less safe than 25 Amps?

The answer is: of course not. Single-rail PSUs have the very same protective circuitry described above as do multi-rail CPUs.

When looking at total amperage, It’s also important to remember that when a PSU is installed into a PC, the current is spread among multiple power cables — to your motherboard, to your peripherals, to your GPU, and so on. You’ll never have a situation where 62 Amps are being drawn across a single power cable. Additionally, any well-designed PSU power cable (that is, the cables provided by any reputable PSU manufacturer) just can’t supply the impedance that would be necessary to overload the cable.

When choosing a power supply, there are a few things to consider: whether you want modular cables, how much power you need today, and how much power you think you’ll need for your next several upgrades. But when choosing between a single-rail or a multi-rail PSU, safety simply isn’t a differentiator.