The phenomenon of limiting a voltage source’s output. Voltage clamping is a feature often found in AC line protection devices and is commonly known as spike protection. The idea is to protect your sensitive electronic equipment from high voltage spikes (which are often of a very short duration) that occur on AC lines. When the voltage shoots up, a device in the line conditioner detects it and “clamps” down on the output, usually by increasing its own resistance, though there are other technologies. If the voltage spike is high enough or long enough these devices will sometimes fail, but their failure is to open, which breaks the circuit and still protects your equipment. Many such devices react differently at different power levels. When current is low they will sometimes clamp at a lower voltage (which is a benefit for sensitive low power gear), but the voltage at which they clamp will raise as current increases. The behavior of voltage clamping devices is specified according to the UL 1449 standard. UL 1449 is a measure of the output voltage during a 6,000 volt, 500 amp spike. It is therefore possible to compare different units so long as their specs conform to this test. The voltage at which a device limits its output is known as its clamping voltage.