let through
To allow an amount of voltage to pass through to the attached equipment.
let through: the voltage a protective device permits downstream
In electrical distribution, to let through means to allow a specified voltage or current to pass from a power source to the equipment connected downstream. A protective device, such as a regulator, surge suppressor, or circuit limiter, determines what voltage is allowed to proceed. If the incoming voltage exceeds the threshold the device is designed to let through, the device blocks or reduces the excess, protecting sensitive loads from damage.
The phrase appears most often in the context of surge protective devices (SPDs) and voltage regulators. An SPD rated to let through 400 volts, for example, will clamp or divert any transient spike above that level. The amount let through is called the let-through voltage and is a critical specification. A device with lower let-through voltage provides better protection but may also introduce more insertion loss during normal operation.
Variants and applications
Plugstrip suppressors, transient voltage surge suppressors (TVSS), and metal oxide varistors (MOVs) all operate on this principle. Industrial power distribution systems use let-through specifications to coordinate protection across multiple stages: a main panel suppressor might let through 800 volts, while a sub-panel device lets through 600 volts, ensuring that the closest protective device to the load reacts first. Manufacturers publish let-through voltage at specific current levels, usually in the range of 6,000 to 10,000 amperes.
The term also applies to current-limiting devices. A fuse or current limiter selected to let through only a certain peak current during a fault will prevent downstream equipment from being exposed to the full fault current available at the source. This is why a 20-ampere breaker will not let through more than its rated current under normal circumstances, protecting both the conductor and the load.
Understanding let-through voltage is essential when specifying protection for computer systems, medical equipment, and control circuits that tolerate only narrow voltage windows. A regulator that lets through ±10 percent of nominal voltage on a 480-volt three-phase line will hold the output between 432 and 528 volts, which is tight enough for most industrial motors but not for precision instrumentation. Exceeding the acceptable let-through range for a given load is a common cause of premature equipment failure and nuisance tripping.