let-through

Let-through is the residual voltage transient that appears across the load terminals of a protective device during a surge event. When lightning, switching transients, or utility faults send a voltage spike down the line, surge suppressors and power conditioning units clamp that surge to a safe level, but they cannot eliminate it entirely. The let-through voltage is what actually reaches your equipment, measured in volts or kilovolts depending on the event severity.

The performance of any surge protection device is defined by how low it can push let-through voltage. A metal oxide varistor (MOV) rated 600V nominal might clamp an incoming 6 kV transient down to around 1.2 to 1.5 kV let-through. Silicon carbide surge arresters, gas discharge tubes, and coordinated multi-stage suppression stacks all trade off speed of response against let-through magnitude. Faster devices typically allow higher let-through; lower let-through requires slightly slower response time. The engineering problem is balancing these competing demands.

Critical equipment and let-through voltage

Equipment sensitivity matters enormously. A programmable logic controller (PLC) or data acquisition card may have insulation designed for 1.5 kV withstand voltage; if let-through exceeds that, you lose the device. Power supplies and motor drives are somewhat hardier, but still vulnerable if let-through peaks above their input voltage rating. This is why sensitive facilities use layered protection: a primary device at the service entrance handles the bulk energy, and secondary point-of-use suppressors clamp let-through to values the downstream equipment can tolerate safely.

The let-through voltage specification on surge suppression datasheets is measured under standardized test waveforms like the 8 by 20 microsecond impulse or the 10 by 1000 microsecond waveform. Real transients in the field are messier and less predictable than test pulses, so actual let-through may exceed the rated specification during extreme events. Coordinating multiple suppression stages reduces let-through more reliably than relying on a single device, because a second stage clamps the first stage's let-through output before it reaches the load.

The term itself reflects the physics: the transient energy passes through the protective device and arrives at the load side. It is distinct from voltage drop across the suppressor, which is a steady-state measurement, and distinct from insertion loss or attenuation of normal operating signals. Let-through voltage is purely a surge phenomenon, and minimizing it is one of the primary performance metrics used to select surge protection equipment for critical industrial control systems, telecommunications racks, and precision measurement apparatus.