- Clamping Voltage Definition: Clamping voltage is defined as the maximum voltage allowed to pass through a surge protector before it limits further voltage, protecting connected devices from surges.
- Purpose and Operation: Surge protectors use clamping voltage to suppress excess voltage, ensuring devices remain safe from power surges.
- Breakdown Voltage: Breakdown voltage is defined as the minimum voltage at which an insulator starts to conduct electricity, allowing current to flow.
- Clamping vs Breakdown Voltage: Clamping voltage prevents excess voltage from passing, while breakdown voltage is the point where current starts flowing in a diode.
- Let-Through Voltage: Clamping voltage is also known as let-through voltage, indicating the maximum voltage a surge protector lets through to connected devices.
What is Clamping Voltage?
Clamping voltage is the residual voltage an SPD lets through while conducting a surge. An electrical circuit breaker interrupts current. A surge protector uses that clamp level so downstream equipment sees a limited surge voltage.
On a datasheet the clamp figure (UL 1449 voltage protection rating, VPR) is the voltage measured during a specified surge test. The protector does not hold every surge to a hard ceiling: residual voltage rises as surge current rises. The device is wired ahead of the load to limit spikes on AC (and, in other products, DC) supplies.
When the incoming surge is large enough to drive the protector into conduction, the voltage at the protector terminals stays near the clamp/VPR for that current instead of following the open-circuit surge peak.
That limited residual is what people mean when they say the surge was clamped. It still stresses the load, so the rating must sit below what the equipment can survive.
Take a 120 V RMS outlet. The sine-wave peak is about 170 V. UL 1449 voltage protection ratings for that service are far above the peak, because the test uses a high surge current.
Choose a VPR below the withstand of the protected equipment and above normal peak line voltage, so the protector stays off during ordinary peaks.
A 220 V clamp is not a UL 1449 class for 120 V AC. The lowest common VPR marking on 120 V plug-in protectors is 330 V, then 400 V and 500 V.
During the UL test surge, a 330 V VPR device is measured at 330 V residual at the test current. A larger surge current produces a higher residual on the same MOV or TVS.
Independent labs measure VPR, nominal discharge current and other UL 1449 ratings on production samples.
Clamping Voltage vs Breakdown Voltage
Breakdown voltage of an insulator is the voltage at which the dielectric fails and a large current then flows through the failed path. That event is usually destructive for insulation.
A semiconductor diode sits between an insulator and a conductor in its I-V curve. Silicon and germanium diodes are built for a controlled reverse breakdown when that is the design intent (zener or TVS).
In reverse bias a rectifier diode blocks until reverse breakdown. Past that voltage, avalanche or zener action lets reverse current through the junction.
For a TVS, breakdown is where reverse current starts to rise. Clamping voltage is the higher voltage at the rated surge current (Ipp). An MOV’s 1 mA varistor voltage is also not the same number as its UL VPR at kiloamperes.
Clamping Voltage vs Let Through Voltage
Makers often print the same residual as clamping voltage or let-through voltage. UL 1449 calls the lab measurement voltage protection rating (VPR).
Let-through at the protector terminals is that residual. Voltage at the equipment can be higher because of lead inductance between the SPD and the load.
What is a Good Clamping Voltage?
Pick a clamp/VPR from the withstand of the protected circuit and from the AC (or DC) voltage the SPD must ignore in normal service.
The VPR tells you how high the residual was in the UL surge test. A lower VPR on the same service voltage usually means a lower residual in that test. 400 V is a middle 120 V class, not a universal cap.
MOVs and TVS diodes conduct on a nanosecond scale. Wiring inductance often sets the voltage the load actually sees more than the device’s quoted response time.
A lower VPR closer to peak line voltage gives a lower residual but wears the MOV faster and can conduct on ordinary swells.
Underwriters Laboratories UL 1449 lists many 120 V VPR markings. The three lowest common consumer classes are 330 V, 400 V and 500 V. 330 V is the lowest of those classes, not a single required value.
