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What is the difference between the industry standards Joules and Let-Through Voltage?


What is the difference between the industry standards Joules and Let-Through Voltage?

Product Line:

All APC Surge and Back-UPS products.


All APC Surge and Back-UPS products.

Cause / Resolution:

Joules vs. Let-Through Voltage


A Joule is a unit measurement of energy (Watt-seconds). For example, 60 joules is equivalent to a 60 watt light bulb that is on for one second.

Effective Clamping Voltage is the peak voltage which is measured at the output of the surge protection device after a surge condition occurs. This voltage is the total voltage which is passed to the protected  equipment and is, typically, the let-through voltage plus the peak AC voltage (i.e. 169Vpk for a standard 120VAC power). Per IEEE, ANSI C62.41 and UL1449 standards, the surge protection device is subjected to a 6000V, 500A unidirectional surge to determine the effective clamping voltage level. UL's best performance rating is 330V.

Let-Through Voltage is actually the difference between the effective clamping voltage and the peak AC voltage. For example, a UL 330V rating indicates that the let-through voltage to the protected equipment is < or = 330V minus 169 Volts peak (for a standard 120 VAC power).

Metal Oxide Varistor (MOV's are used extensively by the majority of surge protection device manufacturers, including APC, for shunt-mode suppressors. MOV's are electrical components which are used to divert surge current and clamp surge voltage to a lower level.


The intent of a surge protection device (line cord connected type) is to protect equipment from a harmful overvoltage caused from a surge condition. The perception has been that a higher device joule rating is analogous to a better performing product. We believe that let-through voltage is a better indicator of performance than joules.

A surge protection device with a higher joule rating does not necessarily mean that the let-through voltage is better. In other words, the amount of energy that the surge protection device can absorb (i.e. joules) does not mean that it is providing the ""best"" protection for your equipment. A joule rating is indicative of the MOV rating only. It does not take into account the other parts of the surge protection device which may be effected by a surge (i.e. fuses, printed circuit board traces, wiring, etc.). Let-through voltage takes this into account since it represents the suppressed surge voltage at the output of the device.

There are two approaches which may be taken to increase a surge protection devices joule rating:

1) Use MOV's which have a higher joule rating. Typically, MOV's with a higher joule rating also have a higher surge clamping voltage which will result in a higher let-through voltage to the protected equipment.

2) Increase the number of similar rated MOV's (cascaded in parallel with each other). The only problem with this approach is that the MOV's are not impedance matched. Therefore, it is feasible that one of the MOV's will shunt more surge current since its impedance may be lower. The other MOV's will also shunt surge current but it will not be evenly distributed since the impedance's may be different. As a result, the surge protection devices' total joule rating may not be relevant since the MOV's do not shunt the same amount of surge current.

Therefore, based on the above and the intent of the surge protection device, we believe that a surge protector should be selected based on the lowest let-through voltage capability and UL's best effective clamping voltage rating of 330V (i.e. UL1449 330V).

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