How Surge Protectors Work

By: Tom Harris  | 

Surge Protector Ratings

A Belkin SurgeMaster II mid-range surge protector with connections for phone lines
A Belkin SurgeMaster II mid-range surge protector with connections for phone lines

On a listed surge protector, you should find a couple of ratings. Look for:

  • Clamping voltage - This tells you what voltage will cause the MOVs to conduct electricity to the ground line. A lower clamping voltage indicates better protection. There are three levels of protection in the UL rating -- 330 V, 400 V and 500 V. Generally, a clamping voltage more than 400 V is too high.
  • Energy absorption/dissipation - This rating, given in joules, tells you how much energy the surge protector can absorb before it fails. A higher number indicates greater protection. Look for a protector that is at least rated at 200 to 400 joules. For better protection, look for a rating of 600 joules or more.
  • Response time - Surge protectors don't kick in immediately; there is a very slight delay as they respond to the power surge. A longer response time tells you that your computer (or other equipment) will be exposed to the surge for a greater amount of time. Look for a surge protector that responds in less than one nanosecond.

You should also look for a protector with an indicator light that tells you if the protection components are functioning. All MOVs will burn out after repeated power surges, but the protector will still function as a power strip. Without an indicator light, you have no way of knowing if your protector is still functioning properly.


Better surge protectors may come with some sort of guarantee of their performance. If you're shopping for more expensive units, look for a protector that comes with a guarantee on your computer. If the unit fails to protect your computer from a power surge, the company will actually replace your computer. This isn't total insurance, of course -- you'll still lose all the data on your hard drive, which could cost you plenty -- but it is a good indication of the manufacturer's confidence in their product.

If you're interested in learning more about these issues, and finding out all the ways surge protection technology can fail, check out some of the sites listed in the links section below. Surprisingly, surge protectors are an extremely controversial piece of technology, and they have sparked a great deal of debate on the Web.

Surge Protector FAQ

What does a surge protector do?
The main job of a surge protector is to protect devices from power surges, or an increase in voltage significantly above the designated level of electricity. If the surge is high enough, it can cause some heavy damage on electronics and appliances in your home.
Are all power strips surge protectors?
Surge protectors and power strips look very similar, but not all power strips are surge protectors. In fact, many are just glorified extension cords.
Is a whole-house surge protector worth the money?
Whole-house surge protectors are called "surge arrestors". These protect all the circuits in your house from a certain range of voltage surges, but only ones coming from outside sources like utility company problems, lightning, and transformer switching. They only cost $300-600 including installation and you may qualify for a discount on your home insurance, so it can be well worth the cost.
What are the types of surge protectors?
There are four common types: basic power strips, better power strips, surge stations, and uninterruptible power supply (UPS) strips. The cost, protection rating, and additional features increase from basic to UPS.
Do wall outlets have surge protection?
Standard electrical outlets (including three-prong) don't have protection against power surges. Surge protectors are primarily sold in the form of power strips or bars, although you can buy single-outlet surge protectors that sit flush with the outlet.

Originally Published: Jan 5, 2001

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