For most of my life I have worked in electronic equipment repair (I'm retired now), and have seen tons of damage done either by poor or worn out surge protection, or the lack any surge protection to start with. The best way to protect your equipment is to pull the plug when a storm approaches, but not always possible if we are not at home at the time. The information in this post is to help you with selecting a surge suppressor.
Unless you are going to spend a lot of money, like over $100, your surge suppressor will most likely be what is known as the MOV (metal oxide varistor) variety of protection. There are other sure protection devices, like transient voltage suppression diode (TVS) and thyristor surge protection device (TSPD) but more expensive, and less common in home use. If you pay less than $10 it's still uses MOVs, but probably poor 1-way protection. More on types of protection later.
Some people erroneously think all power strips are surge protectors. Unless the UL label says "transient voltage surge suppressor" the UL rating is there only to rate them for their performance as an extension cord. If there is no UL label at all it's likely junk regardless of what else the package says, so don't buy it. However the UL label only means that the item has been rated to meet minimum standards.
All MOV surge protectors have three ratings, clamping voltage, energy absorption and response time. All of these ratings are very important to protecting your electronic equipment.
Clamping voltage - This tells you the voltage at which the MOVs kick in to conduct electricity, shunting it away from your equipment. In power strips there are three UL ratings for clamping voltages. These are 330V, 400V and 500V, with 330V being the best. Because this is the voltage that the MOV clamps at, it is also the voltage that will be allowed through to your connected equipment until the surge passes.
Keep in mind that your nominal 120 volt AC house current is measured as a RMS (root mean square) voltage. The actual peak of the constantly changing AC voltage, which is the important factor here, is substantially higher around 170V, and somewhat closer to the clamping voltage of a 330V MOV. Modern switch-mode power supplies in todays electronic equipment will compensate spontaneously to this temporary increase allowed by the MOV, protecting the delicate circuits they supply.
Energy absorption - This rating is much energy in joules (watt seconds) the surge protector will theoretically absorb before it fails. The higher the number here the better, but it should be at least 400 for power strips. I would recommend at least 600 or more. Higher energy absorption surge suppressors either have physically larger MOVs or multiple MOVs wired in parallel.
Response time - There is a slight delay as an MOV responds, or begins to shunt to the power surge. This rating is in nanoseconds or picoseconds. Look for one with a response time of < 1 nanosecond (billionth) or better yet picoseconds (trillionths). The longer this delay the longer your equipment will be exposed to the full unclamped the surge.
Type of protection - It is very important factor to look for the type of protection. You may have read packages that say 2-way or 3-way surge protection (if it doesn't say it's probably only the minimum 1-way protection). If this sounds confusing, it simply means how many ways the incoming power lines are protected, and is also a factor in price. Chances are if it's less that $10 it is only 1-way low energy absorption protection. Still 1-way is better than none.
- 1 way (Hot to Neutral) - Good
- 2 way (Hot to Neutral, Hot to Ground) - Better
- 3 way (Hot to Neutral, Hot to Ground, Neutral to Ground) - Best
MOV surge suppressors have a finite lifetime and will always wear out. When, is determined by how many, and how large the surges are that your surge suppressor is exposed to. Generally you will not know when they wear out unless they short circuit, which commonly only happens from lightening damage, or a very long and large spike in line voltage. The short circuit ends with the MOV burned up, and effectively removed. Your equipment will still work after they wear out, but you just no longer have any protection. As a rule of thumb the higher the joules (energy absorption) rating the longer the surge protector will last. At any rate it's a good idea to replace MOV type surge suppressors every few years.
Most people know better, but I have seen some people use those three prong adapters. These are mostly used by people in older homes not up to code, but don't use them. Using one will open up the ground, effectively removing the all surge protection from the circuit. Fastening the wire coming out of the adapter under the outlet plate screw won't help because the screw often either isn't grounded properly, or worse, is made of PVC or some other material that is either a poor or non-conducting material.