Originally Posted by Chu Gai
Joules are a very rough indicator of longevity. I wouldn't overly obsess over this but I'd expect something that has say 2,000 joules to last signficantly longer than something that had 200 joules.
Joules are an irrelevant metric for Transient Voltage Surge Suppressors (TVSS). The Joule rating only predicts the total amount of energy that can be absorbed by the TVSS; it does not tell you how quickly it can absorb it, nor how effectively it maintains a safe Voltage for your equipment.
The things you should care about are:
1) Clamping Voltage. This is the minimum Voltage at which the TVSS begins to conduct (which is what clamps the transient). This tells you how much residual Voltage your equipment will see. Lower is better, but only to a point. If the clamping Voltage is too low, the TVSS may conduct during a steady-state overVoltage condition; this results in premature TVSS failure. [Caveat: This is the clamping Voltage at the TVSS terminals, so it does not guarantee that this is the Voltage your equipment will see.
2) Peak kA. This is the amount of current that the TVSS can handle during a transient event. This tells you how rapidly the TVSS can absorb the energy in the transient; if you exceed the kA rating during a transient, the TVSS fails, and your equipment gets the rest of the surge. [The Peak kA rating also gives you a rough idea of the lifespan for most common TVSS designs.] Higher is better. [Caveat: The wiring to your TVSS must be short and non-inductive enough to allow the Peak kA to actually get to the TVSS. If it is not, then you are not getting the full protective benefit of the device
3) Clamping time. This tells you how rapidly the TVSS device begins to conduct, which tells you how much of the transient rise time will be let through to your equipment. Faster is better. [Caveat: A slower device mounted at the service entrance will often outperform a faster point-of-use device, due to system impedance
Some other background info:
TVSSs of the sort used in almost all consumer products are MOV-based, which is perfectly fine for just about any home use - IF SELECTED AND INSTALLED CORRECTLY. MOVs are sacrificial devices; as they absorb transient energy, they degrade over time. This degradation slowly increases the clamping Voltage, and eventually results in catastrophic failure of the MOV device (invisible from the outside of the device). Thus, an older TVSS may not be providing effective protection even though it still appears to be operating correctly.
Grounding is the single most important part of surge protection. If you don't have a solid earth ground, or if the impedance between the TVSS and that ground is too high, the TVSS will not be able to operate effectively.
Most properly designed devices today have internal surge protection sufficient to handle a reasonable level of transient energy (within the CBEMA/ITIC limits). In fact, anything with a CE mark will have to have been tested to meet the Low Voltage Directive (LVD), so it will have passed a basic EFT test. HOWEVER, this does not mean that a TVSS doesn't help. The equipment power supply probably has MOVs in it; since the MOVs have a finite lifespan, using a TVSS to absorb the bulk of the surge energy will incerase the life of your equipment's power supplies.
Transients come from many sources, not just lightning. Switched capacitors, noisy motors, arcing fluorescent tubes, etc., can all cause transients on your power lines.
The absolute best transient (surge) protection will be accomplished through the following:
1) Put the primary TVSS unit at the service entrance or main panelboard, tied directly to the main building ground. The wiring should be as short as possiblem, and it should be connected either directly to a dedicated breaker in the panel or the main feed. Putting the TVSS at the main service entrance will reduce the Voltage let through to the equipment far more effectively than putting the TVSS in the equipment rack, because the impedance of the in-wall wiring works in concert with the TVSS to reduce the let-through. Look at it this way: Assume the TVSS lets through a 1 ms 800V spike. If the TVSS is in the rack, your equipment will see 1 ms at 800V - exactly what the TVSS lets through. OTOH, if the TVSS is at the panel, the impedance in the in-wall wiring will reduce the remaining spike by some amount, so your equipment will see something more like 0.7 ms at 500V. A local rack-mount TVSS can help to reduce the remaining transient levels, but the whole-house system is the main and best source of protection.
2) The main TVSS unit should cover all signal lines from outside the building, including anything that is connected to your equipment. All of these should be tied to the same groundnig point. It does no good to tie your power line to ground if a transient takes the whole ground plane up by 1500V and the cable stays at 0. Your equipment will be just as fried as if the 1500V spike had come in on the cable line. [Caveat: DO NOT install a second ground system into your house - this can be extremely hazardous. High-speed, high-energy current flow operates very differently than the "normal" electrical circuit theory everyone understands - don't risk it unless you have serious experience in grounding and bonding distribution systems.
3) Series filtering is a good idea, and is a "premium" adder to many TVSS devices. However, it may not make any difference if you didn't have a noise problem to begin with.