crossy@home, on Nov 6 2007, 09:40 AM, said:
The only way to completely protect your valuable equipment is to unplug everything including the antennas.
That is very true, and we might do it from time to time in extreme situations like very heavy storm activity directly overhead. Many times though our equipment stays connected, for better or for worse.
This is an interesting application area (surge protection). Generally not well understood, but has high interest value (I need something to protect my equipment). Probably a marketer's dream, hopefully tied in with a well engineered product.
I'm sure there are many products out there that don't provide much protection, even though they say they do. And some that could work reasonably well, but may not because of the way they are installed, or have to be installed (just plugged in).
Surge arrestors come in many different designs and types, but all are intended to do much the same thing. Clamp the voltage from going too high before the circuit is damaged. And in clamping, be able to effectively bypass a high current as required for the relatively short period of time as the transient occurs.
That circuit to be protected may be the power line distribution system, or it may be the TV. Or other things in between.
Products like
High Voltage Surge Arrestors,
Surge Arrestors/Suppressors,
Telecom Power-Surge Arrestors,
Metal Oxide Varistors all have their place in the chain of protection.
There is a lot of energy in a
lightning strike. The
energy of an average 3 mile-long lightning strike is one billion to ten billion joules. Typical currents from 5,000 to 20,000 amps.
Hopefully the power company has done all the major things it can do at on its side to clamp/dissipate strike energy on high voltage power lines, transformer sub-stattions, pole-mounted transformers etc.
As consumers, all we can really do is put further protection at our switchboxes/switchboards (directly where the power arrives, and where there is a good earth). The effectiveness of the voltage clamping at the switchbox will be improved (compared to elsewhere in the house) because the incoming power line(s) can be held down voltage-wise by a device connected to them and earth.
This device will need to respond very quickly to a rising surge, and clamp and dissipate energy for as long as needed (usually a fairly short time). That is why surge arrestors are rated in Joules (watt-second, power over a time period). They can take a higher current for a shorter period, or a lower current for a longer period.
So in a normal electricity distribution system, surge arresting devices are cascaded throughout. While there was a comment above about having multiple devices, and concern that protection would only be equivlent to the weakest devices, I don't think that would be correct in practice.
If you imagine a 'test' switchboard for a moment, with a couple of incoming power lines. These lines are all clamped to earth by a whole lot of different surge protection devices, from gas-filled arrestors, all the way down to Metal Oxide Varistors. They all have slightly different characteristics, including response time, clamping voltage, and energy rating.
Apply a large transient, with a certain amount of energy. Depending on the voltage and current present, and the characteristic of the rising transient, various devices will operate, some before others. You will most likely get the smaller rated devices working first, and that may be the last thing they do. If you apply too much energy to a MOV, for example, it will just go open-circuit.
However, some of the devices will clamp and work, if the surge is within reason. Many of the small ones will be gone though.
In a real life situation, what you have are a whole lot of different devices, providing protection at different points in the system. As the tarnsient travels through the system, hopefully they will all trigger and clamp where needed. So what comes down the line is a smaller and smaller transient. And the effectiveness of a device is actually helped by having all that cabling up stream of it, because that introduces resistance. This resistance has voltage developed across it as the surge arrestors desperately clamp the transient, and so reduce the energy they have to dissipate. This is why if all arrestors are in parallel in the test example above, the little ones have no chance. They will vapourise.
In household surge arrestors, the active devices are almost always varistors (literally variable-resistors, as the voltage increases, they look more and more like a short circuit). Metal Oxide Varistors. MOVs.
And they are usually installed between all wires in the outlet (Active-Neutral-Earth). So they try and clamp a surge between any two wires to an acceptable level. They have response times (like 1nS) and an energy rating (Joules).
A surge arrestor at the switchboard will be more effective, hopefully clamping a transient to earth before it enters the house wiring. But an outlet board may help in some situations.
As for the insurance coverage for devices protected by products, that is no doubt good marketing. It would be interesting to see figures on claims versus sales after a period of time. That isn't to say that some products are better than others, I'm sure they are. But in selling these products, differentiators are needed. Good build quality, other filtering for noise/interference, protection insurance.
Not unlike 5 year warranties on hard drives. Hopefully they are better made because of this, but in the end, if your data has gone, all they need to do is replace the hard drive. Believe me, that is the
inexpensive part. I suppose in this situation, they have negotiated a coverage policy the costs of which are offset against potentailly increased turnover.
And that must be a risk they are managing. If there is one thing that is unpredicatable, it is lightning.
0
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or am i still too hungover from last night to correspond whats going on... 
