OK on the grounding rods, I might put one more in by coming 8' away from the house and doing another 8' rod, that's something you can do at your leisure.
A joule is a unit of energy over time. Your house measures energy use in Kilowatt hours (the energy you use drawing 1000 watts for 1 hour). A joule is the energy (or "work" if you want) done when 1 amp flows through 1 ohm for 1 second. So the units are not kilowatt hours but watt seconds. The joule rating of a surge suppressor is a maximum peak rating. The "work" done when suppressing the surge (by shunting it to ground) is generate heat. Some of the heat is in the wire, some in the device, and some in the ground. So the bigger the suppression device, the bigger the hit it can take and the more heat it can generate without burning itself out. In reality, the peak current can typically only be sustained for very short periods of time (like a thousandth of a second or less). So you can see where the marketing types have lots of fun talking up 5,000 and 10,000 and 20,000 joule protection when in fact the device is a small disk about .75 to 1.25 inches in diameter that can only have a sustained power dissipation of a few watts.
So for the non marketing world, the higher the joule rating the bigger of a spike it can safely take to ground. Higher joule rated devices also tend to have lower resistances when they shunt the spike, and therefore dissipate less heat, which is in part where the higher rating comes from. And at higher currents, the wires and ground also share in dissipating the energy by turning it into heat.
An MOV is a Metal Oxide Varistor. It came about in the 1930's or so, and GE made great strides in mass producing them during the 1950's at the Schenectady NY facility. Companies all over the world make them now. The MOV is placed from the line being protected to ground. When the voltage across the MOV reaches a certain level, the MOV start to conduct current. This process is pretty abrupt, and happens instantaneously. A MOV that is happy seeing 130V AC across it can start to conduct around 150V and by the time the voltage is up to 300V it wants to conduct hundreds of amps. This is how it "eats" the spike and shunts it to ground. Now if you do the math, think about this poor little 1 inch disk (about 1/8 inch thick) with 300 volts across it and 100 amps going through it. That's a peak power of 30,000 watts. Sure looks like a job for a 20,000 joule device. So if the device can conduct that 30,000 watts for 100 microseconds (about the time of the peak ringing of a lightning induced fault), then the actual work or heat dissipated by the device will be about 3 watts. The disc will quickly heat up, then cool down, and survive the event.
In reality, it's better than that because the drop on your wires with all that current from the fault will also help dissipate the current, as will the current through the ground. This is why there is all the discrepancy regarding the 30,000 watts peak on a 20,000 joule part, the times, etc. I guess I'm saying accept the hand waving argument here, and realize that spikes are short time events that product high currents that the high joule rated MOVs can shunt to ground.
Long answer for the three letter "MOV?" question isn't it :laughing: ?
As for how often do you replace the device, there are two more things to learn about MOVs. The 1st one is that when they fail, they fail to a short. So you always need some fusing or circuit breaker feeding them. When you see the plug strips with the neon bulb or led that says "Protected when lit", that means that the internal non replaceable fuse that is in series with the MOV has no blown yet and the MOV is good. If the MOV fails, the light goes out, and it's time to replace the plug strip (or whole house surge suppressor). The second thing is that every time an MOV is kicked, it looses a little life. So the lifetime is a function of how many spikes it has seen, and how big they were. Big spikes mean more wear and tear. The higher the joule rating, the bigger the spike it can take, and the longer the life for any given use. Houses are filled with little semi-low energy voltage spices when motors turn on and off. The MOVs can handle these and have a very long lifetime.
So for normal use, the protectors can last for decades. If you get wollaped hard, they can get used up in one pop. This is why it's good to have some way to know if it's OK, be that an indicator on the surge suppressor module or a popped circuit breaker on an raw MOV. You should check it every now and then, especially when you have those fun close to the house lightning pops
.
There are other devices that can be used, most notably a gas tube. They are more expensive and are not used for most residential surge protection systems on the AC. They are used for telephone where the energy levels are smaller and the small wire gauge means lots of drop on the wire during a spike.
A summary for those who could care less about the theory of it all:
1) More ground rods better. For you, I'd add one more, this is a judgment call with a YMMV. Local knowledge of soil types and moisture come into play here.
2) Higher joule rating on the device is better- can handle bigger spikes and lasts longer, but it cost more.
3) Device should have a indicator to show if it's OK or not.
4) These should be long life devices, but if they get kicked enough it will shorten their life.
I think the electrician doing the install is good, and I hope what I've told you can help you select a good device.
Hope my too long explanation helps, if you've got more questions ask. If you look again at the picture from NSbound, it will make a lot of sense now. Looks like a good unit to me!
Pete
