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House protection against lightning


Dear Friends,

As a High Voltage electrical engineer with over 20 years experience, I was always hit with the problem of protecting my circuits and equipments against surge voltages. Think about working here for the nuclear industry at a plasma radiation source in Tulahoma, Tennessee, where this machine was generating a pulse of over 1 Terrawatt in 300 nanoseconds with over 1 million Volts and you need to protect your 5V electronic equipment that is located at 12 inches apart against those 1 milllion volts!

The same thing happen into a house bombarded by lightning, the only thing is that the equipments from the house are far away by lightning and not at 12inch. The lightning is a voltage discharge (plasma) between two bodies charged at two different voltage potentials (the earth that is zero volts and lets say a cloud static charged at 2 million volts) Because the ionization of the atmosphere becomes so intense at one point the electric field between the earth and the cloud breaks off producing plasma arcing or lightning. This lightning discharges tremendous quantities of energy, mega joules of electric energy and if you or your house are luky enough to be around this path you can get lots of damages.

There are many ways to protect your house against lightning. If you have a metallic roof on your house make sure and connect it to a lightning rod to ground. The best is to take a cooper rod ��in thickness and clamped in a grounding electrode system. Good grounding electrode systems recognized by National Electrical Code are:

So the first thing to protect your house and your electronics from your house is to make sure your service panel is very well grounded and the neutral wire is very well grounded to such grounding electrodes. This depending on the quality of the work done by the electrician when he wired the house. This means how well are the electrical contacts tighten in to the panel. Eventually if the aluminum splices in the panel are well protected against oxidation. If the grounding electrode is short to the point of contact in the earth. But the lightning strike can come into your house not only through the 120V electrical cables but also the telephone and cable TV or computer line path. This is why so important to keep all these cable shields grounded to the electric ground system and eventually use some metal oxid varistors (MOV-s) at over 50V working voltage, for telephone and cable TV. The metal oxid varistors are components that have the resistance almost infinite when apply a voltage within working range. When apply a surge voltage over the working voltage the resistance of the MOV decreases when voltage increases. The means at a lighting surge an MOV shorts out the input leads. The disadvantage of MOV is that during the lightning process might permanently short out and get damaged so needs to be replaced. But with several dollars you can save hundreds spent on TV , telephone or other electronic equipment that has link though a wire with outside world.

The most effective way in protecting your house against lightning is to install at the service panel surge protectors in a hardwired mode. This one might cost you one hundred dollars but is very cost effective in protecting your branch circuits from the house. This type of surge protectors might be installed in the service panel or outside service panel against it. The advantage of installing inside service panel is that the connecting leads can be mounted very short making very effective, but has a disadvantage that when fit by a big strike of lighting can explode and can make a mess in the panel. Another type of surge protector can be installed outside in a rugged metallic box flush mounted with the panel and keeping the connecting leads as short as possible in order to increase the speed of answer for the surge protector.

Another type of effective surge protectors are the plug in surge protectors. They can be plugged individually at the point of entry for each equipment: telephone computers, TV but please buy only UL accepted type ones because many are on the market that don�t fit the needs. This type of protectors cannot do the job of hard wired surge protectors mounted in the service panel. The cannot capture hundreds of joules of energy as does one surge protector mounted on the panel. The plug in protectors are more for protecting the electric noise from the house generated by some appliances from the house such as well pumps and dishwashers.

To me the most effective way in isolating electronic equipments against noise and lightning is installing on the branch circuits to be protected separation transformers. These are type of transformers with the same input/output ratio and are very effective in protecting 120V branch circuits. This might sound industrial method but is very effective. The disadvantage is that separation transformer are all the time in circuit consuming some energy even when nothing is fed and are bulky and the circuit need to be dedicated for residential type.

Thank you,

Emil Chirvasuta

KDE Electric manager

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