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Electrical Tip of the Day: Testing wires for lightning damage – MegOhm test

Here in the Lafayette area it is summer storm season, and it has been a very bad year for high yield lightning strikes.  We have a number of jobs going on right now where we are working with clients to get damaged wiring in homes replaced.

The question that always comes up is:  “How do I know if there has been damage, is there a way to test the wires in the walls?”.  Thankfully, the answer is YES.  We do in fact have a way to test for this.  Its called an insulation resistance test, also know as a MegOhm test or sometimes referred to in the trades as “Megging”.

The theory is simple really – the electrical resistance of the insulation on a conductor or in a piece of equipment can be tested and measured by isolating the conductor from ground, applying a high voltage over time, then measuring the results.  In most residential situations we use a 300 to 500 volt DC test over 10 minutes.  When done with care and by knowledgeable electricians it is a very reliable test.

So – here is the process for preforming a megohm test in a home:

1) ALL electrical current to the dwelling must be shut off

2) All appliances, light fixtures, heating and cooling equipment, dimmers, GFCI’s, AFCI’s, low voltage systems and any “plugged in” or “hard wired” equipment must be disconnected from the circuit(s) to be tested

3) The best practice at this point is to then isolate each individual wire – taking apart every splice, junction, and connection in the circuit to be tested

4) Next, the testing of the insulation can be preformed – this part actually goes pretty quick

5) After testing any conductor that fails to meet the testing standards should be considered damaged and marked for replacement

So, now a word of caution is in order.  If you have had a major strike and the  electrician, general contractor (or restoration company) helping you out with the repairs has not told you about this process, then GET ANOTHER COMPANY.  MegOhm testing is really the only way to know if damage has occurred, and even then it is not a fool proof testing process. You can’t be sure by visible inspection alone, and you can’t visibly inspect most of the wires in a home.  Lightning is crazy stuff, it has a super high voltage and travels very fast.  If your home has been hit tell your insurance agent you want it tested!  We also HIGHLY RECOMMEND that any home that has had a strike install Arch-Fault breakers on all the branch circuit wiring, as well as make sure proper fire alarms are in place (and a quality A-B-B fire extinguisher is located on each floor).

Here are some pictures of lighting damage from a few projects we have going on now

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