Insulation Testing After Direct Lighting Strike

TheGingerElectrician

Master Electrician Electrical Contractor, TN
I recently had a customer who had a direct lighting strike to his home and it went to each corner of the house via the flood light circuits. It even blew off a flood light on the opposite end of his house. There is definitely damage to one of the flood light switch legs from this but I don't know where. We will be replacing that particular wire. I'm assuming an insulation test on the affected circuit would be extremely appropriate even for the flood lights that are working. Anybody have experience in this specific area and advice to give? Thanks!
 

hbiss

EC, Westchester, New York NEC: 2014
I think a megger test of all the wiring in the house is appropriate.

-Hal
 

GoldDigger

Moderator
Staff member
For wiring in conduit, it is likely that an insulation damaging arc to the conduit will leave a residual contact or partially conductive film that will likely show up in a megger test. In the case of NM a megger test will likely show up conductor to conductor arc damage, but is less likely to detect the results of a discharge to a nearby object or surface. Unless the surface involved is solidly grounded there may not be a way to megger that path.
JMO.
 

TheGingerElectrician

Master Electrician Electrical Contractor, TN
For wiring in conduit, it is likely that an insulation damaging arc to the conduit will leave a residual contact or partially conductive film that will likely show up in a megger test. In the case of NM a megger test will likely show up conductor to conductor arc damage, but is less likely to detect the results of a discharge to a nearby object or surface. Unless the surface involved is solidly grounded there may not be a way to megger that path.
JMO.
So you are saying you don't think that an insulation test would prove useful? In the case of Romex the ground wire is in the same jacket with the other current carrying conductors. You don't feel that the circuit affected could be tested properly? In addition, are you saying that if the insulation was damaged but not touching something with an effective ground fault current path that the megger test wouldn't pick that up? What is the point of a megger test for wire insulation in this case?
 

GoldDigger

Moderator
Staff member
So you are saying you don't think that an insulation test would prove useful? In the case of Romex the ground wire is in the same jacket with the other current carrying conductors. You don't feel that the circuit affected could be tested properly? In addition, are you saying that if the insulation was damaged but not touching something with an effective ground fault current path that the megger test wouldn't pick that up? What is the point of a megger test for wire insulation in this case?
If the fault within NM is to the EGC, then a megger test would have a good chance of showing it. A megger test can detect degraded insulation that does not conduct measurably at the nominal voltage, but not the result of an arc over a larger distance unless a conductive trail of char is left behind.
I guess the point is to find faults that are likely to continue to worsen under use with the working voltage applied, not to identify all forms of concealed damage. A megger test could be quite useful if the wiring is in a flooded conduit, for example.
 

hbiss

EC, Westchester, New York NEC: 2014
Funny thing with lightning strikes. If it were my house I would insist on a complete rewire because there is no real way to be 100% sure.

-Hal
 

ptonsparky

Senior Member
And when you do the merger test, what benchmark is appropriate for a pass or fail?
Good question. It would be best to have had testing done prior to the damage as a bench mark for comparison.

I don’t have the answer. Others will chime in.
 
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