As an amateur radio operator for 60 years now I have been aware that a major worry vis-a-vis lightning is not so much the very rare "direct hit" that blows up the house, but the far more common "near miss" in which a very large electromagnetic pulse will spread at the speed of light whenever lightning strikes nearby. I have been taught that this pulse (EMP) will induce a substantial voltage on antenna structures but also on power lines coming into the house. In this context, how does having the neutral wire (grounded at the home's AC service panel) provide ANY protection against a voltage surge on the AC supply going into the house? I am confused about this despite much reading on this great web site.

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    \$\begingroup\$ What makes you think neutral adds any protection? \$\endgroup\$ – PlasmaHH Apr 8 '18 at 20:43
  • \$\begingroup\$ Read Lightening Protection PDF for the usual safety protection of people inside. But if you are wanting to protect sensitive electronic devices from EMP, I'm not sure you can take any reasonable measures to prevent that. There are unreasonable measures that work, but they are expensive (multiple stages of Faraday cages, in layers.) Also, read this and associated links: EDN, EMP Damaged TV Tuner Teardown \$\endgroup\$ – jonk Apr 8 '18 at 21:06
  • \$\begingroup\$ What "makes me think the neutral adds any protection" is the frequent mention I have found in some sources that the neutral being grounded "offers lightning protection". \$\endgroup\$ – NK7B Apr 8 '18 at 21:45

The induced pulse from a nearby lightning strike is a common-mode signal with respect to the power line, which is a balanced (differential) connection to the pole transformer. (I'm assuming that you're in the USA, with the usual 120-0-120 "split phase" power connection.)

The neutral is grounded at both ends — i.e., at the pole transformer and at your service entrance. At least one connection is required to keep this link from "floating" to arbitrary voltage levels with respect to ground. Two connections serves to short out the common-mode surge from lightning strikes.

In other words, if the neutral (center tap) was grounded only at the pole transformer, the common-mode surge induced along the length of the cable would enter the house, rather than being shorted to ground. Similarly, if the only ground was at the service entrance, then the pole transformer would experience the surge, possibly exceeding its ratings and causing an internal breakdown.



simulate this circuit – Schematic created using CircuitLab

The best protection is an MOV and a good line filter.


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