From a comment on How can I protect equipment against a lightning strike?

Most of the damage from nearby strikes comes from strong voltage gradients over the ground or between the ground and things near it, causing arcing, causing things near the ground to effectively be grounded. You don't need a whole Faraday cage to protect against this; just a ring of wide copper strap around a building will do, forming a sort of two-dimensional Faraday cage. Commercial broadcast stations do exactly this, and this is part of why they are able to stay on the air despite multiple strikes per year and never disconnecting the antennas. – Phil Frost - W8II Feb 27 '14 at 12:22

How wide?

Is the wide copper strap near the foundation?


A copper ring that encompasses the entire room and is located near the ceiling of the room is typically referred to as a halo ground. This ring is bonded directly to the external, buried ground ring for the structure. The purpose of the halo ring is to protect the contents of the room from strong electromagnetic fields from an EMP (e.g. lightning) that may otherwise induce currents in enclosures and other metal in the room.

In some installations, the halo ground is also used as the method to provide SPGS (single point ground system) connections since large equipment cabinetry is also subject to EMP induced currents. I was recently in a large state patrol communications room associated with a 200+ foot tower and more than 10 transmitters and repeaters that used this method of SPGS.

One and one half inch (38 mm) wide, solid (not woven) copper strap, with a minimum thickness of 26 AWG (0.0159 inch / 0.405 mm) is commonly used as a halo ring or SPGS conductor as it has far less inductance than say #4/0 wire. Wider and/or thicker copper is acceptable as it will increase current handling and further reduce inductance. The copper strap also makes it much easier to fashion drop connections to the various equipment cabinets.

The halo system is bonded by a minimum of one #2 AWG solid copper wire for every 25 feet of halo. These wires drop straight down the interior wall, exit at the base of the wall and are bonded to the exterior buried building ring. For a halo of 100 feet or less. It is recommended that a drop wire be installed in each corner of the room.

There are various standards and best practices documents pertaining to ground systems - some of which are in direct conflict with each other. You may enjoy reading Erickson's guidance in this area.

  • $\begingroup$ The impedance of the conducting path leading from an elevated "Halo" TO whatever serves as the actual EMP/r-f/a-c ground such as one or more conductor(s) buried in the earth has a large bearing on the performance of such protection systems. Is there a recommended upper limit for the impedance of such systems, including the impedance of the buried conductors? $\endgroup$ – Richard Fry Jan 30 at 11:04
  • $\begingroup$ @RichardFry I added some industry best practice information to my answer. $\endgroup$ – Glenn W9IQ Jan 30 at 11:41
  • $\begingroup$ The question seems to refer mainly to the exterior ring. Do you have any guidance on how it should be constructed? $\endgroup$ – mrog Jan 30 at 16:57

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