I was chief engineer at a TV station in
USA.
Everything was bonded to a single point ground.
Inside the building we used 6" flat copper strap.
We took many strikes to the 2000' tower, but never had an
equipment damage.
With a single point ground, during a lightning strike, every thing
elevates to the same level, at the same rate.
There is no damage as everything is referenced to the same point.
With a multi point ground there will be a difference in potential
between equipment between the multiple points.
The is where damage occurs.
There will be a difference in potential between the grounds.
Reference MIL-HDBK-419, available on the web for more information.
Motorola generated their own site selection document (R-56), Our
FAA (Federal Aviation Authority) as well as others generated their
own standards documents, all based on MIL-HDBK-419.
Properly single point grounded sites suffer minimal damage because
of the single point ground.
A multi point grounded system invites lightning damage.
Glenn
On 11/16/2024 1:52 PM, Mike EI9FEB
wrote:
On Sat, Nov 16, 2024 at 05:10 PM, Glenn Little wrote:
How do you prevent damage from a lightning event if
everything is not
referenced to one ground (earthing point)?
The potential difference between the two, or more, earth
points can be very
substantial.
This potential difference is what causes the damage.
Glenn
A single "ground" makes little or no difference. You need all
aerials and power cables, Cable TV, phone cables disconnected. A
strike even 100m from the utility cables (even buried
electricity, phone, cable TV etc) can induce 1000s of volts and
destroy front ends of TV, radio, DSL modem, cable modem,
analogue dial up modem. Even blow the tops of the RS232
interface ICs on the motherboard (amazingly a plug-in RS232 card
and new modem worked once they repaired the exchange, the Mobo
survived).
There are reasons for a single electrical safety earth do to
with ELCB / RLCB / RCCB tripping and safety. As mentioned, the
UK mostly is a different scheme (3rd wire with earth to
substation) to Ireland (an earth spike per meter box and no 3rd
wire). Unlike USA, UK & Ireland homes are only single phase.
The houses alternately use the three phases in built up areas,
so don't share any electrical device connection with a neighbour
without suitable isolation or barrier earth (shared TV aerials
or sat dishes).
Radio earth connections want to be short to earth spike so
there may be more than one.
Solar systems on a roof need the frames to a single earth to
minimise static damage, useless on a direct or close strike. The
battery negative should not be earthed except in certain
circumstances. The neutral on an off grid PV inverter needs a
single earth before the trip switch. Earthing (other than the
isolated metal frames) of PV systems connected to a grid need an
expert.
A Satellite dish or Aerial pole should go to a local earth
spike to minimise static (dry wind) or damage from nearer but
not direct hit lightning. The satellite dish electronics (ODU,
LNB etc) should not be directly earthed.
It's complicated. But a single earth doesn't help with
lightning. Disconnecting everything works best, but not fool
proof if you get a direct hit.
See also EMP.
--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA-LM ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
