[TowerTalk] station grounding question

[Eric Scace K3NA]

on 05 Nov 23 08:03 Ian White GM3SEK said the following:
> [...snip...]
> 
> Don't worry - INSIDE the station, you are not expecting large lightning 
> currents in the RF ground bonding, so the grounding bolts on the backs 
> of the rigs are already quite big enough.
> 
> The purpose of ground bonding inside the station is to keep all the 
> equipment at the same potential as the panel, and you have already done 
> the right thing by using a wide, low-inductance strap for almost the 
> whole length of the run. What you use for the last few inches as a 
> jumper can hardly matter.

    Unfortunately, most residential buildings are nearly transparent to lightning-induced currents 
--  or any other kind of electrical activity.

    Envision your home without any of the non-metallic materials.  Since most of us do not live in 
an enclosed metal shell/screen (i.e., a Faraday shield), all the metallic bits and pieces of the 
building, plumbing, wiring, computers and radio equipment are simply hovering in the space just 
above the ground, as far as lightning and RF are concerned.

    Our buildings keep the rain out and moderate the climate -- but do essentially nothing to 
protect us against electrical activities.

    With this mental picture in mind, one can more easily grasped how exposed our equipment and 
wiring is to electric-magnetic fields of all kinds, including those produced by nearby lightning.

    Note that the bonding system must be low impedance at the frequency of the currents in question. 
  As frequency goes up, this becomes more challenging to achieve -- which is why wide copper 
strapping is helpful and braid is must less helpful.  Strap is used for its low impedance, not for 
its current carrying capacity, in this application.  (Bus bar also has low impedance, but is thick 
because it is used for a different purpose: delivery of high-current at supply voltages.)  Lightning 
radiates strong signals across a very wide part of the electro-magnetic spectrum.  (Notice how 
'bright' a nearby bolt of lightning is way up in the visible light portion of the spectrum.  It has 
similar 'brightness' or power all the way on down to DC.)  A nearby strike will induce currents 
across a wide range of frequencies on the metal, wiring and electrical equipment contained in that 
nearly-invisible-to-RF house... and the strength of those currents at each frequency can vary widely 
depending on the impedance & resonance at each frequency.

    The low-impedance earth bonding system helps in two respects:
    a) keeps adjacent chassis floating at similar electrical potentials, reducing the current flow 
between the pieces of equpment.
    b) provides a path to earth for most of the induced current which is of lower impedance than 
other, more damaging routes (e.g., via signal cabling).

    (a) above also reduces the danger of electric shock to operators when a fault occurs within a 
piece of equipment for whatever reason, since fault current flowing into the chassis will lift its 
potential and that of the neighboring bonded equipment similarly.  (Fault current should be 
short-lived if the protective systems [fuses, etc] have been properly designed.)

    Note that no bonding system can keep equipment at the SAME potential to its neighbors, to the 
'entrance panel' (which is also floating in space), or to the earth.  There is always SOME 
impedance, greater at higher frequencies, and therefore SOME potential difference (voltage).  A 
well-designed and -implemented system will keep these impedances low and the differences small
across a wide range of frequencies down to DC ... but it can never be zero.

    As a result, the higher the current flow (induced or fault), the greater the differences between 
various bits of equipment and cabling.  And, the higher the frequency of these currents, the greater 
the differences across distance -- i.e., the voltages at one physical spot will difference from 
those a few meters away.  With sufficiently high induced or direct current, the differences will be 
too great and equipment damage will occur.  With lower impedances across a wider frequency range, an 
installation will have a greater tolerance for these currents before damage occurs.  But every 
installation will have a point at which damage can occur, so nothing is 'lightning proof'.  We build 
for a degree of lightning and fault tolerance to hopefully cover most events (e.g., nearby strikes), 
and accept that we will have to rebuild when overwhelmed by an extreme event.  (That's what 
insurance is for.)  Those in lightning-prone areas will have to invest more heavily in better (and 
more costly) techniques to reduce failures to an acceptably infrequent level, or re-build more often.

    Braid has much higher impedances than strap and, as Frank has pointed out, its use should be 
avoided whenever possible in the construction of an earthing system.

    Sorry this was a bit long.  I was in a rush and didn't take time to make it brief.

73,
    -- Eric K3NA