[TowerTalk] Tower grounds
jimlux at earthlink.net
Wed Jun 13 07:58:34 PDT 2012
On 6/13/12 6:21 AM, K8RI wrote:
> On 6/13/2012 7:53 AM, David Robbins wrote:
> But the rise time or Dv/Dt of lighting is much lower in frequency than
> that. OTOH there is no really typical lightning strike as they cover a
> very wide range of rise times and power. The only thing predictable
> about lightning is its unpredictability.
it would be the (very) rare lightning strike with a rise time faster
than 1 microsecond. The vast majority of "lightning induced
transients" (which includes both direct strikes and induced transients
from nearby) are much slower.
While the power spectrum of a lightning impulse does extend up to VHF
(otherwise, why would 40 be so noisy these days), there's not a lot of
energy there. From a lightning protection standpoint, energy is really
what's of concern. (once you've got past the simple transient breakdown
One thing to bear in mind is that standards like NFPA 780 (lightning
rods, etc.) and NFPA 70 (NEC) are concerned about personnel hazards
(shock and electrocution) and structural damage (not starting a fire,
mostly), not transient protection.
and none of the safety standards care a whit about RF loss or antenna
>> Legally there can be only ONE ground. Most building codes require that all equipment be grounded to the same building power entrance ground for safety purposes. Fortunately doing this also helps with preventing voltage differences between equipment and you in the shack due to rf currents on coax shields or other cables. Note, it does not prevent the currents, nor will it 'drain' them away to 'ground'... it just keeps all the equipment in the shack at the same potential so you don't get bit when you touch two different things.
> This works only if all the circuits (including phone, coax, Telephone,
> TV antenna, and what ever else) are the same length and follow the same
> path after they enter the house or pass the ground. Quite often it's not
> possible to do things this way as the antenna system, (towers and
> antennas) are on the opposite side of the house from the electrical
> system entrance. Very few homes are wired in a fashion to make this
> possible. Most older homes have circuits connecting outlets in one room
> to lights in another, daisy chain fashion. You may find circuits that
> alternate lights and outlets running the periphery of the home and this
> is not going all the way back to "knob and tube". My single point
> ground is on the back of the house where the important stuff enters with
> a direct, large cable taking a direct route to the house electrical
> ground. There is no practical route for a peripheral ground around the
> house to the service ground as it'd be well over 100 feet long going
> either direction. Legally there is only one safety ground if they are
> all tied together and this service entrance ground must be within a few
> feet of the entrance. It's usually of such a nature that it only serves
> as a safety ground.
Yes.. and at 60 Hz, the length of the wire ("bonding conductor")
doesn't make a heck of a lot of difference. All they're concerned about
is that it is mechanically sound and can carry enough current to trip an
overcurrent protection device. The requirement for AWG 6 is more based
on mechanical strength so that it's not inadvertently broken, more than
resistance, inductance, or ampacity. Ditto the "shortest practicable
route" requirement. Ditto the "one continuous conductor" requirement.
For lightning it serves as a tie point rather than a
> sufficient ground for lightning. The single point ground for the
> station is adequate for most lightning strikes. For the tower I run a
> ground out radially from each leg for about 80 feet. Adequate ground
> rod spacing is about twice the rod length (see above polyphaser link)
> and the radial grounds are tied together for a total of over 600 feet of
> bare #2 CadWelded(TM) to 32 or 33 8' ground rods There are two parallel
> ground lines from the tower to the SPG with ground rods every 16 feet
> (+/-) and polyphasers on each coax.
From a transient protection standpoint, things that are close together
physically should be tied together electrically by short wires
(inductance is more important than AC resistance) to limit the
Your "tie point" term is a good one. You don't much care if that tie
point floats up to 1000V during a transient, as long as everything
floats up together.
Where we get into trouble is when there are clamping or triggered
breakdown devices that limit differential voltage (between things that
aren't bonded) When the clamp "closes the switch", the "wiring diagram"
changes. There are situations where a spark gap type device (one that
has a very much lower "on" voltage than the trigger voltage) can
actually aggravate problems. One problem is that the switch can be very
fast and carry significant current, so the fault (which could actually
be a 60Hz line current fault) becomes a fast di/dt generator, inducing
transients in connected or nearby victim circuits.
The other problem is when the clamping device creates a preferential
path for high fault currents that wasn't intended. Imagine a scenario
where you have a very low impedance connection to earth ground that
can't carry a lot of current (ground lead on an oscilloscope probe, for
instance). The primary ground is higher impedance for the transient, but
can carry a lot more current (a bus bar). Oscilloscope input is
connected to bus bar, shunted by 1 Megohm input Z of scope + vacuum
spark gap overvoltage protection.
Big transient with a lot of energy comes along (say from a big capacitor
discharge in a Marx bank or a lightning strike). The spark gap switch
between bus bar and oscilloscope ground fires, creating a lower
impedance path for the transient, through the spark gap and oscilloscope
lead. In this particular case, the ground wire vaporized, negating the
effectiveness of the spark gap clamp, causing the HV transient to
propagate into the scope where it found other devices that clamped the
transient internally, and dissipated the energy by melting and
vaporizing them. HOWEVER.. the system was "safe", in the sense that the
case of the oscilloscope never had high voltage on it, relative to the
potential operator standing touching it.
It was a bad experimental system design.. there should have been a big
series resistor between bus bar and oscilloscope input. that would limit
the current in the event of a internal short inside the oscilloscope,
which is essentially what the clamp spark gap is.
Same sort of problem crops up in HV power supply design. You need to
design for the case where the output is shorted to ground, and if you
have a series pass regulator that can't take the whole voltage, it dies.
> The ground system on my towers would be a very poor RF ground for a
> vertical antenna. OTOH all equipment in each station is bonded
> together. Also the soil is quite acetic which means no bolted ground
> rod clamps. After a year in the ground you can usually pull the clamps
> right off the rods. When we put in an underground electrical service
> the ground rod clamps for the service entrance could be lifted right off
> the rods as if some one had already removed the screws, yet the
> electrician put back the same arrangement. The electrical inspector
> told me that was code, but I could go ahead and cad weld them.
I think that's the reason why compression clamps can't be buried, but
have to be in inspection wells.
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