On 7/11/13 6:08 AM, Bill Weinel wrote:
Hi Skip,
Theoretically, you'd think ground potential should be the same everywhere.
This is not true, because a point current source in the ground at some
distance between two separated ground points will cause a different voltage at
each ground point caused by the resistance between the voltage source point
and the ground point by Ohms law [E=IR]. (Unless the distances happen to be
exactly the same, and the ground resistance is uniform, the voltage measured
at each point will be different.)
Differential ground voltage rises are caused during a lightning strike by the
surge arriving at two separately grounded circuits at slightly different times
caused by the distance of each ground from the point source. This causes
significant voltage differences (sometimes on the order of hundreds of volts)
between the two circuit grounds for a number of milliseconds and thus causes
damage to any loads connected between those circuits.
Considering that the overvoltage transient/fields propagate at close to
the speed of light, the difference in time is more like microseconds for
any reasonable sized house. (Even the Spelling/Ecclestone 50k square
foot mansion is only a few hundred feet across, and that's <1
microsecond light time)
Difference in ground potentials can result from a couple of causes:
current flow through a resistive medium (e.g. the soil under the house).
Lightning hits or a power line falls in the back yard, and there will
be a potential difference between front and back yards due to the
current flow and the IR drop. This is often called "step potential", as
in the potential difference when you take a step. No time difference to
speak of.
If you've got ham gear in the back of the house "grounded" to a stake in
the back yard, and the electrical system is "grounded" to a stake at the
front of the house, then there's a potential difference between the two
stakes.
if the only path between the stakes is through your gear, then it fries.
If you have an alternate lower resistance path (e.g. a wire that's not
something like 30 AWG magnet wire), then the current will flow through
that wire, rather than through your delicate semiconductor junctions.
Note that the current isn't going to be all that high (no kiloamp
currents), so you don't need BIG wire (the inductance limits the current
and rise time). Your goal here is to make sure that the voltage on
either side of something fragile (semiconductor junctions in the
equipment) is low. Not that your ham rig on the back porch is at the
same voltage as the ground rod in the front rod.
In your case, differential ground voltage rises is likely what took out all
your gear.
If you do some research on the subject, you'll find that lightning prevention
experts recommend that all grounds in a building be bonded together to a
common ground point. This point should also be the entry point for all
services coming into and out of the building. This is known as a single point
ground. The purpose of this is to prevent differential ground voltage rises
during nearby a lightning strike.
Not so much "single point" as "equipotential". You want everything in
the house to move up and down together, and if there is a difference in
voltage between places (due to finite propagation speed), that the
current resulting from that difference goes some place "safe".
Lots of buildings don't have a single point ground, but DO have a ground
grid. Imagine if your house had a solid metal floor. A piece of
equipment is placed somewhere on that floor, and the "exterior wire"
comes in and is bonded to that floor next to your equipment (or the
transient voltage protection is between the exterior wire and the ground
"next to the equipment"). If this is the case, one side of the building
might be at a different potential than the other (as current flows
across that metal floor), but there is a small potential difference
between a wire and the equipment, at any given location.
While almost impossible to do unless your dealing with new construction, the
goal in an existing building is to minimize the ground resistance between
separate circuit grounds by bonding them together with large gauge, low
resistance conductors.
Actually, there's not much need for LARGE conductors. In the transient
case, the inductance dominates and results in potential differences, and
inductance doesn't depend very much on the size or shape of the
conductor. It's all about length.
In the steady state case (60 Hz power line falls on the antenna), the
conductor has to have "low enough" resistance to keep the potential
difference low enough to be safe. The code required green wires and
bonding wires (variously 14, 12, 10, 6, etc. AWG) are more than
sufficient for this. Note that the code requires different sizes more
for mechanical reasons than electrical.
This is typically either copper strap or large gauge
copper conductors. This means interconnecting the electrical, cable, and phone
grounds together to a single grounding point with a heavy gauge low resistance
conductor.
Strap is fine for RF grounds: a vertical antenna for 20 meters should
use strap, because the AC resistance is much less than for the same
amount of copper in a round wire.
Strap doesn't have any electrical advantage for lightning impulses or AC
line safety. Strap might be more convenient mechanically: you can bolt
stuff to a flat strap a lot easier than to a round wire.
The size is more determined by mechanical concerns: a ground bonding
wire outside by itself needs to be bigger than one inside a conduit.
Larger wires are also less likely to "flashover" to adjacent conductors
because the radius of curvature is larger: a reason to use round wire,
rather than flat strip, for lightning conductors, by the way. A 1/4"
diameter copper wire has a higher breakdown voltage than the edge of a
0.020" strip of copper flashing. (5-6 kV vs 1kV)
The bonding wire needs to be big enough to carry the maximum expected
fault current without melting. The worst case for this is not
lightning, but a local medium voltage power line shorting to your
wiring, because it could source several hundred amps for seconds, before
something trips. Lightning has really high peak currents, but the pulse
only lasts 50 milliseconds, so there's not much energy dissipated in the
grounding conductor. (AWG 10 is big enough to handle all but the largest
lightning strokes without melting)
By doing this you can minimize the effect of differential ground voltage issues
during a lightning event.
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