In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection. I know that for digital electronics the
old warning about right angle bends has been shown to be pointless up to
many GHz. What is the deal with lightning? What happens when the
ground wire is bent in a right angle? Has this been verified by
experiment anywhere or by application failures?

--

Rick C

On Saturday, 20 August 2016 21:32:06 UTC+1, rickman wrote:
In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection. I know that for digital electronics the
old warning about right angle bends has been shown to be pointless up to
many GHz. What is the deal with lightning? What happens when the
ground wire is bent in a right angle? Has this been verified by
experiment anywhere or by application failures?

--

Rick C

From the dim dark recesses of my memory, I think it's to do with the passage of a very large current (1000's of amperes?)with a fast rising edge and the intrinsic inductance of the conductor [enhanced by an sharp/r.a. bend] as in e= ?L di/dt.
Now you go research it !

On 8/22/2016 7:05 AM, wrote:
On Saturday, 20 August 2016 21:32:06 UTC+1, rickman wrote:
In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection. I know that for digital electronics the
old warning about right angle bends has been shown to be pointless up to
many GHz. What is the deal with lightning? What happens when the
ground wire is bent in a right angle? Has this been verified by
experiment anywhere or by application failures?

--

Rick C

From the dim dark recesses of my memory, I think it's to do with the passage of a very large current (1000's of amperes?)with a fast rising edge and the intrinsic inductance of the conductor [enhanced by an sharp/r.a. bend] as in e= ?L di/dt.
Now you go research it !

Research what? I've never found anything remotely analytical or even
from experience that talks about sharp corners other than in PCB routes
where the avoidance of sharp corners has been shown to be pointless for
anything short of high GHz signals. High currents don't change the laws
of physics. Inductive reactance is about frequency. If sharp bends in
PCB traces don't have a noticeable impact on sub-GHz signals, will sharp
bends have an impact on lightning?

This seems to be a rule of thumb thing. Lots of people advise to not
have "sharp" bends or "90°" bends. Few define what that means. Some
say 8 inch radius, some say 6 inch radius, another says 1 inch radius.
I also see recommendations for the size of grounding wire all over the
map. One says #2 wire, another #4 wire, another #6 wire and another
refers to NEC which he claims says #6 is fine.

I like to do things "right", but I like to know what rules of thumb are
based on.

--

Rick C

rickman wrote:

On 8/22/2016 7:05 AM, wrote:
On Saturday, 20 August 2016 21:32:06 UTC+1, rickman wrote:
In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection. I know that for digital electronics the
old warning about right angle bends has been shown to be pointless up to
many GHz. What is the deal with lightning? What happens when the
ground wire is bent in a right angle? Has this been verified by
experiment anywhere or by application failures?

--

Rick C

From the dim dark recesses of my memory, I think it's to do with the
passage of a very large current (1000's of amperes?)with a fast rising
edge and the intrinsic inductance of the conductor [enhanced by an
sharp/r.a. bend] as in e= ?L di/dt. Now you go research it !

Research what? I've never found anything remotely analytical or even
from experience that talks about sharp corners other than in PCB routes
where the avoidance of sharp corners has been shown to be pointless for
anything short of high GHz signals. High currents don't change the laws
of physics. Inductive reactance is about frequency. If sharp bends in
PCB traces don't have a noticeable impact on sub-GHz signals, will sharp
bends have an impact on lightning?

This seems to be a rule of thumb thing. Lots of people advise to not
have "sharp" bends or "90°" bends. Few define what that means. Some
say 8 inch radius, some say 6 inch radius, another says 1 inch radius.
I also see recommendations for the size of grounding wire all over the
map. One says #2 wire, another #4 wire, another #6 wire and another
refers to NEC which he claims says #6 is fine.

I like to do things "right", but I like to know what rules of thumb are
based on.

Electric field intensity is increased at sharp points, and can cause
arciing which would not otherwise occur. Could this be the problem the
advice is aimed at? I have no idea if a right angle bend in a thin
conductor has that much effect on field intensity, but it is amenable to
maths I think.



--

Roger Hayter

On Sat, 20 Aug 2016 16:32:05 -0400, rickman wrote:

In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection...

You should not expect the answers to the exam questions to represent
an absolutely correct explanation. Some of the answers have
exceptions to overly simplistic answers. Some answers only apply for
specific frequency ranges. A few make no sense at all if you've had
any experience with the topic. Long ago, I did my best to fix some of
these oddities, and gave up for reasons I don't want to explain. You
should have little difficulty finding other problems in question pool.

Basically, if you want to pass the exam, you have to swallow your
pride and forget most of what you know about physics, and just supply
the conventional wisdom answer that is expected. Sharp bends are
considered bad because the narrowing at the bend causes the cross
sectional area of the wire to be reduced at the bend. Given a large
DC current, such as a lightning bolt, this can cause the wire to fuse
at the bend. You can test this yourself by just bending a piece of
wire at a right angle. Pass sufficient current through the wire and
it will fuse at the weakest point, the right angle bend.

A right angle bend makes a rather poor 1/4 turn coil, but that's
allegedly enough for the wire to induce substantial currents in any
nearby conductors. The damage caused depends on what is nearby,
usually the coax cables on a tower. There's plenty of induced
currents from just the grounding wire, but the right angle bend
allegedly makes is slightly worse. I find this somewhat difficult to
believe, because a wide radius right angle bend would produce the same
induced currents.

There's also the phenomenon where a lightning bolt "leaps" off the
right angle corner bend, to a nearby grounded object. I've never seen
this happen, but I've heard it mentioned often enough to suspect that
it might be real. Since I don't understand it, I won't try to explain
it.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 8/23/2016 2:30 PM, Jeff Liebermann wrote:
On Sat, 20 Aug 2016 16:32:05 -0400, rickman wrote:

In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection...

You should not expect the answers to the exam questions to represent
an absolutely correct explanation. Some of the answers have
exceptions to overly simplistic answers. Some answers only apply for
specific frequency ranges. A few make no sense at all if you've had
any experience with the topic. Long ago, I did my best to fix some of
these oddities, and gave up for reasons I don't want to explain. You
should have little difficulty finding other problems in question pool.

Basically, if you want to pass the exam, you have to swallow your
pride and forget most of what you know about physics, and just supply
the conventional wisdom answer that is expected. Sharp bends are
considered bad because the narrowing at the bend causes the cross
sectional area of the wire to be reduced at the bend. Given a large
DC current, such as a lightning bolt, this can cause the wire to fuse
at the bend. You can test this yourself by just bending a piece of
wire at a right angle. Pass sufficient current through the wire and
it will fuse at the weakest point, the right angle bend.

A right angle bend makes a rather poor 1/4 turn coil, but that's
allegedly enough for the wire to induce substantial currents in any
nearby conductors. The damage caused depends on what is nearby,
usually the coax cables on a tower. There's plenty of induced
currents from just the grounding wire, but the right angle bend
allegedly makes is slightly worse. I find this somewhat difficult to
believe, because a wide radius right angle bend would produce the same
induced currents.

There's also the phenomenon where a lightning bolt "leaps" off the
right angle corner bend, to a nearby grounded object. I've never seen
this happen, but I've heard it mentioned often enough to suspect that
it might be real. Since I don't understand it, I won't try to explain
it.

Yes, I have already figured out that some of the questions and answers
are a bit simplistic.

There is a term for knowledge that is learned by doing (experimental)
rather than by science, but I can't recall what it is. I believe there
is a lot of that in Ham radio. Not that it is bad. RF is complex
enough that an ounce of testing can be worth a pound of math.

--

Rick C