I'm a long-time reader of this NG and so realize that there is a lot
of knowledge of EM principles on tap here. I'm pondering a non-ham
issue from my workplace (a power plant) and could use a little
education or peer checking of my ideas.

In the industry, there is occasionally a need to construct scaffolding
below the main generator, up to the area where the main leads exit.
Typically there are three leads (busses, phases) energized at 22kV
phase to phase and carrying around 28,000 amperes. These busses are
contained within individual aluminum ducts, creating sort of huge
coaxial cables (three of them). The busses run through the turbine
building and out to the transformer yard. The ducts are cross
connected with welded plates at the ends and maybe at one or two
intermediate locations to allow induced currents to circulate.

Here's the issue. Occasionally while erecting or removing such a
scaffold with the unit at full power, there will be reports of arcing
at the ends of scaffold poles (aluminum tube, I think) and of scaffold
members becoming hot. There are lots of anecdotal stories of such
events, and also of measuring from several to maybe 100 amperes in
scaffold members, ground cables, structures and permanent piping in the
vicinity.

Now, question or questions. Sifting through various stories and
recommendations, I see recommendations to assure that the scaffold is
well grounded. I'm not sure why this would help, assuming the
currents (and arcing) are the result of magnetic induction. One
recommendation is to ground only at one point, which makes sense to me.


I also see a lot of cautions about "static charge". It seems to me
that any static charge would be the result of an electric field. And
given that the busses (and generator) are enclosed in grounded metallic
housings, this should not be a possibility. Is this correct?

One person recommended non-metallic scaffolding, which I like. But
another wanted to assure that persons on that scaffold were grounded,
which I'm not too sure about. I don't think it would be harmful in
most cases, but I'd be concerned if the person and his ground
happened to complete a current loop. Does that make sense? Again, I
assume the grounding is for static charge concerns, which I don't see
as valid.

It seems to me that structures built in the vicinity of strong 60Hz
magnetic fields should either 1) avoid creating loops, including open
loops that could have high voltage across the open ends, or 2) make
sure the loops are intact and can carry the resulting induced currents.
Of course, #2 still makes disassembly while at power a problem.

Thanks for any opinions. I now return you to SWR, E-H, CM and G5RV.

73--Nick, WA5BDU

I think you are on the right track. the problem in locations like that is
not 'static' charge, but induced current. induced current requires a loop.
typical scaffolding with metal frames that typically look like big square
loops would make great pickups for those fields. some people may be
confusing the arcs caused by breaking a loop with static charge, which it
definitely isn't.

I kind of like the non-metallic scaffolding idea, that would be a great way
to avoid most of the problems.

The one thing to definitely avoid is to ground the personnel... consider
this, two grounded people on an insulated platform touch, thus completing a
loop that consists of them and their ground wires... could be a shocking
experience. they would also end up in the middle of a loop any time they
touched another piece of metal that happened to have a path to ground.

with the cables at least partially shielded by the conduits I'm not sure if
the fields would be high enough to cause heating of things like bracelets
and neck chains... but it might be something to test out and see if
prohibiting that type of stuff would be in order. it should be fairly easy
to test when you have access to a setup near the generator by using a clamp
on ammeter on the legs of the scaffolding... preferably somewhere that it
forms a loop rather than on a dead ended pipe.


wrote in message
oups.com...
I'm a long-time reader of this NG and so realize that there is a lot
of knowledge of EM principles on tap here. I'm pondering a non-ham
issue from my workplace (a power plant) and could use a little
education or peer checking of my ideas.

In the industry, there is occasionally a need to construct scaffolding
below the main generator, up to the area where the main leads exit.
Typically there are three leads (busses, phases) energized at 22kV
phase to phase and carrying around 28,000 amperes. These busses are
contained within individual aluminum ducts, creating sort of huge
coaxial cables (three of them). The busses run through the turbine
building and out to the transformer yard. The ducts are cross
connected with welded plates at the ends and maybe at one or two
intermediate locations to allow induced currents to circulate.

Here's the issue. Occasionally while erecting or removing such a
scaffold with the unit at full power, there will be reports of arcing
at the ends of scaffold poles (aluminum tube, I think) and of scaffold
members becoming hot. There are lots of anecdotal stories of such
events, and also of measuring from several to maybe 100 amperes in
scaffold members, ground cables, structures and permanent piping in the
vicinity.

Now, question or questions. Sifting through various stories and
recommendations, I see recommendations to assure that the scaffold is
well grounded. I'm not sure why this would help, assuming the
currents (and arcing) are the result of magnetic induction. One
recommendation is to ground only at one point, which makes sense to me.


I also see a lot of cautions about "static charge". It seems to me
that any static charge would be the result of an electric field. And
given that the busses (and generator) are enclosed in grounded metallic
housings, this should not be a possibility. Is this correct?

One person recommended non-metallic scaffolding, which I like. But
another wanted to assure that persons on that scaffold were grounded,
which I'm not too sure about. I don't think it would be harmful in
most cases, but I'd be concerned if the person and his ground
happened to complete a current loop. Does that make sense? Again, I
assume the grounding is for static charge concerns, which I don't see
as valid.

It seems to me that structures built in the vicinity of strong 60Hz
magnetic fields should either 1) avoid creating loops, including open
loops that could have high voltage across the open ends, or 2) make
sure the loops are intact and can carry the resulting induced currents.
Of course, #2 still makes disassembly while at power a problem.

Thanks for any opinions. I now return you to SWR, E-H, CM and G5RV.

73--Nick, WA5BDU

On 23 Aug 2006 11:32:31 -0700, wrote:

I'm a long-time reader of this NG and so realize that there is a lot
of knowledge of EM principles on tap here. I'm pondering a non-ham
issue from my workplace (a power plant) and could use a little
education or peer checking of my ideas.

In the industry, there is occasionally a need to construct scaffolding
below the main generator, up to the area where the main leads exit.
Typically there are three leads (busses, phases) energized at 22kV
phase to phase and carrying around 28,000 amperes. These busses are
contained within individual aluminum ducts, creating sort of huge
coaxial cables (three of them). The busses run through the turbine
building and out to the transformer yard. The ducts are cross
connected with welded plates at the ends and maybe at one or two
intermediate locations to allow induced currents to circulate.

Here's the issue. Occasionally while erecting or removing such a
scaffold with the unit at full power, there will be reports of arcing
at the ends of scaffold poles (aluminum tube, I think) and of scaffold
members becoming hot. There are lots of anecdotal stories of such
events, and also of measuring from several to maybe 100 amperes in
scaffold members, ground cables, structures and permanent piping in the
vicinity.

Now, question or questions. Sifting through various stories and
recommendations, I see recommendations to assure that the scaffold is
well grounded. I'm not sure why this would help, assuming the
currents (and arcing) are the result of magnetic induction. One
recommendation is to ground only at one point, which makes sense to me.


I also see a lot of cautions about "static charge". It seems to me
that any static charge would be the result of an electric field. And
given that the busses (and generator) are enclosed in grounded metallic
housings, this should not be a possibility. Is this correct?

One person recommended non-metallic scaffolding, which I like. But
another wanted to assure that persons on that scaffold were grounded,
which I'm not too sure about. I don't think it would be harmful in
most cases, but I'd be concerned if the person and his ground
happened to complete a current loop. Does that make sense? Again, I
assume the grounding is for static charge concerns, which I don't see
as valid.

It seems to me that structures built in the vicinity of strong 60Hz
magnetic fields should either 1) avoid creating loops, including open
loops that could have high voltage across the open ends, or 2) make
sure the loops are intact and can carry the resulting induced currents.
Of course, #2 still makes disassembly while at power a problem.

Thanks for any opinions. I now return you to SWR, E-H, CM and G5RV.

73--Nick, WA5BDU

I'm no expert on this but it sounds to me like the magnetic fields
around the busses are inducing eddy currents in the scaffolding as
opposed to actual current flowing around the square sections in a
loop. The conduits surrounding the busses will be "thin" skin-depth
wise at 60 Hz so a large amount of field gets through. I'm actually
kinda curious as to why the conduits themselves arn't getting really
hot :?.

I'd suggest posting on sci.electronics.design, I'll bet somebody there
knows about this...

H.

The conduits (bus ducts) don't get hot mainly because they have greater
total cross sectional area than the bus itself. They're about a half
inch thick aluminum and maybe three feet in diameter. All joints are
continuously welded.

Some documents I've found suggest 95% shielding efficiency, supposedly
less than 100% due not to skin effect, but to resistivity.

Regarding the other responder's comments on jewelry and so on, here's
my anecdote. Not so long ago I climbed up near the main generator
output bushings to view a cooling air damper through a window in the
housing. This is sort of unusual with the plant at power and it made
me a bit nervous. I stayed about five minutes, but after I climbed
down I discovered that my (analog) watch had gained an hour!

Now I can't find that watch. My theory is that it's off in the future
just an hour out, but I can't catch up to it.

73--Nick, WA5BDU

(... this being the internet, I guess I'd better state that I was only
kidding about the location of my watch.)


Howard Eisenhauer wrote:

I'm no expert on this but it sounds to me like the magnetic fields
around the busses are inducing eddy currents in the scaffolding as
opposed to actual current flowing around the square sections in a
loop. The conduits surrounding the busses will be "thin" skin-depth
wise at 60 Hz so a large amount of field gets through. I'm actually
kinda curious as to why the conduits themselves arn't getting really
hot :?.

I'd suggest posting on sci.electronics.design, I'll bet somebody there
knows about this...

H.

On 24 Aug 2006 10:24:57 -0700, wrote:

Not so long ago I climbed up near the main generator
output bushings to view a cooling air damper through a window in the
housing. This is sort of unusual with the plant at power and it made
me a bit nervous. I stayed about five minutes, but after I climbed
down I discovered that my (analog) watch had gained an hour!

A 60Hz Fields Exposure Dosimeter