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grounding question

R

Ron & Joan

Jan 1, 1970
0
I'm in charge of distribution of this condo project. We have a 600/347V main
service.
I just installed a 600v - 208/120v transformer and everything is groovy.
The star point (X0) on the secondary side is grounded with uninsulated
copper wire which by code it should be.
I just measured the ground wire and it is carrying 10 amps.
I know that this must be the unbalanced neutral current, but is this ok for
a ground wire to carry that much current uninsulated?
Is this normal? I didn't think that ground wires are supposed to carry
current normally (only under fault conditions).
 
B

Bill Shymanski

Jan 1, 1970
0
*Where* are you measuring 10 amps? Between the load and the transformer
star point, or in the grounding conductor connecting the star point to
ground? If you have a little unbalanced current in the neutral, that's
normal - depending on the size of the service a 10 amp unbalance may be
very good. But if you've got 10 amps circulating in the *grounding*
conductor, there's something else wrong in the system - at least an
extra connection in the neutral out in the wiring system.



Bill
 
R

Ron & Joan

Jan 1, 1970
0
Phil Scott said:
if you touch it, with your feet wet and grounded you might
share the load, but only in direct proportion to the
resistance of your body, many thousand ohms.. to the hopefully
big fat copper wire... virtually zero ohms...takes all of the
current... unless :)) the ground connection or conductivity
goes bad in dry rocky soil or whatever (seasonally)... and
then the current goes though your body to ground.

right, that does make sense. I guess that's why they want that ground plate
buried well.
Better contact through the conduit possibly, creating a
continuous ground even if the conduit separates or corrodes...

In this case, the wire is free air.
Dividing the load to several instead of just one step down
transformer will allow smaller and safer interupt devices...
two instead of one for instance halves the size of the feed to
each step down transformer..

This project has 5 transformers (600-208/120) spread throughout the building
(and currently two 600v-480v for the cranes).
The farthest being about 800' away. The closest is right beside the main
service.
I thought this is done more for voltage drop, but maybe the engineers are
thinking about safety.
do know that you need a high voltage qualified meter to use
at the mains on the 600vac of course. I forget the way its
called out...category2 or 3 maybe.

Yeah, I heard about meters blowing up in people's hands because of the
voltage.
I have some kind of Fluke that supposed to be rated for 750v but I never did
check the category.
Too late now, it works.
it safer... you would use a 10 or 15 amp breaker on that

I've never heard or seen a 10amp breaker. (I live in Canada)
in contrast 100' away..you run for the 110 you use , 20 amp
breakers and size 12 wire usually.
Phil Scott

You can't run a 20 amp breaker on an ordinary outlet.

ps. Thanks for your email, I'm still digesting some of the theories.
 
Shouldn't that be white, as it is actually the Neutral, not Ground?
The bare ground you have is carrying the current on it to
ground. And will take the path of least resistance... the
copper wire to ground...

if you touch it, with your feet wet and grounded you might
share the load, but only in direct proportion to the
resistance of your body, many thousand ohms.. to the hopefully
big fat copper wire... virtually zero ohms...takes all of the
current... unless :)) the ground connection or conductivity
goes bad in dry rocky soil or whatever (seasonally)... and
then the current goes though your body to ground.


its a good question...I wonder about things like that
myself...what if a transformer coil shorted internally.. that
could be a real lively ground.

What ~are the advantages in not insulating the ground anyway?
Better contact through the conduit possibly, creating a
continuous ground even if the conduit separates or corrodes...
is probably the reason..same logic in the distributed system.
It is a ground after all...so it is inherently safe to that
degree.

Also. with the true ground, it may be called upon to take a lightning
strike to ground potential. High frequency signals (like lightning) do
not like insulation nor do they like conduit - they act as a choke to
these signals.
 
J

John Gilmer

Jan 1, 1970
0
If what you are talking about is about 10 amps of "leakage" in, what?, a 200
"service" it could be the sum of a lot of "minor" leakages. Note that a 10
amp fault to ground would not trip any breaker.

If you have the time, you can use your "Amprobe" to see if one or two
circuits have significant leakage while most have next to none or whether
everything "leaks." (Easy to do: just clamp around all the conductors
(other than the green or bare one) going out of the box through one opening.

But that's why the grounding exists in the first place: to get that current
back to where it belongs without making metal objects "hot."

(If it turns out that only one circuit has most of the leakage, it would be
a good idea to take off some covers and check for crosses between the boxes
and neutral wires and even a connection of a lighting circuit between HOT
and GROUND.)
 
R

Ron & Joan

Jan 1, 1970
0
Ron & Joan said:
I'm in charge of distribution of this condo project. We have a 600/347V
main service.
I just installed a 600v - 208/120v transformer and everything is groovy.
The star point (X0) on the secondary side is grounded with uninsulated
copper wire which by code it should be.
I just measured the ground wire and it is carrying 10 amps.
I know that this must be the unbalanced neutral current, but is this ok
for a ground wire to carry that much current uninsulated?
Is this normal? I didn't think that ground wires are supposed to carry
current normally (only under fault conditions).

well you guys were right.

I've been busy with other parts of the project and I finally had time to
recheck this ground thing.
The transformer is hooked directly up to one panel. This panel is feeding
temporary panels around the job site including some permanent circuits.
I checked all of the ground wires (fortunately this is a concrete building
and we are using coreline - not emt, so all outgoing conduits have ground
wires).
One ground has current on it. It was hooked up to a temp panel that some
other electrician hooked up. He didn't take the neutral bond screw out and
that was the whole problem.

Thanks to all the answered. This problem had lead to some more questions
that I'll be posting at a later time.
 
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