earth ground and Common ground! 3

[box23]

Can earth ground be tied to a common ground from a DC power supply?

 

[itsmoked]

Absoluteley!

Sometimes it would be better that way, sometimes it would not. (Because of noise paths.)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[logbook]

"earth ground" is the thing you get from the mains supply. Also known as the "protective earth". It is used to protect against electric shock as fault currents go down it rather than through any ugly bags of water (humans) who happen to be toughing the circuitry when a fault occurs.

It is usual for electrical systems to be earthed at some point rather than to be "floating". It is generally safer to earth internal supplies rather than let them float with respect to the mains supply.

Consider this: if a transformer is connected to mains on one side and the other side is left floating, what is the voltage stress applied to the insulation? If the output potential floats it may eventually over-stress the insulation on the transformer. Not good.

If there is more than one mains related earth in any system then you get into "earth loops". These can be very bad as they give noise/hum/stray currents. Handling earth loops correctly is an activity which many engineers spend much of their working lives coming to terms with.

 

[machmech]

Hello itsmoked/logbook

Since you answered the OPs question I would like to jump in with another.

If for example a large transformer is placed opposite side
of building from switch gear and the Electrician uses the
metal ceiling beams for ground connection on that transformer.

How does he know this is a good enough path? Can you use VOM
from ceiling light hot to beam and verify current flow, or
is there a NEC required procedure for this? Megger reading perhaps?

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[logbook]

There are all sorts of rules about protective earth conductors. I assume this is a serious high power transformer we are talking about. If you were to use the metalwork as the protective earth you would need to ensure the integrity of the earth path with warning labels so it didn't get disconnected. You would have to ensure all the metals were galvanically compatible. You would have to ensure any joints were corrosion free.

Basically this is a heavily regulated area and I wouldn't be comfortable recommending this action. There may well be all sorts of rules which make it illegal, but I can't specifically name any clause just yet. Typcially you might do an earth resistance check on the dead system using a simple multimeter and really long leads, initially doing an ohm-zero on the test leads. This would be necessary but no where near sufficient.

 

[itsmoked]

I think I would whack that, so called, electrician up side the head with the NEC as it is heavy enough to possibly knock some sense into him.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

Thanks for the replies, I'll approach him tongue in cheek
and ask about it. These guy's are really good to have
around and it's possible I don't see some significant
connection somewhere.

Beside that he's like you Kieth if I hit he'll whoop the tar
out of me!

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[waross]

I'm away from my code book just now but I seem to remember a provision requiring that grounding conductors must be copper. Can anyone verify this for us.
Thanks
respectfully

 

[itsmoked]

waross not limited to copper. In some cases it is even proscribed in lue of other metals.

And...I'll be danged.. Better hold off whacking your sparky machmech. You can use steel building framing for grounding. It must be well connected. (bolted or welded I would imagine) This is similar to using water piping which is allowed.

However it seems you must "bond" the transformers to the building frame. I don't believe bolting the transformer to the frame cuts it, since mounting bolts don't specifically guarantee a good bond.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

Yes sir, I pulled panel on transformer confirmed it
was grounded to frame and from there to ceiling we
have 5 hooked up this way through out building The
sparkys scrape the paint off beam or joist and bolt
a wire lug to it. In addition the conduit serves as
ground also.

Now at switch gear we have ground from gear to very
large water main and from there to ceiling. Outside
on main transformer several grounding rods where
installed, I think more then normal due to sandy
soil, not sure did not ask. Ground rods and water
pipe are roughly 8 feet apart.

So now I wonder if the potential for a ground loop
condition does exist under these circumstances.
I would think that water main will take care of
any stray current.

Yes I know some days I should just quit thinking!


Thank's Keith, Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[dpc]

These ground connections at the transformer are provided because the transformer is considered a "separately derived system", per Article 250 of the NEC. The transformers are probably delta-wye and the ground is for the wye winding. This ground connection is not only allowed, it is required. Building structural steel can be used if it meets the requirements in Art. 250.

This does not eliminate the need for an equipment grounding conductor in the incoming feeder to the transformer primary. This could be a green ground wire or metal conduit.

Under normal conditions, these are non-current carrying conductors.