If you're continually hit by lightning, be careful.
For safety, it usually makes sense to isolate between
sections and keep grounding locally. That way a "hit" is
just that. If you reference someplace *and* have hard
connections, well,..just picture the potential currents.
Also be careful thinking your ground rods are a good
reference. Years ago I was asked to solve a problem with
an oil rig located in the midwest being continually killed
by strikes. They kept losing the buried pump, very
expensive and very difficult to replace. They did
everything right - power lines to the pump had all the
dummy lines adjacent that were grounded with all the
arrestors in place and all the ground rods done right.
Just made no sense.
I had given up and was chatting with the wildcatter, he
mentioned how this well seemed to be jinxed from the start.
How they had hit a big buried salt plain, etc. ! ! !
Turns out that salt plain was just under the surface
(relative to drilling distances) and extended around for
miles.
THAT WAS THE TRUE GROUND REFERENCE IN THIS AREA. Solution
take the ground rods down to THAT plain and problem was
solved.
The lesson - be careful of what you think is a ground
reference.
- Robert -
On Wed, 6 Apr 2005 07:53:56 -0400
"Chris Wells" <[email protected]> wrote:
> In our prescreening lab we have a ~9M square metal floor
> plate that we tie direct to the Generator when doing
> IEC61000-4-4, -6 testing etc.
> The Generators and coupling devices are bonded high
> frequency wise to the plate and are also referenced to
> safety ground as per the NEC going back to the
> transformer earth grounding pad. In my view that is
> adequate for these high energy tests.
> the return path of the test signal is back to the plate,
> and all the gear is well within the permiter of the
> plate.
>
> However I am getting pressured to add localized ground
> rods next to the floor plate.
> I do not see how tying this plane to the earth has much
> EMC meaning unless we were doing emission uV type
> measurements.
> For that work we have a large GTEM which sits on the
> plate as well.
> I do see that a redundant local ground would violate NEC
> and that fault or surge currents would take the parallel
> path to my redundant ground rods causing risk to
> equipment and personel.
>
> We are on a hill and have been hit by lightening many
> times.
> We are also a power lab and are testing 3 phase equipment
> (480V P:P)
> As such I must be mindful of the safety topic perhaps
> more than others.
> If we were to instal an isolation transformer at the room
> that serviced all the gear and DUT, then we could
> establish a local ground that would satisfy both NEC and
> EMC type testing but the cost is an issue that would
> have to be rationalized and explained clearly to upper
> management. ( the transformer would have to be rather
> large and the building is leased)
>
> Right now I can not support the argument for either local
> ground rods or the isolation transformer.
> So please help me understand what the rational is for
> earth grounding locally the reference plane with ground
> rods.
> Is it like I thought, a uV issue with emission tests or
> does it actually have some bearing on high energy EFT, RF
> conducted type tests?
>
> If you can copy my work email that would be
> [email protected]
> IT won't let me get the list at work :-(
>
> Thanks in advance!
>
> Chris Wells
> Eaton Electrical
>
>
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