Need Help With Ground System Design

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DANIEL LAWLOR

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My Dear Colleagues:

My Employer, A Municipality, Has Launched A Program To Upgrade
The Grounding System For Its Signal-light Traffic Controllers
In Hopes Of Reducing The Incidence Of Electronic Failure.

We Have Already Upgraded The Protective Devices (tvss Etc)
However Our Grounding Methods Are Inconsistent And Our
Measurements Show 10 Ohms Or Higher Depending Upon The
Intersection In Question.

An Engineer That I Met At A Training Course Told Me To Install
A Separate Set Of Rods (a Triad) As A Clean Ground For The
Electronics Cabinet And Then To Bond It To The Service Ground
On The Pole.

Does This Make Sense To Anyone?

Most Of Our Traffic Controllers Are Installed At Urban Intersections Surrounded By Ashphalt And Concrete With Many
Buried Utilities So We Want To Be Sure Before We Start Hammering
In Ground Rods.

Furthermore, In Many Cases We Hit Bedrock Close To The Surface.
Would Laying The Rods Horizontally In The Concrete Make Any Sense?

Any Advice Would Be Most Welcome.

Thank-you All
 
I'd have the electronics manu. do an autopsy on the failure before you spend a lot of $$$ on digging.
 
Do the failures occur after lighting strikes? If your electrical supply is grounded and your "site" (be it a pole or cabinet mounted on a pedistal) is not grounded or poorly grounded and all metal enclosures along the way bonded to the ground then lightning strikes are likely to find their way to ground via your equipment. You may also have ground loops that like to use your equipment as the path to dissapate strikes.

I also find that installations of surge suppession in multiple locations at the same site is a give a way of poor utility service (electrical supply) grounding.

I'd still slam at least one rod in per cabinet location with exothermic welds. One Hilti TE-76 with a 2" and piercing chissel and a 5/8" rod driver will sink your $15 rod for $1200 (on sale this week at the depot).
 
Sorry, I didn't answer your question directly...

If possible, one rod per cabinet location at least, more is better but isn't always effective or necessary. If your soil conditions are poor and your ground resistance is hi then you may need special ground kits to decrease soil resistance.

You should ground from the top down tying in equiment along the way to the ground rod. Metal poles should be exothermically welded to at least #2 stranded to the ground rod at the base.

Keep in mind, lighting will commonly strike aerial devices and you can easily see a 28+KV rise and fall on your ground and/or equipment but the key is to make ALL of your equiment (ground circuit if you will) rise and fall together and not have multiple grounds or have one that is better than the other.

Hope this helps.
 
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