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reference:grounding

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Electrical System Grounding

Grounding Network

This section needs to be developed to explain:

  • The relationship between the power (safety) ground and grounding networks
  • Single point of entry and interconnection of power ground
  • Recommended grounding practice

Basic grounding and lightning protection for a home or small office:

Start with at least two ground rods, a threadless tapered coupling, a ground rod driving sleeve, an Erico Hammerlock for connecting to the ground rod, and at minimum #6 bare wire. Solid #6 is common, I prefer stranded just because its less likely to break (solid wire will break if it is nicked and exposed to flexing or vibration). Personally I would use #4 for home or #2 for an office.

Drive at least two coupled ground rods to get depth. If you are in a heavy lightning area, use two sets on opposite sides or ends of your building. Bring each ground rod connection independently to your electric panel ground bus.

If you live on sand like we do, use three ground rods (in several locations I've driven 3 rods deep, and they still pushed easily!). Ideally you want to get down to the water table.

If you cannot drive ground rods because the ground is too rocky, dig a trench at least 2ft (or as deep as possible) and at least 20ft long and lay a #2 bare copper wire in it. You could also smash up some charcoal to mix in with the backfill. If you go this route I think it better to lay a ring all the way around the building. You must use exothermic welding (cadweld), brazing, or compression connectors for underground connections (in order of preference, and compression connectors should probably be augmented with brazing). Solder connections are not safe on ground wires, because the solder might melt during fault conditions. Solder is also not good underground because lead and tin are sacrificial anodes and will pit and be gradually eaten or dissolved by galvanic action. This is also why tinned underground wires are not acceptable.

Another option is to cast your bare ground wire in concrete; this is called a “Ufer” ground. It is very effective in rocky soil, very dry soil, or right on rock. The wire should be completely embedded 2“ inside the concrete. If you're putting in a building, lay this right in the foundation footings. It needs to be no more than 2” from the soil, and its ok for it to contact the rebar. Longer is better, it can even be a ring in the footer all the way around the building. A ufer ground is now the preferred primary ground terminal for new construction in the USA, mainly because its permanent and can't be undone. The tail on the ufer ground should be long enough to connect to your ground bar or electrical panel. Six inches of the copper wire should be insulated, covered with heat-shrink, or taped where it exits the concrete – three inches inside and three inches outside – to prevent the pH of the concrete from damaging the wire. (Inside the concrete, galvanic action does not continue once the concrete has cured.)

If you want to do it right but less expensive than a commercial ground bar, buy a piece of copper bar from McMaster Carr (listing below). Mount the ground bar on ceramic insulators, or use blocks of hardwood for standoffs. Its preferable that it not contact the wall.

For surge protection, install a large MOV across the AC lines in your electric panel. You shouldn't parallel any of these unless they have been “matched”. (Connecting two identically rated MOVs in series is ok if you need a different voltage rating.) You should also connect an MOV from each ungrounded AC line to ground at your panel. Connecting these through the largest size breaker is a good idea in case they catastrophically fail. Keep the connecting wires as short as possible (24“ or less) and as straight as possible.

Run the AC power to your electrical panel through a ferrite core to act as a choke. Wrap the wire around the core once if its big enough. All the power wires entering the building need to pass through the same core – don't separate them. This will help keep “noise” like lightning outside of your house.

Run every circuit leaving the panel box through a small ferrite core. Again, you need to run both the neutral and line through the core together, making a loop around the core if possible. These will help keep power line noise like lightning in the box so the MOVs can clamp it.

One of the AC lines should be bonded to the ground wire in your panel. With a three-wire North American system (120 - neutral - 120 for 120/240v) it should be clear which wire should be grounded.

With a two-wire 230v system it may not be evident, so you will have to do some research. First, trace as much of the outside wiring as you can to see if either wire is connected to a transformer ground. Then, check the voltage between each wire and ground. If you can get a consensus between the two tests, you know which one should be bonded. Normally you should have a ground bar and a neutral bar, with a wire between them. Its better not to put all the grounds and neutrals on the same bar so that it remains clear which is which.

Inside the house, make sure that every outlet has a ground wire connected directly back to the panel.

By the way, the copper plating on the ground rod is for corrosion protection, not so much for conductivity. Copper oxide is as good a conductor as elemental copper, but iron oxide is a very good insulator.* That's why its important to have a thick copper plating that won't crack off if the rod gets bent while driving.

* Silver oxide is an even better conductor than elemental silver, and aluminum oxide is an even better insulator. Which is why I really really dislike aluminum wire and connections. If you don't use a zinc-loaded conductive grease on your aluminum connections you are or will be in bad shape, like a fire. You can't just assemble them either, you have to use a wire brush on the wire after laying on the conductive grease. Its not worth the cost savings in the wire.

Sample Bill of Materials

Ground Rods & Connections

Master Ground Bar

McMaster Carr - www.mcmaster.com

Basic Ground Bar

  • 8964K411 Copper (Alloy 110) bar, 1/8” x 2” x 12” – 318 kcmil equivalent1)

Better Ground Bar

  • 8964K22 Copper (Alloy 110) bar, 1/4“ x 2” x 24“ – 636 kcmil 2)

Professional Ground Bar

  • 88865K315 Silver-plated copper bar, 1/4” x 2“ x 3ft 3)

Mounting Hardware – Drill 9/32” holes for bolts & standoffs

  • 94335A151 Ceramic standoff, 3/4“x1”dia, threaded for 1/4“-20 (optional - could use painted hardwood block)
  • 71375K79 Ground bar mounting kit (~$25)

Cable Connection Hardware

  • One each bolt, nut & lock washer, & 2 washers per connection
  • 92240A539 Stainless steel hex head cap screw, 1/4”-20 x 5/8“, pk 50
  • 92673A113 Stainless steel hex nut, 1/4”-20, 7/16“ width, 7/32” height, pk 50
  • 90107A029 Stainless flat washer, 1/4“ screw, 5/8” OD, pk 100
  • 92146A029 Stainless lock washer, 1/4“ screw size, pk 100

Preferred: Belleville (cupped) washers instead of split washers - either item:

  • 90895A029 Stainless Serrated Belleville Washer 1/4”/M6 Screw, pk 25
  • 93501A029 Black Finish Steel Serrated Belleville Washer 1/4“/M6 Screw Size, pk 100

Transient Voltage Suppression

Otherwise known as a “surge protectors”.

www.alliedelec.com, www.mouser.com, www.digikey.com

For 240v circuit

For 120v circuits

If everything else is out of stock and you have to have something:

Resources & References

In our area we are allowed to dig a trench 20' long, install some 3” concrete dobbies to hold the wire off the dirt, lay a bare ground wire sized for the system [minimum #4 copper for concrete-encased electrode], get an inspection, then pour concrete over the ground wire. the concrete must surround the wire 3“, bottom, sides and top, so you have at least a 6” x 6“ x 20' long concrete buried 18” deep. (it takes about 10 to 12 dobbies to keep the wire off the ground)www.electriciantalk.com

Connections

  • CADWELD (Exothermic Welding)
    • One Shot Plus can be shipped by air
    • One size could be used with sleeves to enable multiple wire sizes
    • $10 each
    • Control Unit/Igniter $135
  • Eritech Hammerlock
    • Irreversible ground rod bond
    • Hammer strike locks the conductor to the ground rod
    • $11-16 each
1)
kcmil: 1/8“ x 2” x 1273 = 318,250 circular mils
2)
kcmil: 1/4” x 2“ x 1273 = 636.5 circular mils
3)
Silver plating reduces oxidation resistance with age; although copper oxide is still a good conductor, silver oxide is a very good conductor. In contrast, Aluminum oxide is a very good insulator.
reference/grounding.txt · Last modified: 2018/05/28 22:29 by 127.0.0.1

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