Main keywords for this article are 3-point fall of potential method, Site Grounding Inspection Points.
Site Grounding Test Conditions
Site Grounding Inspection Points
- Verify that ground rods are copper-clad steel not less than 0.625” diameter and a minimum of 8’’ length.
- Verify that CDOT approved other ground rod type if copper-clad steel was not used.
- Verify that grounding electrode conductor is not less than #6 AWG.
- Verify that braided ground straps consist of non-insulated tinned copper flat braid wire not less than 0.5” width and 0.07” thick (for bonding between a cabinet frame and busbar only).
- Verify that insulated grounding electrode conductors are Type THWN with green jacket color.
- Verify that grounding electrode conductor was installed in a continuous run without splices or joints.
- Verify that connectors are exothermic weld for below grade connections and areas exposed to moisture.
- Verify that connectors are UL listed compression connectors for above grade installations and where safety dictates.
- Verify that ground bus is copper bar stock with stainless steel stand-off brackets and mounting hardware.
- Measure resistance of the first installed grounding electrode and test with earth ground resistance tester (3-point fall of potential method). If results exceed 10 ohm, a second grounding electrode needs to be installed.
- Verify that second grounding electrode is at least one electrode length away from the first grounding electrode and connected via bonding jumper.
- Measure grounding electrode system resistance with earth ground resistance tester (3-point fall of potential method). CDOT will accept 25 ohm or less after two grounding electrodes have been installed.
- Verify that oxidation and antioxidant compound was used at connection points to ground bus.
- Verify that stainless steel hardware was used and torqued to a specific value based on hardware grade, material, and size.
- Verify that connections to ground bus utilize a flat washer and Bellville washer for each bolt.
- Verify that all exothermic welds have slag removed.
- Verify that testing is performed prior to connecting to utility ground.