Consider the Mine Safety and Health Administration (MSHA) Standard 56/57.12028 (Testing Ground Systems). The intent of this standard is to ensure that continuity and resistance tests of grounding systems are conducted on a specific schedule. These tests will alert the mine operator if a problem exists in the grounding system which may not allow the circuit protective devices to quickly operate when faults occur. With the exception of fixed installations, numerous fatalities and injuries have occurred due to high resistance or lack of continuity in equipment ground systems. These accidents could have been prevented by proper testing and maintenance of grounding systems.

Grounding systems typically include the following:

• equipment grounding conductors - the conductors used to connect the metal frames or enclosures of electrical equipment to the grounding electrode conductor;
• grounding electrode conductor - the conductors connecting the grounding electrode to the equipment grounding conductor; and
• grounding electrodes - usually driven rods connected to each other by suitable means, buried metal, or other effective methods located at the source, to provide a low resistance earth connection.

Operators shall conduct the following tests:

• equipment grounding conductors - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subject to vibration, flexing or corrosive elements;
• grounding electrode conductor - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subject to vibration, flexing or corrosive elements; and
• grounding electrodes - resistance must be tested immediately after installation, repair, or modification, and annually thereafter."

The rest of the standard indicates certain instances where the testing of equipment grounding conductors and grounding electrode conductors is not required (not subject to vibrations; fail-safe ground wire monitor used to continuously monitor the grounding circuit), and that a record of the most recent resistance tests must be kept. Note that, in all situations, installed grounding electrodes (ground rods) must be tested on an annual basis. You can access the MSHA regulations at http://www.msha.gov.

What does this standard mean? All mines must test (and keep a record) their grounding electrodes. Depending on the specific requirements and test environment, AVO offers four ground testers (DET62D, 250260, DET5/4R, DET/5/4D, and DET2/2). Contact Jeff Jowett, Applications Engineer at AVO International (215-283-2291) for a recommendation on choosing the correct instrument.