Valuable equipment installed on rooftops requires careful and specific grounding in order to be protected and to function as intended. The proper grounding of typical rooftop installations like air conditioning equipment, radio frequency and microwave towers, and cellular sites dictates that it be: “Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.” [National Electric Code (NEC), Article 100; emphasis supplied.] Merely running a copper conductor to the nearest convenient point emphatically does not ensure conformance.
To ensure conformance and adequate protection, the ground connection(s) should be evaluated with respect to grounding and bonding, plus lightning protection. Each of these considerations has its own requirements.
Bonding is the most visible element of ground protection, and a common source of problems. It is tempting to just run a wire to a convenient contact point, like exposed building steel or the system ground, and consider the equipment “grounded”. But the NEC defines “bonding” as: “The permanent joining of metallic parts to form an electrically conductive path that will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.” [Article 100; emphasis supplied.] This is more rigorous than mere basic continuity.
Remember that a simple continuity reading from a common electrician’s multimeter is not rigorous enough. It establishes only that there is an electrical path. Since a multimeter makes this measurement with typically only a few milliamps of test current, it doesn’t tell much about the ability of a protective system to accommodate a fault current. A corroded or loose connection, or stranded wire with only a few strands intact, may still pass a multimeter test. A low-resistance ohmmeter that employs a high test current can establish that a robust connection exists, and certainly provides a reliable test across a bond. But testing the entire length of the grounding conductor may require prohibitively long test leads that load down the test circuit.
Once a good bond has been verified, to perhaps building steel or the water pipe system, it is frequently assumed that a ground has been established. But this reasoning has led to the existence of many “floating” facilities that do not have the ground protection that is assumed. With “pre-fab” or modular construction, a beam may not be connected to continuous building steel. A perfect weld to a floating beam, in terms of ground protection, is nothing more than an electrical dead end. Continuity must be verified to building steel, again by extending leads and performing a low-resistance measurement, but difficult access can hamper this.
A useful “shortcut” that applies in many situations, though not all, is to check the grounding conductor with a sensitive milliammeter. Most commercial facilities exhibit some “leakage” to ground. It may be an unpleasant surprise that this leakage is several amps! In such instances, a significant electrical imbalance needs to be corrected. But the presence of milliamp levels of leakage at least indicates that a continuous ground path exists.
Don’t overlook the obvious. Grounding must terminate with a low-impedance contact to the earth, not simply a bond to building steel or water pipe. The only rigorous test is one employing a dedicated ground tester, according to the recognized Fall of Potential Method. You must implement this from the street level.
Installers of rooftop equipment are always on the lookout for a comprehensive, one-shot diagnostic test. No such thing exists. However, a type of instrumentation popular in Europe, the AC Loop Impedance Tester, can verify in a single operation that a ground connection is complete. This instrument uses utility power to simulate a ground fault by briefly connecting a known resistance between “hot” and ground, and measuring the voltage drop around the loop. An impedance measurement is given for the entire ground clearance circuit, including elements that the operator may have overlooked, back to the utility transformer. The measurement is compared against industry-standard values normally provided.
The loop test is applicable to any energized rooftop equipment, and can be performed in minutes. If everything is properly sized and connected, the test will provide welcome affirmation. If not, however, the problem must still be sorted out by individually troubleshooting each element in the ground clearance path. The NEC [Article 250] remains the best guideline on acceptable methods and goals, while the National Fire Protection Association covers the special considerations of lightning protection in NFPA 780.
Jeff Jowett is a senior applications engineer with AVO International, a manufacturer of electrical test and measurement equipment. He has written extensively on the subject of ground testing and ac loop testing. He can be reached at (800) 723-2861.