Code Corner Grounding Ð How John Wiles In the last installment of Code Corner, the "whys" of proper grounding including safety, performance, and legal issues were discussed. Grounding a renewable energy system requires a knowledge of what the codes say, how the system is configured, the internal design of the components, and sometimes the strength of Hercules to bend the conductors and drive the ground rods. Grounding the equipment and the system are two different aspects of grounding which, when integrated, provide for human and equipment safety and higher levels of performance. The Earth Ground The National Electrical Code (NEC)¨ requires that electrical power systems have a good contact with the earth. Some state codes actually require that a measured, low-resistance contact with the earth be obtained. The NEC, however, only requires an 8-foot "manufactured" rod be driven into the ground at no greater than a 45¡ angle from the vertical. Ground rods are available from electrical supply houses and building supply centers. Suitable alternatives might include a metal well casing, or a concrete-encased, steel building foundation. If solid rock exists near the surface, then it is permissible to bury a grid of copper conductors or pipes in the soil. These should be as deep as possible and cover as wide an area as possible. In any case, the connection between the grounding cable (usually copper) and the ground rod (copper coated steel or steel) must be made with an approved (UL listed) connector or welded. This will minimize corrosion and mechanical and electrical deterioration of the connection. Drilling and tapping well casings or steel beams may be required. Grounding Systems It may be necessary to use more than one ground rod to meet both the requirements of the NEC and the performance and safety requirements of other electrical systems. PV, wind, ham-radio, and radio telephone systems may each require separate ground rods. When more than one ground rod is used, it is suggested and the NEC requires that they all be bonded together to form a grounding system. They should be bonded with at least as heavy a cable as is being used to carry current to and from the various pieces of equipment. Better performance will be realized if the largest practical conductors are used. The bonding cables must be connected to the ground rods with separate approved clamps or welding. Only one conductor may be used per clamp. Equipment Grounding The exposed metal surfaces of all equipment used in the production and use of electrical power must be grounded. The NEC does not make any exceptions for low-voltage systems, and systems less than fifty volts are specifically addressed. The equipment requiring grounding may range from power sources (metal frames on PV modules and cases on wind and hydro generators) to the appliances that provide useful work (pump housings and the cases on blenders). While it is possible to run an equipment grounding conductor from the PV module frame, or the wind machine tower to the central ground rod located near the batteries, better lightning and surge protection might dictate a modified approach. If ground rods are placed as near as practical to the lightning-prone metal surfaces (PV array frames and wind machine towers), then lightning induced surges can more easily be bled off to ground. Of course to meet the code, this ground rod must be bonded to the main ground rod. It might be wise to separate the bonding conductor from the current carrying conductors to minimize induced surges (separation allowed in dc systems but not ac systems). Exposed metal surfaces of switch boxes, fuse holders, charge controllers, inverters, and battery boxes must be connected to the equipment grounding conductor. In the nonstandardized dc world of renewable energy, equipment can be found that does not have a provision to attach the grounding conductor. In these cases, a hole must be drilled, and the paint scraped off to insure a good connection. It is always a good policy to check with the equipment manufacturer to determine where the hole can be drilled, and if case- and negative-conductor grounding will effect the performance or the warranty. The size of the equipment grounding conductor should be based on the size of the overcurrent device protecting the conductors between any two pieces of equipment. The NEC gives details in Table 250-95 -- for example; a circuit fused at 30 amps would require a number 10 AWG equipment grounding conductor, and a 400-amp fused circuit would require a number 3 AWG equipment ground. If the current carrying conductors are oversized to decrease voltage drop, then the equipment grounding conductor must be increased in size proportionately. Bare, uninsulated conductors or conductors with green insulation or green markings (number 6 AWG and larger) may be used for the equipment grounding conductors. The actual connections can be daisy-chained from one piece of equipment to the next and then to the ground rod. They may also be individually connected to a common point, and then to the rod. Separate multiple connections to the ground rod (each with a separate clamp) are allowed. All connections should be inspected periodically (every three months or so) for mechanical and electrical tightness and corrosion. System Grounding Grounding one of the current-carrying conductors is required if the no-load or open-circuit voltage in the system exceeds 50 volts. Below that voltage, system grounding is optional but reasons for grounding all systems were discussed in HP 27. The negative conductor is the most commonly grounded conductor in RE systems. If there is a telephone system powered by the RE system, a dc-dc isolator should be used so that the RE system can have a negative ground and the telephone system can have a positive ground. Contact the Manufacturer There is not much standardization in RE equipment when it comes to the electrical interface. The manufacturer is the best source of information on equipment connections in a grounded system. Current-carrying conductors that are grounded should have white insulation and should be electrically continuous throughout the entire system from power source to load. That generally means no switches, no relays, no fuses, and no internal signal processing in the negative lead (a common problem in charge controllers). A properly designed current-measuring shunt with the same or greater ampacity than the conductor will probably be considered to be the same as a conductor. An electrical inspector expects that all conductors colored white will be at the same potential above ground -- that is zero volts. When there are switches, relays, or transistors inside charge controllers in the negative lead, these devices might violate the continuity of the grounded conductor and create an unsafe condition. One Connection Only In a grounded system, there can be only one connection between the negative conductor and the ground rod. If there is more than one connection (either intentionally or unintentionally), parallel paths for current flow will exist. Currents will flow in the normal negative conductors and in the uninsulated equipment grounding conductors. The system grounding conductor between the negative conductor and the ground rod should be as large as the largest current-carrying conductor in the system -- even if that happens to be a number 4/0 AWG battery cable. Article 690 of the NEC requires that the point of connection to the negative conductor be on the PV output circuits, and suggests that a connection closest to the array will minimize surge problems. Following this suggestion would point to the PV Disconnect Switch Enclosure as the logical place to make the connection. On a direct-drive (no batteries) system this is the ideal place. On a system with batteries and large cables, the negative battery terminal or negative inverter terminal might be a better location since the heavy grounding cable can be connected to the heavy conductor between the battery and the inverter. If, however, the system uses an inverter which has the case connected to the negative dc input terminal, then the system ground connection must be made at this point. To do otherwise, would create the parallel current paths mentioned previously. Summary Keep the system safe, meet the code, make the inspector happy. Use an equipment grounding conductor for all exposed metal surfaces. Connect the negative system conductor once to the ground rod. Use a ground rod and the proper clamps. Access Author: John Wiles, Southwest Technology Development Institute, POB 30001/Dept 3SOL, Las Cruces, NM 88003 ¥ 505-646-6105. National Electrical Code (NEC)¨, National Fire Protection Association, Batterymarch Park, Quincey, MA 02269.