【正文】 center of the building. This arrangement minimizes line loss on feeder and branchcircuit conductors. Some utilities supply a threephase, threewire or threephase, fourwire delta system. The mon voltages that may be obtained from the threewire delta system are 240V, 440V, or 550V. With this arrangement, a transformer must be used to obtain 120V. The usual voltages supplied from the fourwire delta system are 240V, three phase and 120V, single phase. Many large consumers purchase the electrical energy at the primary voltage, and transformers are installed on their premises. Threephase voltages up to 15 KV are often used. The service for this type of installation generally consists of metal cubicles called a substation unit. The transformers are either installed within the cubicle or adjacent to it. Isolation switches of the drawer type are installed within the cubicle. These switches are used to isolate the main switch or circuit breaker from the supply during maintenance or repair. Consumer Loop Systems Although the radial system of distribution is probably the most monly used system of transmitting power on the consumer’s property, the loop system is also employed. When installing any system, overcurrent protection and grounding must be given primary consideration. Electrical personnel who design and i nstall these systems must ply with the NEC and local requirements. Secondary Highvoltage Distribution 15 Large industrial establishments may find it more economical to distribute power at voltages higher than 600V. Depending upon the type of installation and the load requirements, voltages as high as 2300V may be used. Stepdown transformers are installed in strategic locations to reduce the voltage to a practical working value. Sometimes the highvoltage system may be radial, and the lowvoltage system may be connected into a loop. Another method is to have both the primaries and secondary connected to from a loop. （ 1） Secondary Ties Loop System It is frequently convenient to connect loads to the secondary conductors at points between transformers. These conductors are called secondary ties. Article 450 of the NEC gives specific requirements regarding the conductor sizes and overcurrent protection. （ 2） Grounding of Electrical Systems In general, most electrical systems must be grounded. The purpose of grounding is to limit the magnitude of voltage caused by lightning, momentary surges, and accidental contact with higher voltages. System grounds must be arranged to provide a path of minimum impedance in order to ensure the operation of overcurrent devices when a ground fault occurs. Current should not flow though the grounding conductor during normal operation. Directcurrent systems generally have the grounding conductor connected to the system at the supply station, and not at the individual service. Alternationcurrent system, on the other hand, must be grounded on the supply side of the main disconnect at each individual service. For specific information on the location and method of grounding, refer to NEC Article 250. Grounding of Electrical Equipment Metal conduit and cases which enclose electrical conductors must be grounded. If the ungrounded conductor es in contact with a metal 16 enclosure which is not grounded, a voltage will be present between the enclosure and the ground. This presents a potential hazard. Persons ing in contact with the enclosure and ground will plete a circuit. All noncurrentcarrying metal parts of electrical installations should be tightly bonded together and connected to a grounding electrode. Good electrical continuity should be ensured though all metal enclosures. The current caused by accidental grounds will be conducted though the enclosures, the grounding electrode to the earth. If the current is large enough, it will cause the overcurrent device to open. （ 1） GroundFault Protection A groundfault protector is a device which senses ground faults and opens the circuit when the current to ground reaches a predetermined value. A groundfault circuit interrupter is a device which opens the circuit when very small currents flow to ground. There is no way to determine in advance the impedance of an accidental ground. Most circuits are protected by 15A or larger overcurrent devices. If the impedance of a ground fault is low enough, such devices will open the circuit. What about currents of less than 15A? It has been proven that currents as small as 50mA though the heart, lungs, or brain can be fatal. Electrical equipment exposed to moisture or vibration may develop highimpedance grounds. Arcing between a conductor and the frame of equipment may cause a fire, yet the current may be less than 1 ampere. Leakage current caused by dirt and/or moisture may take place between the conductor and the frame. Portable tools are frequently not properly grounded, and the only path to ground is through the body of the operator. The groundfault circuit interrupter was developed to provide protection against groundfault currents of less than 15A. The GFCI is designed to 17 operate on twowire circuits in which one of the two wires is grounded. The standard circuit voltages are 120V and 277V .The time it takes to operate depends upon the value of the groundfault current. Small currents of 10mA or less may flow for up to 5s before the circuit is opened. A current of 20mA will cause the GFCI to operate in less than . This time/current element provides a sufficient margin of safety without nuisance tripping. The GFCI operates on the principle that an equal amount of current is flowing through the two wires. When a ground fault occurs, some of the current flowing though the ungrounded wi