tailieunhanh - POWER QUALITY phần 7

Mỗi vòng lặp đại diện cho các yêu cầu từ hóa quyền lực cao hơn và các khoản lỗ lõi cao hơn. Một thứ hai và ảnh hưởng nghiêm trọng hơn của sóng hài do dòng tần số hài hòa trong các cuộn dây biến áp | On the other hand consider Figure where the motor frame is not bonded to a ground. If the source feeding the motor were a grounded source considerable leakage current would flow through the body of the person. The current levels can reach values high enough to cause death. If the source is ungrounded the current flow through the body will be completed by the stray capacitance of cable used to connect the motor to the source. For a 1 0 cable the stray capacitance is of the order of pF for a 100-ft cable. The cable reactance is approximately 15 600 Q. Currents significant enough to cause a shock would flow through the person in contact with the motor body. NATIONAL ELECTRICAL CODE GROUNDING REQUIREMENTS Grounding of electrical systems is mandated by the electrical codes that govern the operation of electrical power systems. The National Electrical Code NEC in the . is the body that lays out requirements for electrical systems for premises. However the NEC does not cover installations in ships railways or aircraft or underground in mines or electrical installations under the exclusive control of utilities. Article 250 of the NEC requires that the following electrical systems of 50 to 1000 V should be grounded Systems that can be grounded so that the maximum voltage to ground does not exceed 150 V Three-phase four-wire Wye-connected systems in which the neutral is used as a circuit conductor Three-phase four-wire A-connected systems in which the midpoint of one phase winding is used as a circuit conductor Alternating current systems of 50 to 1000 V that should be permitted to be grounded but are not required to be grounded by the NEC include Electrical systems used exclusively to supply industrial electric furnaces for melting refining tempering and the like Separately derived systems used exclusively for rectifiers that supply adjustable speed industrial drives Separately derived systems supplied by transformers that have a primary voltage rating less