. Electric traction and transmission engineering . heline circuit and be superposed upon the normal currents,which surges will cease when the energy has been expendedin heating the conductors, or an arc may be initiatedbetween a wire and ground over an insulator or betweentwo wires. The subsequent maintenance of the arc willbe due to energy supplied by the generator. The currentin an arc to ground is generally intermittent and, if main-tained, may set up resonant currents in apparatus con-nected with the line, since each piece of apparatus has anatural frequency of its own. These resonant curr
. Electric traction and transmission engineering . heline circuit and be superposed upon the normal currents,which surges will cease when the energy has been expendedin heating the conductors, or an arc may be initiatedbetween a wire and ground over an insulator or betweentwo wires. The subsequent maintenance of the arc willbe due to energy supplied by the generator. The currentin an arc to ground is generally intermittent and, if main-tained, may set up resonant currents in apparatus con-nected with the line, since each piece of apparatus has anatural frequency of its own. These resonant currents arelikely to be accompanied by voltages of magnitude suffi-cient to destroy insulation and cause short circuits. The energy of the magnetic field associated with a shortcircuit between line wires is delivered to the line when theshort circuit ceases, and may cause surges similar to those 254 TRACTION AND TRANSMISSION. which result from hghtning. Some writers have thereforeextended the meaning of the term ^^ hghtning to includesuch Fig. 104. 84. Protection from Lightning. — In order to protectapparatus from the high voltages due to lightning it iscommon to insert choke coils, Fig. 104, in series between theapparatus terminals and the line wires so that the incominghigh-voltage wave front may be retarded thereby for ashort interval of time. On the line side of the choke coilis installed a grounded device which conductively connectsthe line with the ground whenever the voltage of the lineexceeds a predetermined value. This device is termed alightning arrester, and its operation, in connection with thechoke coil, quickly relieves the line of excessive means must be employed, however, to prevent themaintenance of a discharge at normal voltage from the lineto ground over the path rendered conductive by the initialdischarge under excessive potentials. In nearly all typesof arresters the circuit from the hne wire to the ground is TRANSMISSION LINES. 2
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