Chapter A General rules of electrical installation design

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© Schneider Electric - all rights reserved J4 J - Protection against voltage surges in LV Three points must be kept in mind: b A direct or indirect lightning stroke may have destructive consequences on electrical installations several kilometres away from where it falls b Industrial or operating voltage surges also cause considerable damage b The fact that a site installation is underground in no way protects it although it does limit the risk of a direct strike General Transient overvoltages at industrial frequency (see Fig. J4) These overvoltages have the same frequency as the network (50, 60 or 400 Hz); and can be caused by: b Phase/frame or phase/earth insulating faults on a network with an insulated or impedant neutral, or by the breakdown of the neutral conductor. When this happens, single phase devices will be supplied in 400 V instead of 230 V. b A cable breakdown. For example, a medium voltage cable which falls on a low voltage line. b The arcing of a high or medium voltage protective spark-gap causing a rise in earth potential during the action of the protection devices. These protection devices follow automatic switching cycles which will recreate a fault if it persists. Normal voltage 230/400 V Fig. J4 : Transient overvoltage at industrial frequency Voltage surges caused by electrical discharge In a dry environment, electrical charges accumulate and create a very strong electrostatic field. For example, a person walking on carpet with insulating soles will become electrically charged to a voltage of several kilovolts. If the person walks close to a conductive structure, he will give off an electrical discharge of several amperes in a very short rise time of a few nanoseconds. If the structure contains sensitive electronics, a computer for example, its components or circuit boards may be damaged. .3 Main characteristics of voltage surges Figure J5 below sums up the main characteristics of voltage surges. Type of voltage surge Voltage surge Duration Front gradient coefficient or frequency Industrial frequency y 1.7 Long Industrial frequency (insulation fault) 30 to 1,000 ms (50-60-400 Hz) Operation 2 to 4 Short Average 1 to 100 ms 1 to 200 kHz Atmospheric > 4 Very short Very high 1 to 100 μs 1 to 1,000 kV/μs Fig. J5 : Main characteristics of voltage surges Schneider Electric - Electrical installation guide 2008 Transient overvoltage Normal voltage 230/400 V t
J - Protection against voltage surges in LV General .4 Different propagation modes Common mode Common mode voltage surges occur between the live parts and the earth: phase/earth or neutral/earth (see Fig. J6). They are especially dangerous for devices whose frame is earthed due to the risk of dielectric breakdown. Fig. J6 : Common mode Differential mode Fig. J7 : Differential mode Ph Ph Schneider Electric - Electrical installation guide 2008 N N Imc Voltage surge common mode Imd U voltage surge differential mode Imd Equipment Differential mode voltage surges circulate between live conductors: Phase to phase or phase to neutral (see Fig. J7). They are especially dangerous for electronic equipment, sensitive computer equipment, etc. Imc Equipment J5 © Schneider Electric - all rights reserved
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