Telephone line protection device for steep electromagnetic pulses - uses low pass LC filter with voltage dependent shunt impedances connected symmetrically in series and parallel with grounded chassis

摘要

Two symmetrical conductors (8,9) link the input terminals (1,2) to respective inductors (14,15) and are connected by spark gaps (10,11) to the earthed screen between the two input stages (3,4). The inductors (14,15) are connected via feed-through insulators (18,19) into an output stage (6) contg. a low-pass filter (24) with 50 Hz cutoff frequency (for power line protection). The normal operating voltage (e.g. 220V) and current (100A) are passed without distortion, but perturbing pulses of higher frequency are greatly attenuated by Zener diode or voltage-dependent resistive shunts (20,21) at the filter input connections. A 10kV peak pulse with 10ns rise time is reduced to less than 100V at 1kV/microsecond at output terminals (33,34). Alternatively the two inductors are replaced by a screened 1:1 isolating transformer.