Logitudinal voltage on cables Bonding of the cable screen to the down-conductor where it leaves the mast and with higher masts every 20 m . Thus, a potential equalization is achieved and the current over the cable screen to earth is reduced, as the down conductor has a lower impedance. Application of coaxial cables with low DC resistance over inner and outer conductor (e.g. corrugated copper tube cables of as large size as possible – larger size means also higher dielectric withstanding voltage). Application of reliable lightning EMP protection de- vices at the entry of LPZ 1. Thus, high partial lightning and induced currents (test pulse 10/350 µs according to IEC 62305) can be led to earth and over-voltages are reduced to a low level (potential equalization). HUBER+SUHNER ran several tests to evidence the ne- cessity of this measure. The cables RG 213, LMR 400, LDF 4-50A (1/2“) and LDF 5-50A (7/8“) were measured in the case of a resistive/inductive equipment input: Measurement of the longitudinal voltage U L over the inner conductor • Here a test surge current of pulse shape 8/20 µs and 10/350 µs was sent into a 1 m piece of cable, inner and outer conductor connected at the input, output screen connected to earth and inner conductor to the oscilloscope input. • Most important result: applying the 8/20 µs test pulse with 25 kA amplitude (half of the assumed load of the model antenna system, as 100 kA is the total lightning current according to PL III) leads to a calculated (if a cable lengths of 10 m is assumed, for example) longitudinal voltage of: The longitudinal voltage U L is proportional to cable length and partial lightning current amplitude! Measurements with lightning currents of pulse shape 10/350 µs resulted as expected in longitudinal voltages of smaller amplitude (due to the lower rise time) but much higher pulse energy. In case of DC selection over the coaxial cable to sup- ply power for remote active electronic circuits in the antenna system, only gas discharge tube lightning EMP protectors can be employed. The residual pulse voltage behind the protector reaches up to several hundred volts over some nanoseconds, dependent on the selected gas discharge tube. This requires additional protective devices for sensitive input circuits of electronic equipment. They can be lo- cated directly behind the gas discharge tube lightning EMP protector (or be a combined arrangement), if the equipment to be protected is nearby. Normally they should be placed at the entry of next protection zone, if a consequent zone concept is being followed (e.g. LPZ 2 – according to IEC 62305 every zone transition requires a separate lightning/surge protection device). The additional protector – here called surge sup- pressor due to its function – reduces the surge pulse voltage to a well-tolerated extent of only a few volts (e.g. HUBER+SUHNER fine protectors). RG 213: LMR 400: LDF 4-50A: LDF 5-50A: 867 V 1438 V 356 V 133 V Coaxial cable Surge pulse generator Outer conductor/screen Inner conductor Oscilloscope Y-port U L 136
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