threshold is not too much. study by the German ZVEI (Goedbloed, 1987) made a statistical Work on common mode transients on telephone subscriber lines survey of 28000 live to earth transients exceeding 100V, at 40 (Goedbloed and Pasmooij, 1990) has shown that the amplitude locations over a total measuring time of about 3400 hours. Their versus rate of occurrence distribution also follows a roughly inverse results were analysed for peak amplitude, rate of rise and energy cubic law. Actual amplitudes were lower than those on the mains. A content. Table 23.4 gives the average rate of occurrence of transients transient ringing frequency of 1MHz and rise times of 10ns to 20ns for four classes of location, and Figure 23.16 shows the relative were found to be typical. Table 23.4 Average rate of occurrence of transients 23.9.2.4 Area class Average rate of occurrence Electrostatic discharge is a further source of transient upset which (transients/hour) most often occurs when a person who has been charged to a high potential by movement across an insulating surface then touches an Industrial earthed piece of equipment, thereby discharging themselves Business through the equipment. The achievable potential depends on relative humidity and the presence of synthetic materials (Figure Domestic 23.17), and the average human body equivalent circuit can be Laboratory Mains lines Synthetic 100V) Anti-static Telecom lines Relative humidity % Figure 23.17 Maximum electrostatic charging voltages (Source: IEC 801 Part 2) represented by a 150pF capacitance in series with 330Q resistance, SO that currents of tens of amps can flow for a short period with a very fast (sub-nanosecond) rise time. Even though it may have low energy and be conducted to ground through the equipment case, such a current pulse couples very easily into the internal circuitry. 23.9.2.5 Effects of interference Maximum transient amplitude (volts) Required reliability of operation depends very much on the applica- tion. Entertainment devices and gadgets come fairly cl