tered depends critically on the distance from the source. Strong r.f. fields occur in 23.9.2.2 Radars the vicinity of radars, broadcast transmitters and r.f. heating equip- ment (including microwave ovens at 2.45GHz). The free space Another serious threat is from radars in the 1GHz to 10GHz range, electric field strength falls off linearly with distance from the trans- particularly around airports. Pulsed 50V/m field strengths can be Electromagnetic compatibility 23/11 found up to 3km from these radars. Again, building attenuation may number of transients as a function of maximum transient amplitude. give some relief, but set against this is the problem that pulsed RFI This shows that the number of transients varies roughly in inverse is particularly upsetting to microprocessor circuits. proportion to the cube of peak voltage. From these considerations a minimum of 3V/m from, say, 10MHz Rate of rise was found to increase in proportion to the square root to 1GHz represents a reasonable design criterion for RFI immunity, of peak voltage, being typically 3V/ns for 200V pulses and 10V/ns with 10V/m being preferred. Immunity from pulsed interference for 2kV pulses. Other field experience has shown that mechanical above 1GHz is extremely hard to quantify. switching usually produces multiple transients (bursts) with rise times as short as a few nanoseconds and peak amplitudes of several hundred volts. 23.9.2.3 Transients As a general guide, microprocessor equipment should be tested to withstand pulses at least up to 2kV peak amplitude. Thresholds Conducted transient immunity is becoming relatively more import- below 1kV will give unacceptably frequent corruptions in nearly all ant, because microprocessor based circuits are much more suscep- environments, while between 1kV to 2kV occasional corruption tible to transients than are analogue circuits. Mains transients, from will occur. For a belt and braces approach for high reliability many sources, are far more common than is generally realised. A equipment, a 4kV to 6kV 