Technical Article

MS-2443

Page 3 of 10

Figure 2. IEC61000-4-4 EFT 50 Ω Load Waveforms

These fast burst transients are coupled onto the communication

lines using a capacitive clamp. The EFT is capacitively

coupled onto the communication lines by the clamp rather

than direct contact. This also reduces the loading caused by

the low output impedance of the EFT generator. The coupling

capacitance between the clamp and cable depends on cable

diameter, shielding, and insulation on the cable.

SURGE TRANSIENTS

Surge transients are caused by overvoltage from switching

or lightning transients. Switching transients can result from

power system switching, load changes in power distribution

systems or various system faults such as short circuits.

Lightning transients can be a result of high currents and

voltages injected into the circuit from nearby lightning

strikes. IEC61000-4-5 defines waveforms, test methods, and

test levels for evaluating immunity against these destructive

surges.

The waveforms are specified as the outputs of a waveform

generator in terms of open circuit voltage and short circuit

current. Two waveforms are described. The 10/700 µs

combination waveform is used to test ports intended for

connection to symmetrical communication lines: for

example, telephone exchange lines. The 1.2/50 µs combination

waveform generator is used in all other cases, in particular

short distance signal connections. For RS-485 ports, the

1.2/50 µs waveform is predominantly used and will be

discussed in this section. The waveform generator has an

effective output impedance of 2 Ω; hence, the surge transient

has high currents associated with it.