Telecom & Networking Interfaces
Protection Design Guide
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IEC 61000-4-4 Electrical Fast Transients (EFT)
Immunity Standard
Electrical fast transients occur as a result of arcing contacts in
switches and relays. EFT disturbances are common in industrial
environments where electromechanical switches are used to
connect and disconnect inductive loads. IEC 61000-4-4 specifies
the EFT threat in both power and data lines. The electrical fast
transient is described in terms of a voltage across a 50 load from
a generator having a nominal dynamic source impedance of 50.
The output occurs as a burst of high voltage spikes at a repetition
rate ranging from 2kHz to 5kHz. The burst length is defined as
15ms with bursts repeated every 300ms.
Each individual burst pulse is a double exponential waveform with a
rise time of 5ns and a total duration of 50ns. A diagram showing the
EFT waveform and the EFT burst repetition rate and burst period is
shown in Figure 2. Four severity levels are defined in terms of an
open circuit voltage as a function of installation environment.
The installation environments are defined as :
1 - Well protected
2 - Protected
3 - Typical industrial
4 - Severe industrial
Table 2 provides the open-circuit voltages for each threat level and
for both power supply and data lines. Short circuit current values
are estimated by dividing the EFT open-circuit voltage by its 50
source impedance. This represents the worse case stresses seen
by the suppression element. Like ESD, EFT can be especially fatal
on data and I/O lines. The fast rise time of the EFT pulses demands
a suppression element with the same characteristics as that which
are required for suppression of an ESD pulse. Again TVS diodes
offer the best solution for suppressing the expected transient
energy while keeping clamping voltages across the protected
elements to a minimum.
Additionally, the extremely fast response time of TVS diodes is
essential for responding to the 5ns rise time of the EFT pulse. Due
to the repetitive nature of the EFT pulses, TVS diodes with slightly
higher power handling capability will be required for protection at
threat level 4.
IEC 61000-4-5 Surge Standard
IEC 61000-4-5 addresses the most severe transient conditions on
both power and data lines. These are transients caused by lightning
strikes and switching. Switching transients may be the result
of power system switching, load changes in power distribution
systems, or short circuit fault conditions. Lightning transients may
result from a direct strike or induced voltages and currents due to
an indirect strike. The IEC 61000-4-5 standard defines a transient
entry point and a set of installation conditions. The transient is
defined in terms of a generator producing a given waveform and
having a specified open circuit voltage and source impedance. Two
surge waveforms are specified : The 1.2 x 50s open-circuit voltage
waveform and the 8 x 20s short-circuit current waveform (Figures
3 & 4 respectively).
Level
Peak Amplitude
Power Supply Port
I/O Signal, Data & Control Lines
V
OC
(kV)
I
SC
(A)
V
OC
(kV)
I
SC
(A)
1
0.5
10
0.25
5
2
1
20
0.5
10
3
2
40
1
20
4
4
80
2
40
Table 2 IEC61000-4-4 Severity Levels
Figure 2 EFT Burst
Figure 3 IEC 61000-4-5 Voltage Impulse
Figure 4 IEC 61000-4-5 Current Impulse
Applications