Analysis is performed of the
currents injected by the IEC 61000-4-4 Capacitive Clamp using current
probes. Surprising results of the current measurements indicate that the Capacitive
Clamp is directional and sends more energy to the support equipment (AUX) than to the equipment
under test (EUT)! The directional property of the Capacitive Clamp also leads to a common lab
error where significantly more energy is applied to the EUT if the
clamp is not properly connected, potentially
causing compliant equipment to fail the test.
: Figure 1 shows a typical IEC 61000-4-4
Capacitive Clamp that is used to inject electrical fast transient (EFT)
noise on I/O leads of equipment. The clamp is symmetrical and can be
fed by the EFT generator from either end. The January 2000 Technical
Tidbit on this site titled "Displaying Measurement Error
discusses how incident and reflected waves on the clamp can cause more
energy to exit the clamp on the end opposite to the generator
connection, The data presented in this article covers a more realistic
case and its implications.
The experimental setup is shown in Figure 2. A four meter Cat 5 data
cable is positioned in the Capacitive Clamp and 150 Ohm terminations
are placed at the ends of the cable. All the conductors are connected
together in common mode. Figure 3 shows the detail of the one of the
terminations and its connections to the Cat 5 cable. In the IEC
61000-4-4 standard, the EFT generator is to be connected to the side of
the symmetrical Capacitive Clamp that is closest to the EUT. In Figure
2, that defines the termination on the right as the "EUT" and the
termination on the left as the "AUX equipment."
Figure 3. 150 Ohm Cable Termination
Figure 4 shows the current flowing in the terminations of Figure 2
when EFT is supplied from the generator. For the measurements, a pair
of matched Fischer F-33-1 current probes
were placed on the Cat 5 cable near the terminations in the same
orientation (same face of each probe to the right). This produces
opposite current directions in the probes so their outputs are inverted
with respect to each other. See the April 2005 Technical Tidbit, "Inductive and Capacitive Coupling - Induced Current Characteristics"
for an explanation of why the current should be in opposite directions as far as the current probes are concerned.
Output from Current Probes on Cat 5 Cable in same Orientation
The surprising result shown in Figure 4 is that about one third
more current is directed to the AUX end of the cable than to the EUT
end. This result is consistent with results from the January 2000
Technical Tidbit on this site titled "Displaying Measurement Error
The more realistic test setup shown in Figure 2 produces slightly
different results than the January 2000 article, although with the same
conclusions. The data in Figure 4 suggests that the Capacitive Clamp
may be specified backwards in the standard. The generator level used in
this experiment was low, a few hundreds of Volts, so as not to overload the
scope inputs or require external attenuators on the scope.
personally seen many test labs connect the Capacitive Clamp to the
generator on the end opposite the EUT. This essentially turns the EUT
into the AUX equipment and therefore applies more current to the EUT
than the IEC 61000-4-4 standard calls for. In some cases, I have seen
much more than the 30% additional current in the data above
(depending on equipment impedances) delivered to the EUT.
error in the test setup is easy to understand. The clamp is one meter
long and a one meter cable is specified to connect it to the EFT
generator which itself is often bolted to the metal test floor. This
setup can often make it difficult to connect the clamp on the proper
end. If your equipment fails this test, be sure to look carefully at
the test setup pictures in the test report! Maybe your equipment really
The Capacitive Clamp used in IEC 61000-4-4 delivers significantly
more current to the auxiliary support equipment than the equipment
under test. It appears that the Capacitive Clamp connections may be specified backwards
in the standard. A common error I have seen made by test labs is to
connect the generator to the wrong end of the clamp. This error turns
the EUT into the AUX equipment and over tests the EUT. Such a condition can potentially
fail a compliant EUT.
of Sunnyvale, CA for the use of their facilities to take the data presented in this article and
tutorial on this subject, covering background as well as more technical details, is available at:
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