Figure 1. An IEC 61000-4-4 Capacitive Clamp

Abstract: 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.

Discussion:  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 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.


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.


Summary: 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.