Figure: Understanding conducted EMI, radiated EMI, and radiated EMI coupling to nearby lines/cables

Bulk Current Injection (BCI) test is a conducted immunity test method. It uses a BCI probe along with other necessary equipment to assess the immunity of electrical and electronic equipment against conducted disturbances.

Conducted disturbances are high-frequency noise currents emitted or caused by electrical and electronic devices during normal operation. These currents travel along connected power and signal lines/cables, potentially affecting the performance or even damaging other equipment or systems sharing the same lines/cables. Also, conducted disturbances can occur when radiated electromagnetic fields from nearby devices are coupled with wires or cables, causing noise currents to flow along the lines. This phenomenon is known as conducted electromagnetic interference (conducted EMI).

The purpose of conducted immunity tests, such as the Bulk Current Injection (BCI) test, is to verify whether the equipment under test (EUT) has sufficient immunity to withstand conducted disturbances. During BCI testing, a BCI probe inductively injects RF disturbance currents (i.e., test signals) onto power and signal lines to determine whether the EUT can operate reliably when exposed to these disturbances. Thus, the BCI test evaluates the immunity of electrical and electronic equipment against conducted EMI caused by radiated electromagnetic fields from nearby devices coupled to wires/cables. This test ensures that the EUT will function properly in real-world environments where such conducted disturbances may occur. 

Figure: Bulk current injection test setup shown

How is the Bulk Current Injection (BCI) test conducted?

The bulk Current Injection (BCI) test setup is shown in the figure, which consists of an EUT, a signal generator, a power amplifier, a BCI probe, a measuring clamp, a suppression device, a spectrum analyzer/RF power meter, and other required auxiliary equipment.  This test setup simulates the conducted EMI caused by radiated electromagnetic fields from nearby devices coupled to wires/cables that the EUT may experience in real-world environments. 

The BCI probe is constructed using split-core ferrites with windings around the core, all housed within a rugged circular outer shield with a 50 Ω coaxial input port. This design allows the probe to be easily placed around power lines or signal cables without disconnecting them. A calibration fixture (not shown) helps to properly position the current probe around the conductor or cable during the testing.

Figure: Shows BCI probe and its construction

The BCI probe operates as a toroidal transformer. When placed around the power line or signal cable to inject a test signal, the probe functions as the multi-turn primary winding, while the power line or signal lead acts as the single-turn secondary winding. When the RF signal generator injects an RF test signal into the probe (via a power amplifier), there will be current flow through the primary winding (i.e., probe winding), generating a magnetic field. This magnetic field induces RF currents in the secondary winding (i.e., the DUT’s power lines/signal lines/cables) based on the principle of transformer action.  

Also, a current measurement probe or monitoring probe is located near the DUT to measure the injected RF current level with the help of a spectrum analyzer or RF power meter. This test setup allows the BCI probe to apply a controlled level of RF current to the EUT without requiring a direct connection to the power lines/cables (i.e., it utilizes inductively coupling). During the testing, the EUT’s performance is assessed for injected test signals. If the EUT operates satisfactorily under the injected test currents, it passes the immunity test (i.e., complies with EMC standards); otherwise, it fails to comply.

Bulk current injection test is conducted according to EMC standards such as IEC 61000-4-6 (commercial), MIL-STD 461 (Military), RTCA/DO-160 (aeronautics), and ISO 11452-4 & SAE J1113-4 (Automotive). These standards specify the test procedure, test setup, test signal levels, and other information for performing the BCI test.