Frequently Asked Questions: Current Monitor Probes
What is an RF current probe and how does it measure current without making electrical contact?
An RF current probe is a clamp-on sensor that measures the RF current flowing in a wire, cable, or cable bundle inductively — without breaking the connection or piercing the insulation. Internally, the probe is a toroidal transformer: the conductor under test acts as a single-turn primary winding, and the multi-turn secondary wound on the ferrite core delivers a voltage to a 50Ω output proportional to the current in the conductor. Because the measurement is inductive, there’s no DC loading, no need to disconnect anything, and no risk of disturbing the circuit under test. This makes current probes the standard tool for in-situ conducted emissions measurements on power cords, signal cables, and harness bundles.
What is transfer impedance and why is it the key specification for a current probe?
Transfer impedance (ZT) is the ratio of the probe’s output voltage to the current flowing in the conductor being measured, expressed in ohms or dBΩ: ZT = Vout / Iprimary. For example, a probe with ZT = −11 dBΩ (0.28 Ω) produces 0.28 V of output for every 1 A flowing in the conductor. Transfer impedance is frequency-dependent, and the individual calibration data supplied with each Com-Power probe gives ZT across the full frequency range. Measurement software or EMI receivers use this data to convert measured voltage (dBµV) into current (dBµA) for comparison against the CISPR, MIL-STD-461, or DO-160 limit line.
Which EMC standards require or use current probes?
Current probes are specified in a number of major EMC standards. CISPR 16-1-2 defines the instrumentation requirements for current probes used in commercial conducted emissions testing. MIL-STD-461 uses current probes for multiple test methods including CE101 (conducted emissions, power leads, 30 Hz – 10 kHz), CE102 (conducted emissions, power leads, 10 kHz – 10 MHz), and CS114 (conducted susceptibility, bulk cable injection, where the probe monitors injected current). RTCA DO-160 Section 21 specifies current probe measurements for airborne equipment conducted emissions. Current probes are also used in FCC Part 15/18 pre-compliance work, automotive CISPR 25, and general RF diagnostic/debug activities.
What’s the difference between a current monitor probe and a bulk current injection (BCI) probe?
Both are clamp-on probes that couple to a conductor inductively, but they are optimized for opposite use cases. A current monitor probe (like the Com-Power CLCE series on this page) is a receive-only device optimized for low-noise, broadband measurement of RF currents. A bulk current injection (BCI) probe is designed to transmit — it’s driven by a power amplifier to inject a known RF current into a cable bundle for susceptibility testing (e.g., MIL-STD-461 CS114, ISO 11452-4, RTCA DO-160 Section 20). BCI probes handle much higher drive power but have lower measurement sensitivity. A common setup uses a BCI probe for injection and a current monitor probe nearby to verify the actual injected current level. See Com-Power’s Bulk Current Injection Probes page for BCI options.
How do I choose the right aperture (inner diameter) for my application?
Aperture size determines what fits through the probe. Pick the smallest aperture that comfortably fits the cable or bundle you need to measure — smaller apertures generally give higher transfer impedance and better sensitivity at a given frequency, while larger apertures accommodate thicker harnesses. The 27 mm CLCE-227 is ideal for individual wires and small bundles. The 32 mm CLCE-332 and CLCE-1032 suit most standard cable bundles in commercial and military EMC testing. The 38 mm CLCE-438 handles medium-diameter harnesses. The 52 mm CLCE-252 and CLCE-452 V2 are built for large harness bundles or multi-cable groups where disconnection is not an option. Always leave some clearance — forcing a tight fit risks mechanical damage to both probe and cable.
What does “split-core” mean and why is it important?
A split-core probe has a hinged ferrite core that can be opened, placed around a cable, and closed around it — without disconnecting the cable from anything. All Com-Power CLCE-series probes are split-core. This is critical for practical EMC work because production cables and wiring harnesses in real systems can’t be removed or threaded through a solid toroidal probe without major disassembly. The split-core design lets the technician clamp the probe onto any point along a cable in seconds, take the measurement, and move on. The mating faces of the ferrite halves are precision-ground to minimize the air gap when the probe is closed, preserving transfer impedance performance.
Why does the specification show different current ratings at DC–60 Hz vs. 400 Hz?
The maximum primary current a probe can handle is limited primarily by ferrite core saturation, which is a function of the magnetic flux through the core. At higher line frequencies, the flux swing per unit time is larger, which drives the core closer to saturation at lower peak currents. That’s why you see, for example, 150 A at DC–60 Hz but only 135 A at 400 Hz for the CLCE-1032. The 400 Hz rating is particularly relevant for aerospace applications (400 Hz is a standard aircraft AC frequency under DO-160), while the DC–60 Hz rating applies to most commercial and industrial systems. Do not exceed the rated current at any frequency — core saturation produces severe nonlinearity and can permanently degrade the probe.
How are Com-Power current probes calibrated and what documents come with each probe?
Every Com-Power CLCE-series probe is individually calibrated on a 50Ω coaxial calibration fixture with NIST traceability. The calibration process measures transfer impedance across the full frequency range and produces a tabulated ZT vs. frequency data set that ships with the probe along with the calibration certificate. You load this calibration file into your EMI receiver or measurement software so that voltage readings are automatically converted into current. Refer to the Current Probe Calibration Fixtures page for compatible calibration jigs. Annual recalibration is recommended to maintain traceability; ISO 17025 accredited calibration is available on request.
Can I use a current probe for conducted susceptibility (CS114 / BCI) as well as emissions?
Yes — specifically as the monitor probe, not the injection probe. In MIL-STD-461 CS114 and similar bulk current injection tests, a dedicated BCI probe is driven by a power amplifier to inject a specified RF current level into the cable bundle, and a second current probe (such as the CLCE-227 or CLCE-332) is clamped nearby to independently measure the actual injected current. The injected level is adjusted until the monitor probe reading matches the standard’s required test level. Using a calibrated current monitor probe for this purpose avoids relying solely on the BCI probe’s calibration and the power amplifier’s output — both of which can drift with load impedance.
Application of RF Current Probes
RF current probes are employed to monitor or measure asymmetrical disturbance RF currents on a wire, cable, or cable bundle without making direct conductive contact with the source conductor. The current is measured inductively by clamping the probe around the conductor(s) to be tested — no actual contact is made with the conductor(s), and the insulation is left in place. Essentially, a current probe functions as a toroidal transformer where the conductor(s) act as a single-turn primary and the probe as a multiple-turn secondary.
The primary applications include conducted emissions compliance testing (CISPR 16-1-2, MIL-STD-461 CE101/CE102, RTCA DO-160 Section 21), conducted susceptibility monitoring (MIL-STD-461 CS114, ISO 11452-4 BCI), pre-compliance debug of switching power supply conducted emissions, and RF current diagnosis on motor harnesses, shielded cables, and equipment interconnects. Because the measurement is non-invasive, probes can be clamped onto production hardware at any convenient point without modification.
Current Probe Calibration Fixture
Current probes are calibrated using a calibration fixture that provides a 50Ω coaxial-type transmission line arrangement. The fixture allows the probe to be clamped around the center conductor, while the outer conductor encapsulates the probe on four sides so that the transmission line characteristics are not compromised. This geometry produces a precise, repeatable RF current in the center conductor that serves as the reference for measuring probe transfer impedance. See the Current Probe Calibration Fixtures page for compatible fixtures and ordering information.
