Clamp-on current monitors

Start with the expected peak current, pulse duration and 10–90% rise time. Multiply the current by the pulse duration and select a model whose peak-current and I·t ratings both exceed the resulting values; for example, a 2,000 A pulse lasting 100 µs has an I·t of 0.2 A·s. The monitor’s usable rise time should be shorter than the measured transition to control overshoot and ringing. Next, multiply the specified droop rate by the pulse duration in microseconds to estimate flat-top deviation. Sensitivity and aperture size should only be considered after these electrical limits have been satisfied.

Check the maximum RMS current, the low and high 3 dB points, and the I/f rating. For example, a 100 A peak sine wave at 50 Hz produces an I/f value of 2 A/Hz and an RMS current of approximately 70.7 A. The selected model must accommodate both figures while keeping 50 Hz within its stated frequency band. Operation close to either 3 dB point introduces additional amplitude and phase error, so margin from the frequency limits is desirable. If the signal contains harmonics, repeat the frequency and heating assessment for the highest relevant harmonic content.

Use a high-impedance instrument input when the full stated V/A sensitivity is required. A 50 Ω termination is useful for pulses with rise times below approximately 100 ns, RF measurements above a few megahertz or installations using electrically long coaxial cables, because it reduces reflections and cable fill-time effects. With a 50 Ω source and 50 Ω load, the measured voltage is approximately half the high-impedance value. Cable attenuation and termination tolerance must also be included in the current conversion. The acquisition system should therefore be configured for either terminated or unterminated sensitivity before measurements begin.

They cannot measure steady-state DC because the operating principle is based on changing magnetic flux. A repetitive unipolar pulse train contains an average DC component even though each individual pulse is time-varying. This average component can move the magnetic core towards saturation, increasing droop and the low-frequency cut-off while reducing the available I·t capability. Biasing can support some applications by resetting or offsetting the core flux, but the required arrangement must be assessed for the particular monitor and waveform. In practice, calculate the average current and pulse area rather than evaluating peak current alone.

Choose an aperture that clears the conductor and its complete insulation system without forcing the monitor against the live insulation surface. The available hole diameters range from 0.5 to 6.0 inches, but aperture size does not itself establish a safe operating voltage. The monitor case is conductive and its painted finish must not be treated as electrical insulation. Never install, open or remove the monitor while the conductor is energised. Creepage distance, clearance, conductor centring, insulation barriers, earthing and the surrounding environment must all be addressed by the equipment or test-system design.

Select a monitor whose usable rise time is shorter than the rise time of the current transition being measured. Across the range, usable rise-time ratings extend from 1 to 250 ns and represent the fastest 10–90% transition that keeps overshoot and ringing within approximately 10%. For frequencies at least one decade away from both cut-off points, phase shift is usually below 6 degrees and amplitude error below 1%. Performance becomes less predictable as the signal approaches the low or high 3 dB boundary. Timing-sensitive work should therefore consider rise time, bandwidth, phase response, cable length and oscilloscope bandwidth together.

The I²t rating represents the transient-energy limit associated with heating in the monitor’s internal secondary circuit. For a constant-amplitude pulse, it can be estimated as current squared multiplied by duration; a 500 A pulse lasting 1 ms therefore produces 250 A²s. The selected monitor’s I²t limit must exceed the calculated value with suitable allowance for waveform uncertainty and repetition. Core saturation may limit secondary energy during some unidirectional events, but it should not be used as the primary protection method. Bidirectional and oscillatory transients require particular attention because their energy is not limited in the same way by one-direction core saturation.

With a high-impedance input, the output voltage is the measured primary current multiplied by the monitor sensitivity. A 1 V/A model produces 10 V for a 10 A current, while a 0.01 V/A model produces 0.1 V for the same current. Adding a 50 Ω termination reduces these outputs to approximately 5 V and 0.05 V respectively. Passing the conductor through the aperture twice doubles the effective sensitivity, but it also halves the available I·t and I/f ratings. Peak-current, RMS-current, aperture and instrument-input limits must therefore be rechecked whenever multiple primary passes are used.