No single sensing technology dominates every criterion.
Current and voltage measurement always involves trade-offs between galvanic isolation, bandwidth, response time, DC capability, accuracy, long-term stability, size, power consumption and cost. This article compares the principal technologies used in industrial and railway electrical systems and highlights where each approach is strongest.
The correct technology is defined by the measurement function and operating environment — not by a universal hierarchy of sensor types.
Passive current-measurement technologies
Resistive shunt
A calibrated low-value resistor is inserted in series with the conductor. The measured voltage follows Ohm's law, V = R·I. It measures AC and DC with very high bandwidth and can achieve high accuracy, but it provides no inherent galvanic isolation and dissipates power through Joule losses.
Current transformer
Magnetic coupling between a primary conductor and secondary winding provides native isolation. A conventional current transformer measures AC only and can saturate under high fault current or DC bias. IEC 61869-2 is the key reference for instrument current transformers.
Rogowski coil
An air-core winding produces a voltage proportional to di/dt. Electronic integration reconstructs the current waveform. With no ferromagnetic core it does not saturate, which is valuable for very high fault currents and wide-band transient analysis, but it does not measure DC.
Open-loop and closed-loop Hall-effect sensing
A Hall element senses the magnetic field generated by the primary current without inserting a resistive element in the power path. Both open-loop and closed-loop architectures can measure AC and DC while maintaining galvanic isolation.
Direct field measurement
The Hall signal is amplified directly. The architecture is compact and cost-effective, but accuracy is more sensitive to Hall offset and temperature drift.
Flux compensation
A secondary compensation winding continuously drives the net core flux toward zero. The resulting secondary current is proportional to the primary current, improving linearity, accuracy and thermal stability at the cost of greater complexity and power consumption.
Current sensing at a glance
| Technology | DC capability | Galvanic isolation | Typical bandwidth | Typical accuracy |
|---|---|---|---|---|
| Resistive shunt | Yes | No | DC to 1 MHz+ | 0.1–0.5% |
| Current transformer | No | Native | ~15 Hz–100 kHz | 0.2–1% |
| Rogowski coil | No | Native | ~2 Hz–30 MHz | ~0.5% |
| Hall — open loop | Yes | Yes | DC to ~100 kHz | 1–2% |
| Hall — closed loop | Yes | Yes | DC to ~300 kHz | <0.5% |
Bandwidth and accuracy values are technology-level indications from the source article, not product-level specifications.
Approaches to isolated voltage measurement
Resistive divider
A resistor network scales the measured voltage to an electronics-compatible level. It is simple and wide-band, but does not inherently provide galvanic isolation and is sensitive to high-voltage resistor drift and leakage in polluted or humid environments.
Capacitive divider
A capacitive ratio performs the voltage scaling without continuous resistive dissipation. Its response depends strongly on frequency and parasitic capacitance, which limits transient accuracy.
Isolated Hall-based voltage sensor
A precision high-voltage resistor converts the voltage into a small current, which is then measured by an isolated Hall current transducer. This combines galvanic isolation with good DC capability and stability.
Optical / Pockels sensing
An electro-optical crystal changes light polarisation according to the applied electric field. The optical link is intrinsically dielectric and highly immune to electromagnetic interference, but the architecture is more complex and costly.
| Technology | Isolation | Frequency dependence | Typical use |
|---|---|---|---|
| Resistive divider | No inherent galvanic isolation | Low | LV/MV, cost-controlled measurement |
| Capacitive divider | No inherent galvanic isolation | Marked | HV capacitive voltage measurement |
| Isolated Hall | Galvanic | Low | MV/HV where isolation and DC capability matter |
| Optical / Pockels | Dielectric | Very low | HVDC, smart grids, extreme EMC environments |
Choose by function, not by habit.
Shunts and Hall-effect technologies can reproduce a DC component; conventional current transformers and Rogowski coils cannot.
Rogowski coils and shunts provide very wide bandwidth for dynamic protection and harmonic analysis.
Magnetic or optical coupling is structurally valuable when power circuits must be safely separated from low-voltage control electronics.
Closed-loop compensation is attractive where low drift and high linearity are important for protection selectivity or precision measurement.
| Application | Measured quantity | Technology direction | Main reason |
|---|---|---|---|
| Protection / selectivity | Current | Rogowski or closed-loop Hall | Fast response, fault-current behaviour |
| Energy metering | Current | CT or precision closed-loop Hall | Accuracy and stability |
| Power quality / harmonics | Current | Rogowski or shunt | Wide bandwidth |
| MV/HV voltage supervision | Voltage | Isolated Hall or dedicated divider | Isolation and control accuracy |
| HVDC / smart grid | Voltage | Optical | EMC immunity and dielectric isolation |
Glossary
Galvanic isolation. Electrical separation between two circuits with no direct conductive path.
Bandwidth. Frequency range over which the measurement chain reproduces the signal within its specified performance.
Magnetic saturation. A core condition in which further primary-current increase can no longer produce a proportional magnetic response.
IEC 61869. International series covering instrument transformers and measurement transformers, including accuracy classes.
Selectivity. Protection strategy in which only the protective device closest to a fault opens, leaving the rest of the installation energised.
Translate the measurement requirement into the right sensing architecture.
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