Current Transformer
Current Transformer
A current transformer scales a high primary current down to the safe 1 A or 5 A signal that meters, relays and protection devices can consume. It is the sensing backbone of every switchboard: billing metering, overcurrent protection, differential schemes and earth-fault detection all depend on CTs that stay accurate across their whole operating range.
We supply measuring and protection class current transformers — solid core, split core and window type — matched to your panel schedule, ratio list and accuracy requirements. As a switchgear manufacturer operating since 2010 with 4,000+ projects behind it, our plant builds CTs into its own cabinets daily and knows the wiring clearances and burden budgets your panel builders work with.
Where a control system needs an analog signal rather than a CT secondary, a current transducer converts the measured current into a conditioned 4–20 mA or 0–10 V output for PLC and meter inputs; a current transducer connection is wired like any analog loop — unlike a CT secondary, it may be opened without danger, because the measurement is isolated inside the device.
Key Specifications & Options
The configuration below reflects the options we build to order for current transformers. Every parameter can be adjusted to match your drawings and site conditions — send us your requirements and our engineers will confirm what applies to your project.
| Parameter | Options / Typical Range |
|---|---|
| Function class | Measuring 0.2 / 0.5 / 1.0; protection 5P10 / 5P20 / 10P10 |
| Secondary rating | 1 A or 5 A |
| Primary current | From a few amps to beyond 4,000 A using window and bar-primary constructions |
| Core construction | Solid (toroidal) for new panels; split core for retrofit without breaking conductors; window type for busbars |
| Rated burden | 2.5–30 VA typical range |
| Insulation voltage | 0.72 kV and 12 kV system classes common; higher classes by project |
| Standard | IEC 61869-2 (superseding IEC 60044-1) |
| Options | Terminal covers, mounting brackets, custom ratios and dual-ratio windings |
Frequently Asked Questions
Measuring or protection class — which current transformer do I need?
Measuring classes (0.2, 0.5, 1.0) are optimised for accuracy in the normal load range — what billing wants. Protection classes (5P, 10P) stay proportional far into fault-current territory — what relays need. Most panels use both on different cores. Send the panel schedule and we split the functions correctly per circuit.
What accuracy class should billing metering use?
That follows your tariff agreement and meter standard: 0.2 class supports high-accuracy revenue metering, while 0.5 class meets most commercial metering requirements at lower cost. We will match the class your metering authority specifies rather than oversell precision.
When is a split core CT worth the premium?
Whenever the installation cannot afford a shutdown — retrofitting metering into a running plant, adding monitoring to existing feeders, temporary load studies. The hinged core clamps around the conductor without disconnecting it. For new builds, solid core remains cheaper and slightly more accurate.
Can you match ratios to our existing panel schedule?
Yes — send the schedule with primary ratings, burdens and class requirements and we manufacture to it, including odd ratios and re-rating of legacy installations. CTs are wound to order, so a custom ratio costs no more than a standard one.
How are current transformers connected?
The primary is the load conductor itself — passing through the window, or wound as turns around the core — and the secondary terminals (S1, S2) connect in series with the meter or relay input, polarity kept consistent so S1 faces the same reference as P1 on the drawings. Current transformer connections are always made through a closed burden: never open a CT secondary while the primary is energized, because the open winding develops dangerous voltage. Bar, window, and wound-primary designs change the physical current transformer connection, not this rule.
What is the CT core?
The current transformer core is the magnetic ring the primary encircles; its material sets the accuracy and saturation behavior. Cold-rolled grain-oriented silicon steel is the economical standard for metering and protection classes, while permalloy and nanocrystalline current transformer cores buy higher precision for precision metering and wide-range measurement at higher cost. Core material and cross-section together determine the accuracy class on the nameplate.
Related Guides
- Toroidal vs EI Transformer: Construction and Selection Trade-Offs — how toroidal core geometry — the same family of construction used for many CTs — behaves electrically
- Grounding Transformer Working Principle: Application and Selection Insights — earth-fault sensing schemes and where zero-sequence current measurement enters the picture
- Distribution Transformer — the power transformers these CTs meter and protect
- Medium Voltage Transformer — the MV/LV interface where protection-class CTs concentrate
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