The current transformer is a specialized transformer that converts a high primary current into a measurable and safe secondary current. Typical secondary currents are 1 A or 5 A. Current transformers provide the input variables for metering, billing, and protection devices at every voltage level. The correct choice of current transformer is a prerequisite for accurate measurement, secure billing, and reliable protection tripping at every voltage level.
How does a current transformer work?
The primary conductor carrying the main current runs through the core of the current transformer, or is wound around it. The resulting magnetic flux induces a current in the secondary winding that is proportional to the primary current – stepped down according to the turns ratio. Current transformers must never be operated with an open secondary circuit, as this could generate extremely high voltages.
Key Characteristics at a Glance
- Standardized secondary values of 1 A or 5 A
- Accuracy classes for metering (e.g. 0.2) and protection (e.g. 5P20)
- Designs: pedestal-mounted, bushing-type, or cable-type current transformers
- Available from low voltage to extra-high voltage
- Low maintenance and long-lasting (typically 30+ years)
- Multi-core transformers for metering and protection applications in parallel
- Wide-range transformers covering a large dynamic range
Frequently Asked Questions
Why must the secondary side of a current transformer never be left open?
With an open secondary side, the transformer’s entire magnetizing current can no longer be compensated for. This creates dangerously high voltages in the secondary winding, which can endanger people and equipment. Secondary terminals must therefore always be short-circuited or connected to a burden.
What should I consider when selecting a current transformer?
Important factors are the turns ratio, the accuracy class, the burden (load), and the short-time current withstand capability. For protection applications, the accuracy limit factor is also decisive, so that the transformer does not saturate even during short-circuit currents.
When are multi-core transformers used?
When a measuring point simultaneously requires highly accurate billing measurement and robust protection functions. One core is then designed for metering (class 0.2), and another for protection (class 5P20). This allows both requirements to be met without compromise in a single device.
How is a current transformer calibrated on site?
Calibration takes place at the factory or with special mobile test systems on site. Accuracy and the turns ratio are verified using known reference currents. For billing-relevant applications, regular verification under weights-and-measures regulations is legally required.
We supply current transformers in standard and custom designs. Let our specialists advise you on the optimal choice – contact us.