Current transformers measure power flow and provide electrical inputs to power transformers and instruments. Current transformers produce either an alternating current or alternating voltage that is proportional to the measured current.

Types of Current Transformers

There are two basic types of current transformers: wound and toroidal.

• Wound current transformers consist of an integral primary winding that is inserted in series with the conductor that carries the measured current.
• Toroidal or donut-shaped current transformers do not contain a primary winding. Instead, the wire that carries the current is threaded through a window in the toroidal transformer.  

Specifications

Current transformers have many performance specifications, including primary current, secondary current, insulation voltage, accuracy, and burden.

• Primary current is the load of the current transformer, is the measured current.
• Secondary current is the range of current outputs.
• Insulation voltage represents the maximum insulation that current transformers provide when connected to a power source.
• Accuracy is the degree of certainty with which the measured current agrees with the ideal value.
• Burden is the maximum load that devices can support while operating within their accuracy ratings. Typically, burden is expressed in volt-amperes (VA), the product of the voltage applied to a circuit and the current.

Applications

There are a variety of applications for current transformers. Some devices are used to measure current in electronics equipment or motors. Others are used in street lighting.

Current transformers with small footprints mount on printed circuit boards (PCBs) and are used to sense current overloads, detect ground faults, and isolate current feedback signals.

Larger devices are used in many three-phase systems to measure current or voltage. Commercial class current transformers that monitor low-power currents are also available. Some current transformers are weatherproof or are rated for outdoor use.

Standards

Others meet MIL-SPEC, ANSI C-12, or IEC 1036 standards. Generally, ANSI class devices are intended for power monitoring applications where high accuracy and minimum phase angle are required. Additional standards can be found on the IHS standards store.

BS 2G 127 - Specification for power and current transformers for use in aircraft electrical power supply systems.

CSA C60044-1 -  Instrument transformers - part 1: current transformers.

IEC 60044-8 - Instrument transformers - part 8: electronic current transformer.

References

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Hoyt Electrical Instrument Works, Inc. | Black Box