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What is a Current Transformer?

By Alan Rankin
Updated May 17, 2024
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A current transformer is a device used in the measurement of electricity. Typically, electric current is distributed at high voltage levels that are dangerous to human beings. A current transformer allows the current to be measured without diverting the current into the measuring instrument, which could be risky for both the device and the person holding it. This is usually achieved by measuring the magnetic field produced by the current rather than the current itself.

There are two ways of delivering regular electric current to power a device: alternating current and direct current, sometimes abbreviated as AC and DC. Direct current is generally used in battery-powered devices, including most automobiles. Power stations use alternating current to deliver electricity to remote locations, as electricity in this form can be sent over long distances without significant energy loss. High-voltage direct current lines achieve the same goal. Both forms are different from static electricity, which is naturally occurring electricity that is too irregular to use as a power source.

The high voltage used in power delivery is dangerous to anything not specially engineered to conduct such voltage. Measuring devices, even if insulated to protect users, could easily be damaged by fluctuations in high-voltage electric current. A current transformer eliminates these dangers by providing an accurate measurement without a direct link to the current itself. The current transformer also allows for standard measurements from a wide range of different power devices. The current used in power delivery systems can vary from device to device, so standard measurements are important for efficient management of complicated systems.

Most power transformers are connected to the power delivery system, such as the transformers that reduce the high voltage from power lines to safe levels for household use. The current transformer, however, does not require a physical connection to the line being measured. All devices bearing electric current create a magnetic field. The current transformer uses this magnetic field to create a current directly proportional to the device or line being measured. A 400-amp charge, for example, might create a 4-amp reading. The instrument user, knowing the ratio is 1:100, can deduce the device’s actual current.

The current transformer is an important tool in electrical engineering. Current levels must be monitored for purposes of safety and efficiency. The measuring devices connected to current transformers allow this monitoring at various locations throughout the system. They can also be used to meter a building’s electrical usage for billing or inspection purposes.

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Discussion Comments

By NathanG — On Jun 21, 2011

@hamje32 - I learned current transformer basics back in college, but I had never actually seen them in operation. I did visit a substation once but we only saw the electrical relays. The relays are used to detect fault conditions, from what the engineer who worked there told me,

I think transformers are used to detect those spikes in electricity, like you said, before they set the relay off. That’s what I remember anyway.

I do agree that being able to measure electricity without sticking your finger in the socket, so to speak, is very useful.

By hamje32 — On Jun 20, 2011

I work in the utility industry, and I’ll tell you right now that a current transformer is one of the smartest measurement tools we have. Not having to directly touch the current ensures that our technicians are always safe. Further, if there is a spike in the electrical current we can detect it from the magnetic field without being subjected to the spike ourselves.

Instrument current transformers basically use a donut hole and wind the electrical wiring through the hole. It’s inside the hole where the magnetic field is created and the readings are measured.

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