A current transformer, or CT, is the component that lets you measure a large alternating current safely, by scaling it down to a small, standardised value an instrument can read. CTs sit behind almost every power quality analyser, energy meter and protection relay. This guide explains what they do, how they work, the main types, and the one safety rule you must never break.
What a current transformer does
Measuring hundreds or thousands of amps directly is neither safe nor practical. A current transformer solves this by producing a secondary current that is an accurate, scaled-down copy of the current flowing in the main conductor. A 1000:5 CT, for example, turns 1,000 A in the primary into 5 A at the secondary, which a meter or analyser can handle comfortably.
This scaling is what makes power quality and energy monitoring possible on real installations. The analyser never touches the full current; it reads the CT's small, proportional output instead.
How a current transformer works
A CT works on the same electromagnetic principle as any transformer. The main conductor acts as the primary winding, often just a single conductor passing through the CT's core. The alternating current in that conductor creates a changing magnetic field in the core, which induces a proportional current in the secondary winding wound around it. The ratio of turns sets the ratio of currents.
Because the primary is frequently a single pass of the busbar or cable, CTs can be made to clamp around an existing conductor, which is what makes them so convenient for monitoring an installation that is already in service.
Types of current transformer
CTs come in several physical forms to suit different jobs, from permanent metering to temporary surveys.
|
Type |
Description |
Typical use |
|
Solid (ring) core |
Closed core; the conductor is threaded through |
Permanent metering and protection |
|
Split core |
Core opens to clip around a conductor |
Retrofit monitoring without disconnecting |
|
Wound primary |
Has its own primary winding |
Lower currents needing accuracy |
Split-core CTs are the practical choice for temporary power quality and energy surveys, because they clip around a live conductor without breaking the circuit. Solid-core types, threaded onto the conductor at installation, suit permanent metering where the highest accuracy is wanted.
Key specifications: ratio and burden
Two figures define a CT for a job. The ratio sets how the primary current maps to the secondary, and it must match both the expected load current and the instrument's input. The burden is the load the CT can drive at its secondary, set by the meter and the wiring; exceed it and accuracy falls. Choosing a CT means matching ratio and burden to the current you expect and the instrument you will use.
The safety rule: never open-circuit a live CT
There is one absolute rule with current transformers. Never open-circuit the secondary of a CT while current is flowing in the primary. With the secondary open, the core saturates and a dangerously high voltage can appear across the open terminals, enough to injure and to damage insulation. Always short-circuit or safely connect the secondary before disconnecting a meter from a live CT.
FAQs
What is the difference between a split-core and solid-core CT?
A split-core CT opens so it can be clipped around a conductor without disconnecting anything, ideal for retrofit monitoring. A solid-core CT is a closed ring the conductor must be threaded through at installation, offering high accuracy for permanent metering.
Why must you never open-circuit a CT secondary?
Because with current still flowing in the primary and the secondary open, the core saturates and a high, potentially lethal voltage develops across the open terminals. Short the secondary before disconnecting any instrument from a live CT.
If you are setting up power quality or energy monitoring, choosing the right CT matters. Talk to us about current transformers and the analysers they feed.