A current transformer (CT) is a, [2] acting as a current step-down transformer--reducing secondary current via respect to its primary.
A CT is crucial in electrical systems to measure high currents by stepping them down to a safer, controllable level.
CT ratio is the inverse of the voltage ratio
CT ratio = Ipri/Isec. Example CT ratio=300/5 A means :
Ipri/Isec= N2/N1=60/1
[3]
Usual CTs
This is a usual CT for low voltage applications.
A conductor run through the CT at 100 A makes 5 A in the ammeter circuit.
[2] E.g. 100 A to the load makes 5 A in the ammeter. We rescale the meter to reflect this.
What happen to the same CT ratio 100:5 but double turns to have 5A?
Now 50A ( two turns) would produce full scale deflection of 5)
Is this X represents the primary or secondary current?
Set a meter to amps and clamp it around a wire where the ammeter is told.
The current displayed in the actual current multiplied by the number of turns within the clamp on ammeter’s jaws.
For example, if the current flowing to the load conductor is 5 amps, and we add one wrap ( two turns around the clamp jaws), the meters will read 10 amps. This feature is useful for measuring small currents.
The ammeter effectively short circuits the secondary rendering it safe.
Removing the ammeter can be dangerous.
Shorting the CT before handling the CT is best
The ammeter effectively short circuits the secondary rendering it safe.
Removing the ammeter is risky. Shorting the CT before handling the CT is best
[3] A Low-voltage CT is mainly for electrical systems working under 1 kV, often for distribution panels, commercial buildings, and industrial facilities.
Such transformers ensure accurate current measurement and system protection, for overload detection, energy monitoring, and equipment safety.
Bar-type CT consists of a solid metal bar with a secondary winding, suitable for measuring large currents, with a fixed primary bar/busbar integrated into the transformer body. This design allows direct attachment to high-voltage conductors or switchgear, ensuring maximum durability and performance in demanding environments.
Advantages: Very accurate even under heavy load conditions.
Robust and durable, capable of withstanding harsh environmental conditions.
Ideal for permanent installations requiring long-term performance.
Compact design, directly integrates into switchgear or distribution panels.
Disadvantages:
Unsuitable for small current measurements.
The fixed primary bar design limits flexibility for modifications.
Hard to replace/upgrade without dismantling its system.
Low-voltage CTs also have their own subtypes:
Wound-type CT have a core to a secondary winding, for easy installation around a conductor. Their primary is physically wrapped around a core via the secondary winding. The primary connects directly to the circuit carrying the measured current.
Downsides: More complex and costly compared to other CTs.
Requires physical integration into the circuit, increasing installation time.
Upsides: High accuracy for low-current measurements.
Sensitive to small currents, making it ideal for precision applications.
Suitable for metering and protective relays in low-power environments.
A split-core CT (clamp-on CT) has a hinged design for easy installation around conductors, can be opened and clamped around an existing conductor, for installation without disconnecting the circuit.
Upsides: Non-intrusive installation—ideal for retrofitting and upgrading systems.
Quick and easy to install on live systems without interrupting operations.
Flexible and portable, suitable for temporary monitoring.
Downsides: Lower accuracy compared to solid-core designs.
The open/close mechanism may degrade over time, impacting performance.
A mini CT is smaller and compact, often for limited spaces, often fit on printed circuit boards (PCBs) or within enclosed devices.
Upsides: Space-saving design, ideal for embedded systems.
Provides precise current measurements for low-current applications.
Suitable for integration into smart devices and metering equipment.
Downsides: Limited to small current measurements (typically below 50A).
Requires careful design and integration into electrical circuits.
Applications:
Smart meters and energy monitoring devices.
UPS systems and power supplies.
IoT-enabled energy monitoring.
Window-type CT (Toroidal CT) are made to be installed on existing conductors without disrupting the circuit, with a hollow core through which the primary conductor (busbar/cable) passes. Its secondary is wrapped around a core to measure the magnetic flux made by the primary current.
Upsides: Easy installation by passing conductors directly through the window.
Versatile—can handle a wide range of conductor sizes.
Cost-effective and widely used in low-voltage applications.
Lets many conductors to go through for higher measurement ranges.
Disadvantages:
Accuracy can decrease at very low currents.
To rise accuracy, conductors may need to loop multiple times through the window.
The ammeter must connect in series to the load, which is hard as current and wire size rise.
A clamp-on ammeter won't need th econductor to be broken, by creating a changing magnetic field in the clamp that creates a current flow in the meter.
To measure current via a CT, the conductor runs through an iron core
AC in the conductor produces a varying magnetic field.
This induces a current in a winding on the core.
The current on the winding drives an ammeter.
E.g. The secondary’s max current is 2 kA. The ammeter has a full scale deflection of 5 A. How many turns does the CT has?
CT ratio= 2000/5 = 400 turns
We use a 5A ammeter but change the scale to tell primary current between:
0-2000 A.
A small voltage drop across the primary terminals or opening the primary may make very high, dangerous voltage on the secondary side of CT.
This is 3 CTs, one per phase for metering, control or protection