Manufacturer of Low and Medium Voltage Switchgear


Current transformers play a crucial role in power systems for primary measurement and control purposes. During normal operation, the secondary load impedance is very low, resulting in a low secondary output voltage. This is similar to a transformer operating under short-circuit conditions. The secondary side is typically rated for full-scale currents of 1A or 5A, with load capacities ranging from 5VA to 100VA. The corresponding load resistance falls within the range of 0.2Ω to 20Ω, and the secondary voltage typically ranges from 1V to 20V.

However, if there is an open circuit in the secondary winding or an abnormal overcurrent flows through the primary winding (such as lightning current, resonance overcurrent, capacitor charging current, inductance starting current, etc.), the secondary side may experience voltage surges reaching thousands or even tens of thousands of volts. This can lead to insulation damage within the secondary system, overexcitation and subsequent burnout of the transformer, and even pose a risk to the safety of personnel.

Consequences of a secondary open circuit in a current transformer:

  1. Steep-wave high-amplitude overvoltage: An open circuit in the secondary winding can result in the generation of steep-wave high-amplitude overvoltages. This poses a severe risk to the insulation of equipment and the safety of workers. The excessive voltage can cause insulation breakdown, damaging the equipment and potentially causing harm to personnel.
  2. Increased core loss and overheating: When an open circuit occurs, the current transformer experiences increased core loss. This leads to excessive heat generation, which can cause overexcitation and ultimately result in the burnout of the transformer. The overheating not only affects the performance of the current transformer but also poses a fire hazard.

  3. Measurement errors and system failure: The presence of residual magnetism in the iron core, caused by the open circuit, can lead to measurement inaccuracies in the current transformer. The increased magnetism interferes with the proper functioning of the transformer, potentially leading to erroneous readings. In critical situations, this can cause the failure of the secondary control and protection systems, compromising the overall safety and stability of the power system.

Reasons for the open circuit in a current transformer:

Defects in test terminals or connecting pieces: Poor contact between the screw rod and the screw hole of the copper connecting piece due to structural or quality issues can result in an open circuit during operation.

Insufficient tightness of terminal joint crimping: In the secondary circuit, if the crimping of the terminal joints for the secondary wires is not tight, heat or oxidation caused by high currents can lead to an open circuit.

Errors during repair or testing: Occasionally, mistakes made by repair and test personnel can result in open circuits. For instance, forgetting to connect the connectors inside relays or instruments during acceptance testing can lead to an open circuit.

Moisture and corrosion in terminal boxes: Outdoor terminal boxes and junction boxes can become damp over time, leading to serious corrosion of terminal screws and gaskets. This corrosion can result in an open circuit.

Incorrect assembly of test terminal pressure plates: If the bakelite material of the test terminal pressure plate in the current circuit is too long, the metal sheet of the rotary terminal may mistakenly press onto the bakelite instead of the metal sheet of the pressure plate. This assembly error can cause an open circuit.

When an open circuit occurs in the secondary side of a current transformer, it is often accompanied by the following phenomena:

Abnormal indications on circuit instruments: Generally, the indications on the measuring instruments will decrease or become zero. An open circuit in the current loop used for metering can cause inconsistent readings on three-phase current meters, decreased indications on power meters, slow or non-rotating indications on watt-hour meters. If the indications are intermittent, it may indicate a semi-open circuit state (poor contact).

Unusual noise, uneven vibration, excessive heat, or smoke from the current transformer itself. However, these phenomena may not be evident at low loads.

Discharge or sparking at the terminals or connections of the current transformer’s secondary circuit.

Malfunction or abnormal operation of protection relays or cabinets. This situation may be discovered and addressed during false tripping or out-of-step tripping events.

Smoke or damage to energy meters and relays. When the active/reactive power meters, transmitters of energy meters or motion devices, and relays of protection devices are damaged or burnt, it can result in an open circuit in the secondary side of the current transformer, as well as a short circuit in the secondary side of the potential transformer.

According to the actual requirements of the power system, our company has developed a new generation of protection devices based on the first and second-generation models. These new devices feature pure digital signal technology and offer the following capabilities:

  • Action contact output: The devices provide an action contact output, allowing for easy integration with other control systems in the power system.
  • Automatic luminous display: The devices are equipped with an automatic luminous display, providing clear visual information about their status and operation.
  • Automatic locking differential protection: The devices incorporate an automatic locking differential protection feature, ensuring efficient response and fault detection.
  • Manual and automatic reset function: The devices support both manual and automatic reset functions, allowing for convenient resetting after a fault or abnormal condition.

The product adopts advanced MCU centralized control technology and high-performance imported microcontrollers, ensuring a long service life and reliable operation for more than 100,000 cycles. The devices offer fast action speed, strong overload capacity, and zero static leakage current, meeting the various protection needs for current transformers (CTs). These new-generation devices are designed to meet the specific requirements of power systems, providing accurate measurement, effective fault detection, and reliable protection for CTs.

Principles of product protection

The device incorporates a high-performance MCU centralized control system, with the core control unit consisting of an imported high-performance single-chip microcomputer and peripheral devices to form a signal acquisition and data processing system. It is connected in parallel to the secondary side of the current transformer (CT). During normal operation, the voltage across the protector remains below 20V. When the single-chip microcomputer detects a conduction voltage below the programmed threshold, no signal is sent from the output pin, and the protector remains in an open state, resulting in zero current flow. This ensures that the device has no impact on the operating values of the circuit or the accuracy of the meter.

In the event of an open circuit in the secondary circuit or an abnormal overcurrent in the primary winding, the voltage generated in the secondary winding exceeds the normal operating voltage (the actual value depends on the CT’s parameters and operating conditions). The single-chip microcomputer in the device’s secondary overvoltage protector receives and compares this signal with the specified conduction voltage value. If the value exceeds the conduction voltage threshold, the output pin of the single-chip microcomputer sends a signal to trigger the instantaneous action of the protector (within 1 ms). The alarm light is activated, indicating an alarm signal and the differential protection is locked. Once the fault is resolved, the protector can be reset manually or automatically (within 30 seconds) and is ready for use again. The device has a service life of tens of thousands of cycles, providing stable and reliable operation.


  • The KC500D current transformer secondary overvoltage protector is extensively employed in power systems. It is designed for the protection of indoor high, medium, and low voltage switch internal current transformers, outdoor current transformers, current transformers in box-type substations, and other similar installations.
  • The KC500D current transformer secondary overvoltage protector is utilized in various windings on the secondary side of the current transformer, including differential winding, overcurrent winding, measuring winding, busbar protection winding, backup winding, and more. It ensures the effective and reliable protection of these windings to maintain the overall stability and performance of the current transformer system.

Wiring principle

In general, current transformers are connected to three phases (A, B, and C), although some may be connected to two phases or even just one phase. The majority of current transformers are connected in a star configuration, while a few are connected in a delta configuration.

When it comes to the protector, it is connected in a star configuration to the secondary windings. The A, B, and C windings of the current transformer are correspondingly connected to the A, B, and C terminals of the protector. The three-phase secondary neutral point (virtual ground N) of the A, B, and C windings is connected to the N terminal of the protector. If only the A and B windings are used, the C phase can be left unconnected without affecting the normal operation of the protector.

This configuration ensures the proper distribution and protection of currents in the current transformer system, allowing for accurate measurement, control, and safety in the power system.

he protector is designed to be directly connected to the AC220V power supply terminal. There are three alarm lines, one at the midpoint and the other two labeled as normally open and normally closed. The user should wire these lines according to their specific requirements and intended use.

The device offers a pair of normally open contacts and a pair of normally closed contacts, each with a capacity of AC277V/10A. These contacts can be connected to various sound and light alarms or used for remote transmission of information to a comprehensive protection device. When the protection is activated, the normally open contact closes, while the normally closed contact opens.

Key Features

  1. High-speed microcontroller control: The product is equipped with a high-speed microcontroller for efficient and precise control of operations.
  2. High-precision A/D sampling: The device utilizes high-precision analog-to-digital (A/D) sampling technology, ensuring accurate and reliable measurement and data acquisition.

  3. Industrial-grade design: The product is designed to meet industrial standards, guaranteeing durability and suitability for rugged environments.

  4. Reliable performance: The device delivers consistent and dependable performance, minimizing the risk of errors or failures.

  5. Complete optical isolation: Both signal input and output are fully optically isolated, ensuring enhanced safety and preventing interference between circuits.

  6. Unique anti-interference technology: The product incorporates advanced anti-interference technology, enhancing its ability to withstand and reject external disturbances.

  7. Multi-signal output: The device offers multiple signal output options, providing flexibility for various applications and system configurations.

  8. Alarm relay output: The product includes an alarm relay output, enabling seamless integration with alarm systems or other external devices.

  9. Blocking differential protection performance: The device provides effective blocking differential protection, safeguarding against potential faults or abnormalities.

  10. Infinite input impedance: The input impedance is infinitely high, ensuring that it does not affect any measurement or protection performance when connected.

  11. Pre-settable secondary side protection voltage: The secondary side protection voltage can be easily pre-set to suit different operating conditions or requirements.

  12. Fault location display: After a protection event, the device displays the fault location, simplifying troubleshooting and maintenance tasks.

  13. Contact signal output: The device offers a contact signal output after protection, allowing convenient integration and utilization within a larger system.

  14. Reset options: Users can choose between manual button reset or automatic reset (30s) after the fault disappears, providing flexibility and convenience.

  15. Compact size: The device features a small form factor, making it suitable for local installations. It can be easily mounted on guide rails and allows for straightforward inspection and debugging.

Product technical standards and parameters

Product Technical Standards
GB/T1408-1989                   Test method for power frequency electric strength of solid insulating materials
GB/T2423.46-1997             Environmental experiments for electrical and electronic products
DL/T620-1997                     Overvoltage Protection and Insulation Coordination of AC Electrical Installations
GB7251.1-1997                      Low-voltage complete switchgear and control equipment
GB1208-1997                       Current Transformer Standard

Selection model and specification

Product Dimensions

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