Overcurrent test

The overcurrent test of ship generators is to ensure that the generator can cut off the current in time when it encounters an overcurrent fault, so as to prevent equipment damage and accidents. This testing process includes the calibration of the overcurrent protection device to ensure that its action time, action current and recovery function meet the design requirements. The overcurrent test not only ensures the safe operation of the generator, but also ensures the stability of the ship's power system.

I. Overcurrent Test Overview

The purpose of the overcurrent test is to simulate overcurrent conditions that may occur in actual operation and to check the responsiveness of the generator and protective devices in the event of a fault. The overcurrent test has the following main aspects:

• (1) Verify the action values and action times of overcurrent protective devices.
• (2) Check the recovery function of the overcurrent protective device to ensure that it can return to normal after an overcurrent.
• (3) Ensure that overcurrent protection devices do not generate false actions and avoid misjudgment of current fluctuations under normal operating conditions.

II. Overcurrent test principle

In order to carry out overcurrent testing, a range of test devices are required to simulate overcurrent conditions and to check the performance of protective devices. The main test devices include:

• (1) Current Generator:Used to simulate different overcurrent situations.
• (2) Switch Tester:Used to check the response of an overcurrent protective device, focusing on recording the time from when the protective device is triggered to when it is actuated.
• (3) Current meter:Used to measure the magnitude of current, monitor current in real time and record data.
• (4) Overcurrent protection device:Used to protect generators from overcurrent damage, such as relays or circuit breakers.

Figure 6- 1 Overcurrent test device schematic diagram

In practice, when testing overcurrent protective devices, the protective device should be disconnected from the power supply and load. As an example, marine frame air circuit breakers have three main working positions: connection position (working position), test position and disconnection position.

(i) Connection position (working position)

The connection position is the position of the circuit breaker when it is in normal operating condition. In this position, the main contacts of the circuit breaker are fully closed and current can pass through the circuit breaker into the load circuit to ensure that the electrical equipment can operate normally.

In the connected position, the circuit breaker is in full contact with the main circuit of the power system, allowing current to flow and supplying power to the electrical equipment. In the connected position, the circuit breaker will continuously monitor the operating status of the circuit, and in the event of faults such as overloads and short circuits, the circuit breaker will automatically disconnect the circuit to prevent damage to the electrical equipment. This is the regular operating condition in the power system, in which all load devices can work continuously to ensure that the system can operate stably.

(ii) Test location

The test position is the position of the circuit breaker when it is undergoing a functional test or fault simulation. In this position, the main circuit contacts of the circuit breaker are disconnected, but the control circuit remains connected. In this state, the circuit breaker is unable to energize the load equipment, but can perform various tests and function verification.

In the test position, circuit breakers can be tested by external test equipment (e.g. main switch calibrators, simulators, etc.) to check that they are able to operate according to predefined parameters. Faults in the electrical system, such as overloads, short circuits, etc., are simulated to ensure that the circuit breaker is able to disconnect the current in time to prevent the fault from spreading under these conditions. Testing in the test position verifies that the circuit breaker's protection characteristics can respond correctly, including delayed protection, instantaneous protection, overload protection, etc.

(iii) Disconnection position

The disconnected position is the position in which the circuit is isolated from the power supply when the main circuit contacts of the circuit breaker are completely disconnected. In this position, current cannot enter the electrical equipment through the circuit breaker and the entire circuit is completely cut off.

The disconnect position is used to completely cut off the power supply, completely isolate the electrical equipment from the power source, ensure that the equipment will not be electrified during repair, maintenance or troubleshooting, and protect the safety of the operating personnel. In the event of a serious fault or other emergency, the circuit breaker can be quickly placed in the disconnect position to cut off the power supply and prevent the fault from expanding. When the electrical equipment needs to be overhauled or replaced, the circuit breaker is switched to the disconnect position to ensure that the equipment is completely disconnected from the power supply to avoid the danger of electric shock.

Overcurrent test procedure

(i) Preparatory phase

1. Ensure that the test equipment and measuring instruments are properly connected and check that all equipment is operating correctly.
2. Calibrate the current generator and set the test range for overcurrent.

(ii) Setting of protective device parameters

1. Set the action current value of the protective device according to the rated current of the generator and the technical requirements of the overcurrent protective device.
2. Set the action delay time of the overcurrent protective device to ensure that the protective device responds according to the set time during the test.

(iii) Simulation of overcurrent faults

1. Start the current generator and gradually increase the output current until the set overcurrent threshold is reached.
2. Observe that the overcurrent protection device operates as set.
3. Record the response time and current value of the overcurrent protection device to ensure that the overcurrent protection device will break the circuit within the specified time.

(iv) Recording of test results

Records the action time, current value and recovery function of the protective device when overcurrent occurs.

Four, overcurrent test training operation

(i) Introduction to marine main switch calibrators

The specific principles are as follows:

1. Analog electrical fault signals

• (1) Short-circuit current: simulate a short-circuit fault and verify that the circuit breaker cuts off the current in a very short time.
• (2) Overload current: simulate overloading and test the circuit breaker to cut off the circuit in time to prevent damage.
• (3) Lack of phase or incorrect phase sequence: Analog power supply is out of phase or incorrect, test circuit breaker is correctly disconnected.
• (4) OVERVOLTAGE: Simulate an abnormally high voltage and check circuit breaker response.

2. Automated action time testing

The Marine Main Switch Calibrator accurately measures the action time of circuit breakers. If the action time exceeds the set range, it indicates that the equipment needs to be adjusted or serviced.

3. Protection function verification

• (1) Overload protection: Simulates the overload condition of long time working.
• (2) Short-circuit protection: Simulates the occurrence of a short-circuit to prevent further damage.
• (3) Undervoltage protection: Check for proper disconnection in case of missing or abnormal voltage.

4. Data recording and analysis of results

Real-time recording of electrical parameters and generation of test reports, comparative analysis of whether to meet the requirements of the protection function.

(ii) Hands-on simulator exercises

1. Store energy to the main switch and close the main switch. Check the main switch status indication.
2. On the Parameter Settings panel, set the system parameters (as shown in Figure 6-2).
3. Set the short-circuit short delay parameter: action current value, press the test button, observe and record the result, and then reset.
4. Close the main switch, set the system parameters, and carry out the overload long delay and instantaneous release test in turn.

(a)

Figure 6-2 Marine generator main switch protection device parameter setting interface

Marine Electrical and Electronic Officer (ETO) Practical Training Reference | 2026 Edition