A power distribution system can experience different types of faults. One such abnormal electrical connection is a ground fault . The most common reason for ground fault is reduced insulation. This could be due to moisture, mechanical stresses, or atmospheric contamination. Potential consequences of ground faults are dangerous. Thus, you must be aware of them. And this is where a ground fault test procedure comes in handy.
Image: Grounding wire
Ground Fault and Ground Fault Protection
A ground fault occurs when an electrical current from an ungrounded phase conductor takes an unplanned path and flows directly to the ground/earth. For an electric circuit to function properly and safely there needs to be a grounding wire. The grounding wire provides an easy ground path for any escaping electricity. If an electric box malfunction and the ground connection is incorrect, the electrical current surge can be instant. It can destroy any electrical equipment or harm a person that becomes the path of travel to the ground.
To protect you from electrical shock and safeguard your equipment from damage or fire, install a ground fault protection system that will detect even the slightest faults and shut off the electric current in milliseconds. The National Electrical Code (NEC) sections 230-95 (C) and 517-17 (D) suggest periodic performance testing of ground fault systems.
How Do Ground Fault Protection Systems Work?
The working of a ground fault protection system is based on the current balance between phase conductors and neutral conductors. Whenever there is an earth fault in an electrical system, one of the phases gets in contact with the ground. This results in a short circuit, and current flows through the earth instead of going back through the neutral phase. As the current diverts to the earth, an imbalance is spotted in the circuit, generating a residual current detected by the ground fault protective device.
A ground fault protection system consists of:
- Current transformers (CTs): a sensor detects ground-fault current levels
- A ground fault relay or logic box: to determine tripping current levels value and time
- A trip device: An operating mechanism to trip the breaker or switch
Some ground fault protection systems come with different types of panels. A test panel simulates a ground fault signal to trip test the breaker. In contrast, a monitor panel indicates the system status.
Ground Fault Relay Operating Principles
A current transformer monitors possible imbalance. The common type of methods for ground-fault protection on low-voltage single-source systems using current transformer arrangements are:
This method involves using multiple current transformers connected for each set of phase and neutral conductors to determine the ground-fault current. Apart from CTs, you can also use circuit breakers having electronic trip units and integral ground fault protection.
If an equal number of current flows between the conductors, the two signals will balance it. When an earth fault occurs, there will be an imbalance, and if the ground-fault current exceeds the pre-set current and time-delay settings, it will trip, breaking the circuit.
Another method of detecting ground faults connections is by using a zero-sequence current transformer.
When an equal amount of current passes through the phase and neutral conductors, it results in zero sensor output. The current passes through the opening of a zero-sequence Current Transformer, and the exiting and entering currents will be equal to zero. The current passes via the ground conductor when a fault occurs instead of passing through the zero-sequence CT. This results in an unbalanced magnetic flux condition.
Ground Fault Protection System Performance Testing
For an efficient protection system of a ground fault, you need to make sure that every component within the system is working properly. If sensors, shunt trip, monitors, panels, wiring, power source, etc., have any damage or have incorrect wiring, there can be issues with the fault protection system.
So you must do regular upkeep and electrical testing of the ground fault protection equipment. Make sure to employ qualified personnel for the testing as per the instructions provided with the equipment. Besides, you must test the ground fault protection system on its first installation.
Things to Keep in Mind During Testing:
- First, do the complete field acceptance testing as required by the NEC.
- Secondly, check the neutral main bonding connection.
- Thirdly, check for any improper installation in a sensor and grounding connections.
- Once you inject current through the current sensor, make a note of the pickup and timing of the relay.
- Reduce current voltage supply to 277 from 480.
- Finally, check whether special features like zone interlocks are working properly.
Ground Fault Guard System Testing Methods
There are two test methods for assessing ground fault protection systems:
Simulated Fault Current Method
Testing using a simulated fault current helps to identify the effectiveness of the relay. Further, you also understand the functioning of the sensor and shunt trip. Additionally, you can verify the competence of the control power supply. When you wind a coil around a window-type sensor, it creates a simulated fault current. Once a current passes through the test winding, it creates a secondary current in the sensor. The relay responds in the same way it would respond to a primary current of 1600 amperes.
High-Current Primary Injection Method
The high-current primary injection method tests the reliability of all the system components. This includes the circuit breaker operation, relay or trip unit, control power, wiring, and the current transformers.
In this, you inject a high-current supply through the primary side of the overcurrent device or current transformer. This will duplicate the flow of ground-fault current under various conditions.
Zero-Sequence Earth Fault Protection Systems- Performance Testing
Before performance testing, perform a visual inspection. Ensure that the number of neutral conductors and phase conductors passing through the zero-sequence sensor is correct. Further, make sure that the direction of the conductor is right.
You can conduct these tests to verify the correct operation of zero-sequence ground fault protection systems:
No Trip Test
In this, make sure that the phase conductor and neutral conductors go in a similar direction through the sensor. Connect the supply to point N1 and A1 to a jumper that connects N2 and A2. The main breaker will not trip even when the test current goes beyond the preset pickup and trip time.
Image: zero sequence ground fault- no trip test
Send current to points A1 and N1 to a jumper that connects A2 and G1. When the test current goes above the preset pickup and time delay setting, the breaker will trip.
Image: zero-sequence ground fault- trip test
Residual Ground Fault Protection Systems- Performance Testing
You need to do the following tests:
No Trip Test
First, connect the test current to point N1 and A1 to a jumper between N2 and A2. Now, as the current sensed by transformers sums up to zero, a trip does not occur. Hold the current above the predefined pickup setpoint for a longer period than the pre-set trip time.
Image: Residual ground fault- no trip test
Connect the test current to points N1 and A1 to a jumper that connects points G1 and A2. The earth fault relay will trip when the current exceeds the preset pickup set point.
Image: Residual ground fault- trip test
Half Trip Test
Connect the test current to points N2 and A1 to a jumper that connects points N1 and A2. The relay trips when the current is more than the preset pickup setting.
Image: Residual ground fault- half trip test
Neutral Sensor Trip Test
In this, you connect the test current to two different points i.e. N2 and N1. The relay will trip when the current applied surpasses the preset pickup setting.
Image: Residual ground fault- neutral sensor test
All in all, a ground fault in your electrical system could lead to serious consequences. However, the quality of wires and cables is essential for efficient ground connections. So, if you need any help with high-quality wires and cable assemblies, contact Cloom.