Recently, we had to face an unexpected situation related to three phase network. Our factory was cut by a thief. At first, the thief only wanted to cut the ground wire, but he mistakenly cut the neutral wire. , and as a result, many single-phase equipment in the production line were broken. We discuss the causes and solutions together in this article!
1. Overview of single and three-phase electrical circuits
In the no-fault circuit here, we know that whatever current leaves the line terminal will always return to the neutral terminal, no matter how many loads are attached. A fault free single-phase circuit will always be balanced. The neutral current will always equal the line current.
And a break in the neutral in a single-phase circuit after the circuit breaker will result in zero amps current flow
A perfectly balanced load will remain balanced even with a lost neutral connection. The voltages also remain balanced at ±230Vac.
Compare this to an unbalanced installation with a loss of Neutral, each phase demanding different currents. The voltages will vary by a large amount and can be ±400Vac or more as they try to compensate for the loss of Neutral and returning currents
We known that Kirchoff’s Law said: the sum of currents flowing into that node is equal to the sum of currents flowing out of that node or orther words the algebraic sum of currents in a network of conductors meeting at a point is zero.
That meaning when three phase be balanced, without currents flow in neutral wire. Otherwise, when having mixed loads on each phase it means there could be several different circuits on each phase cause out of balance currents flow along the Neutral
2. Why Devices be Damaged in Three Phase Network lost Neutral Wire?
Now comback to typical three phase distribution network, where usually having both 3 phase and 1 phase loads. Each phase wire is connected to differents phase bar in three phase network, while all neutral wire will be gathered to Neutral copper bar in distribution board.
The any load one phase’s curent flows will from phase at ditribution board through circuit breaker, devices and comback to neutral bar also rightly is neutral of distribution transformer. It cycle unsure that all devices be supplied right voltage and all unbalaced currents will dissipate on neutral bar.
Now we will see what happens when loose neutral wire at distribution board?
A break in the neutral isolates the circuits from neutral at the supply, at this time, current flows is also same above, but when flow to neutral wire, it can’t come back to neutral bar of general network. According to the usual principle, the current will close a loop through the device with the lowest internal resistance and accidentally it goes to another phase, at this time the power supply for that device is taken from 2 phases of 400Vac. Regular 230Vac single-phase devices can only withstand a maximum of about 285Vac, and will be destroyed at a voltage of 400vac.
That is reason why single-phase equipment is destroyed, when the distribution system loses its neutral wire.
3. Protect Devices from Loss of Neutral Wire
To protect single-phase devices from damage in a 3-phase network when the neutral wire is lost, several methods can be employed. Here are some effective protection strategies:
3.1 Neutral Wire Monitoring Relay (Loss of Neutral Protection)
- How it works: A neutral loss relay constantly monitors the neutral connection. If it detects that the neutral is lost or becomes unstable, it can trigger an alarm or automatically disconnect the load to prevent damage to single-phase devices.
- Protection provided: This helps to prevent voltage imbalances, protecting devices from overvoltage or undervoltage caused by a neutral loss.
3.2 Over/Under Voltage Protection
- How it works: Over-voltage or under-voltage relays can be installed to monitor the supply voltage to single-phase devices. If the voltage goes outside of safe operating limits due to a loss of neutral, the relay disconnects the device or sends an alert.
- Protection provided: This stops the device from being exposed to damaging high or low voltages that can result from a lost neutral wire.
3.3 Surge Protection Devices (SPDs)
- How it works: SPDs are designed to protect electrical equipment from transient voltage spikes. In the event of an imbalance caused by a lost neutral, these devices can absorb excess voltage and prevent it from reaching sensitive equipment.
- Protection provided: This provides a buffer against spikes in voltage, which can occur due to neutral loss, thereby protecting sensitive electronics from short-term overvoltage.
3.4 Phase Failure Relay
- How it works: A phase failure relay monitors the three phases of the system. If one of the phases fails (such as a situation where the neutral wire is lost and causes imbalances), the relay can disconnect the affected single-phase loads to prevent them from being damaged.
- Protection provided: It prevents single-phase devices from being exposed to conditions where one phase is significantly higher or lower than the others.
3.5 Voltage Stabilizers / Regulators
- How it works: A voltage stabilizer or regulator can be used for sensitive equipment to ensure that the voltage supplied remains within the safe operating range despite fluctuations caused by the loss of neutral.
- Protection provided: This helps maintain a constant voltage supply to single-phase devices, preventing damage from voltage fluctuations or imbalances.
3.6 Automatic Disconnection Devices
- How it works: Some systems include automatic disconnection devices that immediately cut off power to the load when an abnormal situation like a neutral loss is detected. This can be coupled with a sensor that detects the imbalance in the system.
- Protection provided: This minimizes the exposure of single-phase devices to potential damage caused by voltage imbalances due to the lost neutral.
3.7 Transformer Isolation
- How it works: Using an isolation transformer between the 3-phase supply and single-phase loads can protect against voltage imbalances. The transformer ensures that the load only sees a balanced voltage, even if there’s a loss of neutral.
- Protection provided: The transformer isolates the single-phase devices from any issues in the 3-phase network, providing stable voltage regardless of neutral loss.
3.8 Regular Maintenance and Monitoring
- How it works: Regularly inspect and test the neutral wire, connections, and overall system. Implement real-time monitoring of the system to identify any signs of a neutral fault.
- Protection provided: Early detection of neutral wire problems allows you to address the issue before it causes damage to devices.
By implementing these protection methods, you can help shield single-phase devices from damage in the event of a lost neutral wire in a 3-phase network. The key is to detect the problem early and disconnect or protect the load before it is exposed to unsafe voltage levels.