Neutral Current Diversion Demonstration

Summary

This video takes viewers through a demonstration at the Scottish Power Energy Networks Demonstration Center in Scotland, showcasing the effects of neutral current diversion on a protective multiple earthing (PME) network. The demonstration involves testing various failure scenarios, such as an open-circuit in the combined neutral and earth (PEN) conductor. Experts from Scottish Power Energy Networks explain how to properly isolate and test the system for safety, including testing for electric shock risks, earthing issues, and the use of specific tools like voltage indicators and clip-on ammeters. The importance of adjusting safety procedures for such situations is emphasized.

Takeaways

• 😀 The IET visited the Scottish Power Energy Networks Demonstration Centre to witness the effects of neutral current diversion on a Protective Multiple Earthing (PME) network.
• 😀 A PME network combines Earth and neutral in the same conductor, which is split into two terminals: neutral and Earth, at the service head.
• 😀 The PEN (Protective Earth and Neutral) conductor failure can occur due to various reasons, including corrosion, mechanical damage, or damage from trees hitting overhead lines.
• 😀 During the demonstration, neutral and earth conductors are tested by isolating the consumer unit, removing fuses, and proving polarity with test lamps and voltage indicators.
• 😀 The test rig setup involves two single-phase supplies to two properties, with a common bond through a water pipe, and a shared neutral connection acting as the PEN conductor.
• 😀 The first test ensures the neutral and Earth connections are intact, with positive results for polarity and no issues found in the extraneous metalwork.
• 😀 The next test simulates a failure by disconnecting the service neutral, resulting in the neighboring property still having power despite the issue.
• 😀 The demonstration reveals how a broken PEN conductor can cause issues in the neighboring property, including the potential for dangerous current flow through the metalwork.
• 😀 Key safety tests include verifying the integrity of the Earth and neutral conductors, as well as using a contact and non-contact voltage indicator to check metalwork and the incoming supply.
• 😀 The final part of the test involves introducing additional load and simulating a main neutral break, highlighting the potential effects on the electrical system when the network is unbalanced.

Q & A

What is the purpose of the IET's visit to the Scottish Power Energy Networks Demonstration Center?

The purpose of the visit is to witness the serious effects of neutral current diversion on a Protective Multiple Earthing (PME) network, specifically in the event of a failure, such as an open circuit in the Protective Earth and Neutral (PEN) conductor.

What is a PME supply and how does it function?

A PME (Protective Multiple Earth) supply is an earthing arrangement where the Earth and Neutral are combined in the same conductor from the distribution transformer to the consumer installation. The PEN conductor is split into separate Earth and Neutral terminals once the supply reaches the service head.

How does the PEN conductor function in a TNCS system?

In a TNCS (Terrestrial Neutral Combined and Separate) arrangement, the PEN conductor serves as a combined Neutral and Earth conductor. The system ensures that multiple Earth electrodes along the buried supply cable provide a protective grounding system for the installation.

What kind of test was demonstrated with the test rig at the demonstration center?

The demonstration used a test rig to simulate scenarios where the neutral is broken at the service supply to two adjoining properties. The rig allowed testing of the neutral and Earth connections, as well as the detection of potential failures like open circuits in the neutral conductor.

What are the key tests performed to ensure safety and proper earthing in the system?

The key tests performed include testing for polarity, checking for continuity in the Earth and Neutral connections, testing the extraneous metal work using contact and non-contact voltage indicators, and conducting an Earth PEN test to verify the integrity of the protective earthing system.

What happens if the PEN conductor is disconnected in the demonstration scenario?

If the PEN conductor is disconnected, the light in the consumer's property remains energized, demonstrating how the neutral current diversion can continue even if the protective Earth is interrupted, thus highlighting the risks involved in such faults.

How did the test rig simulate a real-world failure in the PEN conductor?

The test rig simulated a failure by using a switch to break the neutral connection between the properties. This was used to replicate potential real-world issues such as corrosion, mechanical damage, or external factors like overhead lines being damaged by trees.

What was discovered when the neutral was disconnected in the neighboring property?

When the neutral was disconnected, no immediate indication of a fault was visible. However, upon further testing, it was discovered that the system had compromised the equipotential zone, and the current was flowing through alternate paths, which was undetected by normal voltage indicating equipment.

What additional test was introduced to identify issues with the PEN conductor that other tests did not catch?

An additional test was introduced using a clip-on ammeter around the main Earth conductor to identify any current flow that could indicate an alternative path for the current, which could compromise the safety of the system. This test helped in identifying problems not visible through standard tests.

What is the significance of the imbalance in load when the main neutral is disconnected?

When the main neutral is disconnected, an imbalance in load causes a noticeable increase in voltage in one of the properties. This demonstrates how the system can continue operating under an unsafe configuration, highlighting the potential hazards of a disconnected neutral conductor.