Why is the Neutral Wire Grounded at the House as Well as at the Transformer?
Understanding the different earthing systems and reasons for grounding the neutral wire at both the transformer and household level is crucial for electrical safety and system stability. This article explores the purpose of grounding and the various earthing methods in use today.
Introduction to Grounding Systems
Grounding at the transformer and household levels serves different purposes, primarily ensuring electrical safety and reliability. At the transformer, grounding provides an earth reference point for the entire system. At the house, it offers protection to the local electrical system and devices.
Neutral Wire Grounding
The neutral wire, often treated as a return path for current, is grounded at both the transformer and the household level. This grounding serves to protect against leakage and fault currents. However, it is important to note that the manner of grounding can vary depending on the specific earthing system implemented by the electrical distributor.
Different Earthing Systems
There are several earthing systems in use today, each designed to meet specific technical and economic requirements. Here, we discuss three common systems: TT, TN-C-S, and TN-S.
TT System
In the TT system, two earths are present because there is no continuous conductor between them. This configuration is commonly found in remote installations where power is supplied over an overhead pair for some distance. Each house has its own local earth spike, giving the appearance of separate ground points at the transformer and the house.
TN-C-S System
The TN-C-S system, also known as PME (Protective Multiple Earth), merges the neutral with the earth, with multiple earth spikes distributed throughout the system. This approach is used when there is a risk of fault in the neutral leg, ensuring that any diversion of current is safely redirected.
TN-S System
The TN-S system is a classic distribution system where the household earth is typically taken from a lead or steel jacket on the incoming cable. In this case, only one earth spike is needed, as the system is already well-grounded from the start.
Special Cases and Potential Hazards
Some special systems, such as those used on ships and sometimes in industry, have unique grounding configurations. In these systems, both legs are considered "hot" and are labeled accordingly. A ground fault detector is connected between the earth and the neutral line, which can be useful for certain installations but not suitable for domestic use due to its complexity.
It is also important to avoid using an ersatz TN-C system for sheds or workshops without proper earth spikes and neutral leg integrity. If the neutral leg fails open circuit, the return stub becomes live and can pose a significant safety hazard. In this scenario, a second earth spike would significantly increase safety.
Conclusion
Grasping the importance of grounding the neutral wire at both the transformer and household level is crucial for electrical safety and system reliability. Different earthing systems cater to specific needs, and each has its own unique advantages and potential hazards.