Difference Between Earthing & Bonding
What’s the Difference Between Earthing and Bonding?
Basically, electrical safety is based on two related but different principles: earthing and bonding.
While the terms are loosely used interchangeably, they have different functions to perform in ensuring that an electrical system works both safely and efficiently. Understanding the difference between earthing and bonding is paramount at all levels for electricians, especially those that are either in training or have little experience.
In this article, each of these terms will be discussed in detail, outlining the purposes of each and clarifying when and why each is necessary.
Understanding Earthing
Earthing refers to the process of connecting exposed conductive parts of an electrical installation directly to the Earth. It allows fault currents to flow through it and safely dissipate into the ground.
In case of improper earthing, the fault current may flow through the accessible metal parts to inflict serious shock. Earthing protects people and equipment against electric shock and prevents excessive heating of parts that may cause outbreaks of fire.
Purpose and Function
The most important function of earthing is for reasons of safety. In the case of an electrical fault—such as a live wire touching metal casing—then a rapid (usually high) fault current goes to the ground via the earth connection.
This is vital, as it enables protective devices to act, which include circuit breakers and fuses, thereby disconnecting the circuit and reducing harm. This is the basic principle behind ADS (Automatic Disconnection of Supply). This is different to the purpose of bonding, which we will discuss further below
Relevant Regulations
BS 7671 discusses the requirement for earthing on different installations. It has to be taken into consideration that BS 7671 regulation 411.4 provided clarification that in TN and TT systems, there is a requirement for earthing—standards of protective earth conductors, and also the earthing arrangement.
Such regulations ensure that every installation will have an efficient earthing system with low resistance to ground. For domestic situations, this usually signifies connecting the main earth terminal to an earthing electrode, or sometimes a common neutral earth.
This is done either via a local earth electrode (in the case of a TT earthing system), a remote earth shared with the transformer (in the case of TN-S earthing system) or via a neutral-earth connection within the service head (in the case of TN-C-S earthing system)
Applications in Real-World Installations
- SWA Armoring as CPC: Sometimes the armoring itself can be the CPC for the installation; one such example is an SWA cable installation, provided the cross-sectional area meets the requirements of Regulation 543.1.3. Very common in outdoor or submain circuits where the cable is buried or is out in mechanical risks.
- Fire-Resistant Earthing: In some designs, earthing systems are combined with fire-resistant cabling. Section 421 of BS 7671 addresses safe path maintenance at fault conditions in extreme scenarios, such as fire.
Exploring Bonding
Bonding serves a different purpose from earthing: it equalises potential between accessible conductive parts to reduce the risk of electric shock from touch voltage.
Bonding does not connect directly to the ground; instead, it links extraneous conductive parts (like pipes or structural steelwork) to the main earthing terminal. It is usually used where an extraneous conductive part brings a different potential (normally ground/earth potential) into a particular area.
In this case, the bonding creates an ‘equipotential zone’ where all accessible conductive parts, whether exposed conductive or extraneous, are at ‘equal potential’
In cases where someone could touch two metal parts with differing voltages, bonding helps prevent dangerous potential differences from arising, thus reducing the risk of shock
Purpose of Bonding
Bonding ensures that all accessible metalwork within a property maintains the same electrical potential.
This equalised potential prevents shock hazards if someone touches two conductive parts that could otherwise carry different voltages, especially in the event of a fault.
For example, if someone is touching the exposed conductive casing of a washing machine (under fault conditions) with one hand and the kitchen tap with the other, there is a chance that differing potentials could lead to an increased shock, particularly in this type of instance where the shock would travel between the arms, and thus across the heart.
Bonding is particularly important in bathrooms and kitchens, where water pipes, radiators, and other metal items could be exposed to electrical contact.
As you can see, the difference between earthing and bonding is very important to understand, although they both use green & yellow cables to connect, they serve different, yet both crucially important purposes
Types of Bonding
- Main Protective Bonding: Connects incoming metal services like gas and water pipes to the main earthing terminal to equalize potential between these conductive parts and the earthing system.
- Supplementary Bonding: Used in special areas like bathrooms where additional safety is required, as per BS 7671’s Regulation 701.415.2. This ensures that all conductive parts within reach are at the same potential, minimizing the risk of shock.
PME-Specific Challenges
In installations using a PME system, different requirements arise as there is a chance that under a ‘lost neutral’ supply fault, diverted neutral currents can flow through the bonding conductors to buried pipes, structural steel, etc…
In this case the requirement is that larger bonding conductors are used to ensure that the conductor is capable of carrying these diverted neutral currents safely. This is a particularly dangerous instance as if the bonding were not in place, or were damaged, then it could create differing potentials which could cause diverted neutral currents to flow through a person touching exposed conductive parts, giving rise to a dangerous shock risk.
Key Differences Between Earthing and Bonding
Earthing vs. Bonding
While earthing provides a path for fault currents directly to the ground, bonding ensures that all accessible metalwork within an installation is at the same potential (hence: Equipotential Bonding). Earthing primarily protects against electric shock from faults, while bonding prevents differences in potential between conductive parts, reducing the risk of touch voltage.
Common Misconceptions on Difference Between Earthing and Bonding
Electricians often encounter confusion between earthing and bonding due to their similar safety roles. This can be made worse by the fact that terms can be used interchangeably, or incorrectly (for example: earth bonding)
However, the primary distinction is that earthing is directly tied to fault current dissipation, while bonding is about potential equalisation between metal parts.
Bonding doesn’t involve a path to the earth but instead creates a safety barrier by equalising voltage differences.
Importance of Both
A safe electrical installation depends on both earthing and bonding working together. Earthing provides the fault path to ground, allowing protective devices to operate, while bonding protects people by keeping metalwork at an equal potential.
Further Resources
Here are some links to further resources you can use to help learn more about the difference between earthing and bonding:
