What is the minimum resistance value in ohms for an effective earth fault loop?

The minimum resistance value for an effective earth fault loop is critical for ensuring the safety and reliability of electrical installations. A resistance value too high could result in insufficient fault current to trigger protective devices, like circuit breakers or fuses, making an installation more hazardous.

In many standards and regulations, 0.35 Ohms is often cited as the minimum acceptable resistance for effective earthing. This value ensures that, in the event of a fault, sufficient current will flow to facilitate the quick operation of protective devices, thereby minimizing the risk of electric shock or fire.

Values below this threshold, such as 0.2 Ohms and 0.1 Ohms, may also appear in discussions about earthing systems, but they are typically more stringent and not always applicable depending on specific installation circumstances. Importantly, while these lower resistance values could improve safety, they can lead to impractical design and earth resistance configurations in many scenarios.

Choosing 0.35 Ohms as the minimum means striking a balance between safety and the practicality of achieving this resistance in real-world installations, in line with established electrical safety guidelines.