Measuring the Continuity (Resistance in Ohms) of the Protective Bonding Conductor to Water and Gas

Protective bonding conductors (typically 10mm² cable) connect to metallic water, gas, and oil pipes. These connections are often made in an MET or within the consumer unit. It is critical never to remove these conductors while the supply is energized, as doing so can create a dangerous shock hazard if an earth fault occurs, leading to a potential difference between disconnected pipes and other earthed metalwork.

An ohmmeter is used to measure the resistance between the disconnected bonding conductor and the clamp on the water pipe, and then to the pipe itself. The expected reading should be less than 0.05 ohms, representing approximately 27 meters of 10mm² cable. If the connection point is tarnished or painted, it should be cleaned to ensure an accurate reading. The demonstrator achieves a reading of 0.02 ohms, well within limits.

For bonding conductors connected to distant pipes, such as a gas pipe located further away from the MET, a wandering lead (e.g., 20 meters) is used. Before using the wandering lead, its resistance must be nulled out by connecting both ends to the ohmmeter and pressing the test button until a zero reading is achieved. After testing the water bond, it must be reconnected and secured.

One end of the calibrated wandering lead is connected to the gas bonding conductor, and the other to the clamp on the gas pipe and then the pipe itself. The connection point on the pipe should be cleaned for accuracy. The measured resistance should again be less than 0.05 ohms. The demonstration shows a reading of 0.03 ohms. Before restoring power, both the water and gas protective bonding conductors must be reconnected to ensure safety.

Before any testing, the power supply must be isolated using a double-pole switch or by isolating within the consumer unit. Once isolated, the protective bonding conductor for the water pipe can be safely disconnected from the MET. This is often necessary because METs are not always present, and connections might be directly in the consumer unit.