Having recently had my annual assessment, my inspector asked me; “why do you need to earth the gas and the water?”. It was a good question and the key term I missed was “potential difference”.
As a professional electrician, I had always envisaged the potential for a poorly installed cable to lose its insulation and possibly come in contact with a gas and or water pipe. Although this is indeed a possibility, there is a second more indirect way…. The potential for transient voltages or incorrectly wired/crimped connections to make either the water or gas mains another source of potential difference. Either way, in layman’s terms; an electrical fault could flow through the water or gas main and not through the main electrical earth supply which is the desired path.
Imagine if there was a water leak in the kitchen and an electrical appliance had been connected incorrectly? With the live and earth the wrong way around for example, I’ve seen it before, believe me! If there was no RCD protection and or no earth to the water, then any unlucky person could receive a fatal shock. The floor would become live as water is a great conductor and sadly people have been known to die in similar circumstances. Even if the floor was a thick rubber vinyl, the earth path would flow through the person and then through the water main.
So there is the more direct method like my former example, or the more indirect example and perhaps more deadly, like the latter!