Main bonding to water question

[tomlonsborough]

Hi folks,

Just wanted to check and see if bonding is required in this situation:

• There's no bond to water pipework, bond to gas all present and correct
• There is a piece of plastic pipe at the point where the water enters the building, probably around 100mm in length
• There is then a mix of copper and plastic water pipework throughout the installation
• I don't think there's any metal water pipework which penetrates the floor at any point after the service comes into the building
• There is no continuity between any of the copper water pipework and the MET (Megger reads >99.9kOhm)

I'm erring on the side of not required, however if anyone could shed any light otherwise I'd be very grateful!

Tom

 

[HappyHippyDad]

As I understand it, a resistance between extraneous/exposed of 0-1667 Ohms will be fine on RCD protected circuits, and a resistance of >22000 Ohms will be fine because they are sufficiently isolated from one another. The "dangerzone" is between 1667-22000 Ohms where bonding would be required. On an MFT you would use the low resistance Ohm meter function to measure the 0-1667 range, otherwise you would need to use the IR functionality to measure the 22k+ range.

Does that sound about right?

I've never quite understood how we can accept the <1667ohms bit. I understand the maths, in that the RCD will trip within given times if L-E <1667 ohms. However, we use 200 ohms for a rod as this is deemed stable. How can we accept say 1666 ohms when testing for extraneous as this could easily pop over that figure in dry weather.

 

[Pretty Mouth]

I've never quite understood how we can accept the <1667ohms bit. I understand the maths, in that the RCD will trip within given times if L-E <1667 ohms. However, we use 200 ohms for a rod as this is deemed stable. How can we accept say 1666 ohms when testing for extraneous as this could easily pop over that figure in dry weather.

Do you mean testing to see if supplementary bonding is required? If R <= 50/Ia between exposed/extraneous parts, then no need for sup bonding? If so, then the exposed/extraneous parts are already connected to the MET, either bonded or earthed, and so there should only ever be a few ohms between them.

You'd never get close to a fraction of 1667ohms, and if you did then you've probably identified an extraneous that hasn't been main bonded.