Open PEN: why over-bonding is dangerousWhy can over-bonding be dangerous?

Now change the drawing to TN-C-S, the most common system in the UK, where the supply cable's PEN conductor does the job of both neutral and earth. Both the gas and water supplies come in as plastic, and the pipework inside is metal. So no bonding is needed, but someone bonds the pipes anyway, "just in case".

Normal operation: current goes from the star point, through the consumer unit to the load, back along the neutral and down the PEN to the star point. Everything works.

Open PEN fault: the PEN conductor breaks, somewhere on the supply side, outside the installation. It's out of our control and can't be prevented, only its effects. Open PENs can only happen on TN-C-S.

• The load current has no way back through the neutral. The equipment probably stops working.
• The current finds its way into the MET instead, so every exposed conductive part (connected by CPCs) becomes live to touch at up to 230 V.
• Anything that was bonded when it didn't need to be is live too: taps, pipes, radiators and metal window frames.

Indoors, you probably wouldn't feel it, because you're not in contact with earth (like the knife rack). But imagine the aluminium window frame is being cleaned by someone on an aluminium ladder, with someone footing it on the ground. The ladder is effectively an earth electrode. Both people could be electrocuted, and the one at the top could fall.

Please don't test the idea that you can hold a live conductor safely if you're not in contact with earth.