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If i have a cable that I am unsure of, say just cut and hanging there. I isolate, and terminate the conductors in a connector block, to enable testing. If I am happy that that cable is not connected to any source of power. I strip out if possible , or terminate it, as if it was live. Even if...

Always presume live, or can become live. The only way to know for sure is trace them to a junction box. Only strip out cables when you can find both ends. Something that disappears into a wall or void, terminate as if it was live. I've found live lead covered cable in a house that was 'rewired'...

If you love the house buy it. A rewire in the scheme of things is not that big a deal.

No worries at all John, I've been on and off with this for many years, so no rush. :)

Hi EFLI Yes this is an academic exercise, I suppose. But if you don't get the theory clear in your mind, you are just going through the motions. Installing MPB, is just something we get told to do. And I understand the need for it, but not the science behind it, well only in a basic way. But...

erroneous post removed at OP request

With Supplementary bonding all but done away with (for now) and the additional protection of the RCD, now being found to be wanting, because the AC types we have been installing, can't deal with the DC modern equipment uses. You wonder if installations have been made less safe.

Indeed yes. But thats at the maximum (Tns) Ze. I generally never find it that high. Usually less than 0.2Ω.

I put this together. I was listening to the Footie and drinking beer, whilst doing :whistle: but I think my sums are correct

I'm getting it John :) Been reading up on kirchhoff's law. As you say John. This 50V is not really achievable. Its interesting that the very low EFLI which we are encouraged to obtain, increases the touch voltage. And I can see more clearly, the importance of supplementary bonding, which...

I fear I will drive you chaps mad.... I do understand the concept of potential difference. Voltage needs to be different at two points for current to flow. What I am struggling with is. With no fault, all the Extraneous and Exposed CP are at the same potential. MET voltage, as they are all...

Yes this is making sense, but I am still pondering the CPC to the light switch Oh dear ..Sorry John So, how does the voltage appear on the light switch plate - It has to be by the CPC. So there is a voltage on it. So we have a voltage, we have a resistance, so we must have some current. ...

OK. I fear I'm going backwards, but I need to get something clear in my head please John, before I unleash my next confusing drawing. MET potential, and how that affects the connected conductors... Im getting a bit confused by the volt drop in a conductor when no current is flowing. So in CPC...

Excellent, some good learning going on here. Really appreciate your patience, and time.

:) Well no...and well yes kind of. As in say a radial socket circuit. Nothing is flowing if no load attached. You calculate the volt drop based on a expected load. When unloaded, the conductors just sit there waiting to be called into action ( sorry for the not technical terms) There is a...

Thanks again for you help John. Re. MPB I think I've made very hard work trying to understand something I basically understood, Well the concept of it anyway... The better understanding of the actual volt drops will be very helpful going forward. I've never tried to calculate these things...

Oh dear :( Im back to Square 1

OK. This is a simplified version So with bonding we have a touch voltage below 50V. This is because we are not using the external R2 in out calculations. Why the external R2 is left out confuses me, but I think this is why... So we are not talking about EFLI and low impedances here, i.e...

Thanks John I reduced the R2e as the parallel earth paths gave a combined impedance of 0.0875Ω. Yes I am confusing and complicating things... I will have a go at the Ze of 0.8 I think I am mixing up the job of the cpc and the bonding. That fig 5.3 in guidance note 8 is causing me...

Thanks very much for this John...I Think i am starting to get somewhere, then I lose it all again. Here are a couple more diagrams. So in the first diagram, Its the figures you used. The Black type is the resistance The Orange is the volt drop The red is the voltage appearing at each point...