Ok this is making me think a little bit, it may seem a stupid thought/question, but as most of you know, I don't give a f**k and will ask anyway, so here we go.

The fault current that flows in a cpc of a circuit is determined by 230v / R2 and where about in the length of the circuit it occurs right? Assuming a SP supply, 230v is consistant throughout any circuit to earth, whether it's a 6a rated circuit in 1.5mm or a 40a circuit in 10mm, 230v is still going to flow through a given resistance along the CPC of a circuit, the difference being that a 10mm  T&E CPC will normally have less resistance than a 1.5mm T&E CPC, making it's fault current larger. My question is, why can't all CPCs be 1.5mm or 2.5mm if 230v is going to pass through them under fault at their given resistance?

Sorry if the question is obvious but I can't seem to work it out?? This question derrived from me having to sort a ring circuit out where somebody cut through the cable, I sorted it but when I tested the circuit there wasn't continuity on the rings CPC. My QS had already done work here last week, upgraded sockets, mains CU and rewired lights. He new about the problem and issued an EIC, I thought any faults had to be put right first? Anyway, I called him and he said he tried finding the fault but couldn't but was happy because all the sockets were earthed, I raised the point of what if a further earth connection was lost, then some sockets won't be earthed, but he didn't seem concerned? Anyway, I said the CPC is no longer 3.0mm but is now 1.5mm on a circuit protected by a 32a CB, which meant that under fault conditions it wouldn't withstand the current drawn, but when I thought about it, that's when my first question above popped into my head?

So if I'm wrong then please put me right in regards to my original question!

Thanx


Carl

P.S, I looked at the EIC he issued for that job and low and behold there was an R1 + R2 and open ended text Reading on the CPC of the ring?? How?

Edited:

I've had a thought, the CPC has to be so sized to meet the requirements of the max Zs values of it's circuit so that the CB trips in time, and the thermal strain put on the cable under fault, but still it's resistance determines how much current flows, not the size of the CB?

Thanx ding, yes that helps! Maybe I should go back to college, thing is I never have a need to know any of this stuff in the work that I do, but I want to know it, I wanna know a lot of stuff that I will probably never get involved with and that's what frustrates me, I don't wanna do domestic anymore, have had enough of those f****ng scabby tenants thinking that they deserve everything because people like us pay for them through taxes.

As TT would say, rant over.

Thanx again ding, easy to understand explanation!


Regards

Ah rings, you have to love 'em, don't you?

There is not and there never was any real justification for introducing ring circuits for general use.

The original thinking was flawed and sought to solve a problem that did not exist.

Add to that the fact that most (and I do mean most) electricians do not fully understand their limitations and design criteria, certainly don't know how to test them, yet never question their use and you can see why I take issue with the things.

Ban the ring circuit!

(On the other hand, no, don't ban it... I am a sadist; I enjoy taking the pish out of the sparks that don't understand them... it's where half the fun of the 2391 comes from!  )

I'm not exactly a fan of 20A radials either though. It's easy to say that there are unlikely to be more than two big loads on a ring for any signficant time (and afaict short duration overloads are really not that big a deal) in most domestic situations but much harder to design a setup which ensures there will be no more than one big load on any circuit at a time.

And 32A radials mean you need much thicker cable (4mm is marginal) which brings issues of it's own, particularlly if thermal insulation is involved.

One thing to remember is that electricians design circuits based on the continuous rating of cables (since that is all BS7671 specifies) but domestic power circuits are only used on a very intermittant basis. PVC does not fail suddenly at 70°C, it just shortens the lifespan a bit.

Have you ever seen a ring (or even a ring broken by kitchen fitters) with signs of insulation failure due to overloading?