Unfused spur.

The fuse will open before a 1mm2 cpc for all PSSC as well, the copper wire does not melt, it just gets hot enough to damage the PVC insulation irreversibly.

Mike.

Andy, you are making a possible mistake, it could be worse than I expect, but unlikely. The energy let through you are quoting is at the DB with 6kA PSCC (or 3kA) which you will notice is not much more than the 1mm2 rating to only get to 155C. The other points to consider are the PSCC at the point the 1.0 Earth conductor joins, which will not be 6kA or even 3kA, it might be as high as 575A if the ring is short and the DB PSSC high. The by the book calculation is not a theoretical possible maximum value, it is an actual value. The resistance of 2.5 copper cable is 18mOhms/m. The drop /m at 6ka = 108 volts per metre, so for the live only 54 volts per metre and the 1.5 Earth 87 volts per metre. The mains source impedance at 6kA PSCC is 38 mOhms, which is more than doubled for a metre of 2.5 cable, so with 1 metre of 2.5 will be halved approximately. More realistically the supply will probably be 2kA or less, and the cable a few metres. The actual PSCC to be faced will therefore probably be well safe. To cause a 1mm cable to actually melt (which is the danger condition) will need roughly 4 times the energy let through to get to 155C, although this may cause insulation failure. This will be detected by the RCD if it happens. I suggest this is the real scenario, one needs to examine the actual situation (PSCC present at the reduced cable size) not the possible maximum which could occur. A PSCC measurement at the source DB and the socket connecting to the 1.5 cable would also be instructive. You will notice that for design purposes we reduce the PSCC value between the main switchboard and sub-DBs in line with the cable impedance, otherwise a large supply would need impossible sized switchgear everywhere. This is the same situation.

Naughty question to Z, I presume you have the loop impedence figure at your new socket or the R1+R2 value and the Board PSCC so that we can estimate the possible maximum fault current?

I was just reading the thread again and it is TT. The Earth fault current is therefore not a consideration unless there are metal service pipes and other properties which are TN, which seems very unlikely. I would like the measured values though!

Perfectly satisfactory Z. No code, all tested fine, panic over!

The 1.5mm2 6242Y is no longer than 300mm and runs through a brick wall. It starts from a deeply sunk double metal box and terminates at a surface mounted all insulated 13 Amp. I.P. 64 rated single 13 Amp socket. Any physical damage is very unlikely.

It's not just about physical damage - faults can occur from many causes. I'm sure we've all come across back boxes where a wire has come loose and touched something it shouldn't or been nicked by one of the faceplate fixing screws (possibly after Mr DIYer has been loosening faceplates to redecorate or otherwise messing with things).


You might have been thinking of omitting fault protection (3m and all that - 434.2.1) - but I'd be uncomfortable with that personally - squashing things into a back box isn't really in the sprit of (ii) to my mind.

 

The energy let through you are quoting is at the DB with 6kA PSCC (or 3kA) ....  The other points to consider are the PSCC at the point the 1.0 Earth conductor joins, which will not be 6kA or even 3kA, it might be as high as 575A if the ring is short and the DB PSSC high. The by the book calculation is not a theoretical possible maximum value, it is an actual value.

Agreed - but does that help? Lets say the actual fault current was say 600A - what we'd then need to know is how long the MCB takes to open at that current (it could well be slower than for a 3kA fault) - but we don't have that data. The time-current curves don't go above 160A for a B32 and the regs send us back to using energy let-through data - which again normally isn't supplied for 600A. All we know is that it shouldn't be longer than 0.1s - but that's far too long to be useful. BS EN 60898 just gives us a single table of energy let-though data (based on the device's rating rather than the actual fault current) - so we're back to 18,000 A²s again. If Z. had told us the manufacturer of the MCB we could use manufacturer's data rather than generic BS EN 60898 data  - which may or may not give us some more favourable figures (but often they're just reproductions of the BS EN 60898 ones anyway).


Z. asked if it complied - I still say we can't say that it does. It may well be probably OK or unlikely to be an issue in reality, but probablys and unlikelys don't show compliance.  Maybe Z. can provide some more data so we can be certain, but I don't think we're there yet.


   - Andy.

No Andy, it is not that you must assume the energy let through is the maximum, it cannot be unless the fault current is very high. The B32 for instance characteristics in the Regs shows a vertical line on the graph at 600A fault current, which according to the graph is between 10ms and 5 seconds. This is the instantaneous region, and the worst case is your energy let through. You are assuming the worst case can occur anywhere in the characteristic, but this is not the case. You will see the note on the graph, which says the worst case is at the high end of fault current. You do not and cannot have this, so the let-through must be less. Instantaneous operation is usually considered 10ms, not you see an unreasonably quick figure. Therefore the energy let through is I2t Watts, so the current is the important factor.

Now look what you've started Z.?


The property's consumer unit is diagonally opposite to the new single outdoor socket about ten metres away. The all insulated consumer unit is a Wylex make. It is split load. The R.C.D.s are WRS80/2 types. I did not note the M.C.B. model numbers but they are B type of the same era. The supply is TT.


P.S. I consider that any damage to the 1.0mm2 C.P.C. contained in its sheath in the wall is nigh impossible. The metal box has a plastic 20mm grommet installed. The hole in the brick wall is too small for mice to enter. It slopes downward to prohibit rain running into the socket box in the house. Any L to E fault will be at the appliance or appliance flex that is plugged into the outdoor socket. The plug fuse being a maximum of 13 Amps.


Z.

Don't worry Zoomup, it wasn't really you! The case we are considering is as you have probably noticed is really quite complicated, and therefore not in the "normal" range of installation designs. If you had used a bit of 2.5mm, no one would have said a word, but some quite important stuff would not have got an airing here. The discussion has covered two different scenarios, the TT one where the Earth fault current is limited to a low value by Re, and the TN-S one where the fault current is limited by the source impedance and the wiring resistance. You have now told us that each leg of the ring is probably about 15 metres, and so we can calculate the maximum fault current. We also know the ability of a 1mm cable to withstand this current for the time necessary to trip the MCB, and can therefore decide that the installation is satisfactory.


Kind regards

David