Oh… and the circuit will be 30mA RCD protected. :)

This stuff then I think the ‘short circuit current’ column under the tab for the current rating is fantasy or in the wrong units. Use your own analysis.

Should meet the regs routed out of zone, and if it gets hit, well the RCD will go off, that is why we have it.

It is unusual, and I'd be minded to take photos or make a sketch and leave a copy in an envelope by the CU - it will not be what the next person to look at it will be expecting, make life easy for your future self and avoid them ringing you up to say it is an EICR fail when it isn't..

If you are really worried, what is the PSSC, and do you know the make of the MCB /RCD - quite often they publish their own disconnection curves which are quite a bit faster than the minimum, and you can then be reassured that you have some margin.

Mike 

PS

Those tests

the BS standard is:

• Voltage rating: 300/500v
• Range: 1.0mm to 4.0mm2, 2c 3c & 4c plus earth.

Three principle performance criteria called up in BS 8436:2011 are:

 As an aside if you  want a better cable - well one that is rated for a higher voltage, it is worth looking for dual rated Irish and UK cable.

IS273:

An existing Irish standard that was modified to include cable failing safe under nail penetration, this standard addition was based on BS8436, but has a number of important differences: 

differences in the standard are:

• Voltage rating: 600/1000v
• Range: 1.0mm to 6.0mm, 2c 3c & 4c plus earth

The other principle  criteria called up in IS 273 are basically the same as used in BS8436.

Hi Mike, thanks for coming back to me. That is the cable. Apparently it is to that Irish Standard as well according to the website specofication. From previous tests the PEFC at origin will be less than 2kA. Not sure yet what it will be at the 3A MCB by the manifold. Its a good call regarding photos and as fitted drawings and was something I was intending to append to the certificate and by the manifold/consumer unit. What throws me a little about the test criteria is - what if the cable is pinned and experiences a fault >160 amps or whatever figure they specify. I appreciate the duration will come into effect and we're into K2S2 being >I2T - bit difficult to assess if you can't actually get that data - or would this be a given if the shield can tolerate a relatively slow burning lower fault for a full a second? I'll dig out some MCB data later but i fully expect the I2T of it at this fault current to be at the very least less than 10000.

The risk of course is that the foiling  burns back from the nail before the MCB or RCD opens, and opens the fault path, leaving a live nail. That would not be ideal, but its better than nailing T and E where you go straight to the  live nail situation. the RCD then hopefully protects the user if they touch it. That ‘fusing current’ to melt the foil is the I2T we are looking at.

Given the test calls up a 32a MCB, you are some way back down from that.

I'd be vary wary of that table that claims to be in kA… as you say, more likely amps.

As a quick sanity check, the allowable energy let-through for 1.0mm² c.p.c. in a T&E cable would be k²S² - or usually 115² x 1² = 13,225A²s.  The withstand for the aluminium foil in a BS 8436 cable is likely to be significantly higher than that (especially as in this case, as Mike notes, we're worried temperatures that will melt/vapourise the aluminium rather than just permanently damage PVC insulation).

    - Andy.

Scrambled: 
 

It has been suggested to remove coving and run cabling behind to thermostats but this has met firm opposition. There is no access from above other than this.

Battery operated wireless thermostats?

Thanks both. In fairness Atom did come back to me just with an excerpt from the British Standard which mentions using a CPD with an energy let through <42000A2S.

I have seen this figure quoted in other manufacturers literature as well. I have the energy let through data for a B6 MK Sentry (they make a 3A type b as well) which puts the energy let through at 3kA at around 10000A2S which as you said Andy would be fine for a 1mm CPC. What is turning me about a little bit is that the test procedure doesn't seem to make an allowance for a low impedance/high fault current short. Im just struggling as to how I as the designer can ensure fault protection at all points in the circuit or will I have achieved this by virtue of meeting disconnection times and following the british standard? It would seem impractical to use this cable if it can only protect against a short to a maximum current of 100/160 amps!

It makes sense the consideration that we're not necessarily protecting the pvc insulation but rather ensuring that the circuit can disconnect and I suppose at the point in time the cable gets pinned it is knackered but yes the thought of the foil frying post penetration and then the circuit being re-energised is not a comforting one. 

Where did you obtain the information about the K2S2 of the foil being generally greater than that of a 1mm copper cpc Andy? I'd be happy my 3A MCB would take care of any drama if this were the case :)

Thanks again chaps.

All the gear is onsite. It's my problem now! :)

The MCB let-through in I2t == joules per ohm is the maximum, over the full range of prospective fault currents that are between the onset of near instant tripping (5 times In for a B type then) and the upper limit of 6000A or whatever is stamped on the breaker.

This does cover all cases. rest easy. The energy  in joules it takes to vaporise a fixed amount of  however much aluminium, is more or less independent of the rate of arrival of those joules, once you are fast enough to be able to consider the process adiabatic.

That was 160 amps for a second - so perhaps for a 10th of a second 450 amps (times by root 10)

for 1/100 of a second, more like 1.6kA (times by root 100)

for 1/1000 second, more like 4.5kA  (times by root  1000)

etc. All give the same heating power into the metal so the same degree of damage.

Mike.

Thanks Mike, that makes sense and I'm starting to think I might be seriously in danger of overthinking this. 

Do you think I am reading/complying with the british standards correctly if I were to use the MK Type B MCB which at 6kA (its maximum fault current rating) has a maximum energy let through of approximately 13000A2S which is significantly less than the stated maximum for the protective device of 42000A2S and that the cable would be capable of fulfilling its purpose in the event of a ‘penetration event’.

In reality both the fault current and the energy let through will be far lower than this. As I said previously I'm anticipating a PEFC <3kA at origin. The start of the 8436 cabling and 3A device will be remote from the origin as well again bringing down the fault current.

I will be complying with the ECA guidance as well i.e. ensuring use of type b devices and only when pfc is <5kA.

Is there anything I'm missing here or have I got the right end of the stick?

Thanks again for all your help all. It's always good fun trying something new. :)