Is this acceptable under 433.3.1?

[EFLImpudence]

But in reality which would go first? In my very limited experience I would expect the MCB to trip before the fuse blows. Is there any discrimination for faults?

Maybe, but it doesn't matter for the 'paperwork'; the fuse is there.

If I use the actual Zs at the FCU of 0.2 ohm and the quoted disconnection time of 0.1s then I get a much more sensible PFC of 1200A which gives me a minimum CSA of 0.75mm.

I get 3.3mm² with those numbers.

0.2Ω is very low. Is it very near the supply?

 

[JohnW2]

@JohnW2 that's really useful thank you. So in my ignorance, I was being overly conservative. As I don't have an earth loop tester I was using the 3kA break rating of the MCBs and the maximum permissible disconnection time of 0.4s.

Fair enough - but, as I said, that is far far too 'conservative', both in terms of PFC and disconnection time.

If I use the actual Zs at the FCU of 0.2 ohm ...

Well, that certainly 'better' but, as EFLI has said, still very low unless you have a substation in the garden and the FCU is very close to the origin of your installation

..... and the quoted disconnection time of 0.1s then I get a much more sensible PFC of 1200A which gives me a minimum CSA of 0.75mm. So ok. ... Does that sound better?

Again, certainly 'better', but with a fault current of 1,200A (if it really is that high), I still think/suspect that the disconnection time would probably be appreciably less than 0.1sec.

Kind Regards, John

 

[EFLImpudence]

bsr,

Just noticed you said you don't have a loop tester - so how did you come by the 0.2Ω?

 

[JohnW2]

bsr, ... Just noticed you said you don't have a loop tester - so how did you come by the 0.2Ω?

Yes, I wondered that, too - and then further wondered whether he was perhaps talking about the calculated within-installation R1+R2, rather than Zs (i.e. ignoring Ze)?

Kind Regards, John

 

[EFLImpudence]

Yes, that's what I thought.

 

[bsr]

Hi, all done now thank you. Yes indeedy I was measuring R1+R2. Although you told me it doesn't matter anyway I took the opportunity to change the first bit of wire between the FCU and socket to 2.5mm. I had an offcut so it didn't cost me anything except five minutes fishing it through the wall and some earth sleeve.

 

[Adam_151]

Thats the best solution, breaking out the adiabatic to prove that your none standard 1mm spur to FCU from ring is not really in the spirit of how things are generally done (and would likely prove tricky at EICR time)

But generally for breakers operating on the magnetic release, things are happening so fast that finding I²t yourself doesn't really work, you need to use tabulated figure for the I²t, you could go to manufacturer, or you could just use the worst case figures from BSEN60898:

https://ckm-content.se.com/ckmContent/servlet/servlet.ImageServer?id=0158V00000Brza7QAB&oid=00DA0000000abSmMAI

Theres two tables, one for <=16A and one for >16 <=32A

You'll find just about all current breakers are energy limiting class 3 (look for a three in a square), the table makes reference to the 'rated Ka' if the breaker, and the higher ones have higher I²t, now I suspect thats because if you have a 10ka breaker, the figures provided are those for when its exposed to its maximum rated breaking capacity, one would assume that you could use the 3kA figures for a 10ka breaker installed at a location where its exposed to 10ka but nothing actually says that...

So 3kA. B32 MCB. Energy limiting class three gives you 18000A²S. Which for K=115 gives 1.16mm² which is slightly over your 1mm. but you are not actually going to have a loop impedance of 0.076 ohms at your accessory, il'll be a lot lower, no figures given for lower Ka, but you know is its more than a little bit under 3kA it'll work out fine.

You can run into the earth looking a little bit undersized in 2.5mm twin for the legs closest to the DB if you have a commerial installation with high PEFC with the board close to the origin and C type breakers used.