Could somebody do the calculations?
Are recessed power sockets available in the UK
- Thread starter KaraP
- Start date
Someone would need to make up a scenario - supply impedance, loop impedance of the circuit, type of fault, ... If you stick 230V across one end of a 2m bit of 0.5mm cable with a dead short at the other end then there's little doubt a 16A fuse or breaker will pop "quite quickly".
Add a supply impedance, and circuit loop impedance, and the situation might change somewhat.
My out of date BRB gives the resistance of 0.5A flex as 93mR. So say a 2m flex, that's 0R186.
If the circuit is wired in 1.5mm cable*, then that's 29mR/m, or say 0R870 for a 30m run which isn' at all unrealistic for a medium sized property.
Assume a supply impedance of (say) 0R5.
That gives us a total impedance of 1R556.
And a prospective fault current of around 150A. So plenty to trip a Type B on it's magnetic trip. However, it doesn't need a lot of extra resistance (higher supply impedance**, not a dead short, longer flex, extension lead in use, ...) for that to change.
* I can see people doing their utmost to get away with 1.5 since the difference in cost between 1.5 and 2.5 given the quantity that will be used would be significant.
** What would be a typical range of values found for real installations ?
Isn't that over-complicated?
A 16A mcb will trip in 0.1 second @ 80A - a Zs of 2.875Ω @ 230V.
So, as long as Zs is below that, which it must be, then -
(sq.rt.(80² x 0.1)) / 115 = 0.22mm².
So, 0.5mm² would be alright and would still be alright on a 32A mcb.
A 16A mcb will trip in 0.1 second @ 80A - a Zs of 2.875Ω @ 230V.
So, as long as Zs is below that, which it must be, then -
(sq.rt.(80² x 0.1)) / 115 = 0.22mm².
So, 0.5mm² would be alright and would still be alright on a 32A mcb.
Not if you actually include the cable in your calculations.
Now, how much 0.5mm cable does there need to be before the B16 doesn't trip "instantly" ?
Never seen a fault that wasn't a "hard short" ?
I have. 2.875 is very high.
That leaves 2.689Ω for the circuit.
If that isn't met then the arrangement doesn't comply anyway.
Using your figures (if I'm correct) another 7.1 metres.
Although 'instantly' is variable because with a higher current it will be quicker.
No.
Not sure where we are going.
Have we not proved small (0.5mm²) appliance leads would be satisfactory?
Far from it - all you've proved is that with a short cable it should be.
Add in some extra flex and the resistance of the circuit increases, reducing the fault current.. At some point the breaker no longer trips "instantly" but then needs the thermal element to trip. The energy dissipated in the flex during the trip time could be considerably more than your initial calculation.
However, if you are correct, then we've been wasting our time all these years using fuses in our plugs - you even went as far as saying 0.5mm cable would be OK on a 32A MCB.
It is also a rather pointless thread of discussion since we have our BS1363 fittings on RFCs (or sometimes 4mm radials), and they have their different ones on radials. That isn't likely to change any time soon.
Well, exactly. I thought that's what we were discussing.
How long a lead do you (or the French) want on a kettle?
Obviously - same with any cable.
Not wasting time. Elfen safe tea.
It would be no different than 2.5mm² spur on 32A.
Did I do the sums correctly?
Strange how people often think threads pointless after contributing several posts.
I hope not; I'm still waiting in hope of an answer to my question!
Sorry.
No, there aren't any.
No, there aren't any.
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