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Resistance per Meter for copper cable (R1+R2)
wrote:
Hmm, looks like I've opened up a can of worms here !
Here's the full example question and the the bits I've got to.
Consumer unit, 190V A.C., with BS3036 fuse, Ze=0.49Ohms
45m PVC Armoured cable, clipped directly to wall with 5 other circuits
50 Degree C ambient temp, 9Kw load
Cable runs through 100mm Insulation
IB = 9*103/190 = 47.36Amps
In = 60A (From Page 195 Fig3.2A Next biggest to 47A)
OK so far, assuming single-phase, which is not explicitly stated
(although the words "consumer unit" do tend to suggest it's 1-ph).
Correction Factors
1). Cf - A BS3036 device 0.725
Only applies if overload protection is required. Also if the six
circuits of the group are not liable to simultaneous overload you can
use Appendix 4, 6.2.2, equations (7) & (8) which will almost certainly
help towards a smaller cable.
2). Cg - From Table 4A1, (Page 210) Method 1 used
From Table 4B1, 1+5=6 Circuits, Bunched = 0.57
Six armoured cables clipped to a wall will not be bunched, they'll be in
a single layer and spaced by more than one diameter. (Otherwise there
won't be room to get the cleats in!) This gives Cg = 0.9. (If spaced
by more than 2 diameters you can used Cg = 1.0 - no derating.)
3). Ca - From Table 4C2 50°C, Armoured PVC = 0.87
4). Ci - From Table 52A, page 97 100mm = 0.81
Do all six cables go through the insulation, or just this one? The
question isn't very clear. If only this cable, then Cg and Ci won't
both apply at the same time. Then you have to consider the installation
conditions for each section of the circuit in turn and take the highest
It rating obtained.
Iz = 60 = 206Amps
(0.725*0.57*0.87*0.81)
This is getting silly - no-one would install like that, on grounds of
cost if nothing else. You'd adjust the method of installation so that
not all factors applied simultaneously, and not bunch the cables as I
said above.
R1+R2 = mO/m * Length (m) * Temperature multiplier (Table 9C)
1000
Since it's SWA Table 9C won't help with R2 (assuming armour as CPC). Or
if a third core is used as CPC then R2 will equal R1.
So I'm a bit confused as to who's right.
The question seems to leave a lot of necessary detail out. Also what's
the ultimate aim of the problem - to verify that Zs is low enough, or to
verify compliance on all grounds?
--
Andy
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