Using adiabatic equation for CPC on SWA

[Michaelwgroves]

I want to use the armour as my CPC but need to check it complies. The cable will be run from a SP Distribution panel to CU.
As per Selection and Erection Guidance Note 1 table D10a I can see a 10mm XLPE SWA requires a 31.1mm csa to comply, but only has 26mm on 2-core.
20171231_114229828_iOS.jpg - https://1drv.ms/u/s!ApQQCRGg1ka_gvxxrqQ0QslTR-xaHg
So the only way I might be able to use this is to calculate using adiabatic equation.

Is my calculation correct?

Voltage = 230v
Ze = 0.09 (to be verified, taken from old EIC)
I = 230/0.09 = 2,555A
I2 = 6,528,025
t = (Fig 3A1, 100A BS1361 Type 2a > 1,450A) = 0.1s
I2 x t (6,528,025 x 0.1) = 652,802
sq Root 652,802 = 807
k = 46 (TABLE 54.4 steel)
S = 807/46 = 17mm

So looking at table D10a above we can see 10mm XLPE SWA has csa of 26mm, but as I only need 17.5mm I will be fine ?

On a similar note, Table 4E4A advises 10mm 2-core SWA capacity in ducting is 71A
20171231_140220034_iOS.jpg - https://1drv.ms/i/s!ApQQCRGg1ka_gvxzxVXvCVCdScy83Q

But manufacturers notes say in duct I can run their cable up to 89A, I assume in this instance I could use manufacturers notes over Table 4E4A as Table 4E4A is only generic ?
SWA Capacity.pdf - https://1drv.ms/b/s!ApQQCRGg1ka_gvxyaNUN1GXpn1Ob8w

In reality I'll protect with either 50A or 63A MCB, but it's possible I might want to run at 80A in the future. Now I know the cable spec so I know it's safe; I can also leave cable spec with EIC, but is there anything else I should do to prevent another electrician in the future condemning the cable?

 

[Leesparkykent]

They are shown on a logarithmic scale.
Figures along the horizontal axis of the curve represent the continues current rating of the OCPD. The figures along the vertical axis represent time in seconds. find the current multiple of (In) at the bottom of the graph. Next, draw a vertical line to the point where it intersects the curve and then draw a horizontal line to the left side of the graph to find the trip time.

 

[Wilko]

You get 100,000 (10 power 4) ...

Hi - don't mean to be picky but ... time for a drink

 

[Richard Burns]

Okay, found something similar for Bussmann, but I don't understand how to read chart. You get 100,000 (10 power 4) I follow bottom row until I get 100A, then up, but where to stop?
It looks more to me that it should be 10 power 5 as there is where thick black line is?

For the I²t graph for fuses the line starts to become thick at the prearcing point i.e where the fuse starts to melt and ends at the point where the fuse separates (this is the point you want).
1x10⁵ is 100,000.

 

[buzzlightyear]

I would run 3 core SWA .... never happy to rely on the armour as the CPC................

did we not have this in a post weeks ago Murdoch !

 

[Michaelwgroves]

Let down by my maths, but it was good to get explanations of the chart.
Thanks gents,

 

[Octopus]

did we not have this in a post weeks ago Murdoch !

Quite possibly - I haven't changed my mind recently either!