Determining the cas of the CPC

[lofty84]

A circuit is protected by a BS88 fuse and the prospective fault current is 171 A. How do I determine the csa of the CPC assuming 70 oC themoplastic cables with copper conductors.

Can anyone tell me where I need to be looking in the BS 7671 or onsite guide please ?

 

[GrannySmiff]

to work out the size of cpc needed you need to understand the adiabatic equation, look that up.

Here is a thread about it, not sure if its any good as only scanned through it.

http://www.electriciansforums.net/i...al-installations/5179-adiabatic-equation.html

 

[pushrod]

Have a look at reg 543.1.3 formula and explanation is there.

 

[i=p/u]

equation
aint in on-site-guide

 

[j.mawer]

Was there any more information in the question? What about for instance the current rating of the fuse? pg 247-248 will give you the time for the fuse to blow which would be required to use the adiabatic equation to get an optimal CSA

 

[lofty84]

no mate thats all it says.

adiabatic equation = S = the sq root of Isq x t / k

s = minimum size of earthing conductor (CPC)?

= square root of 171 sq x 0.1 / 115

= 0.47mm2

Ive no idea if that is right but I have had a go, what has confussed me is the value to put in for t. Im not sure why I chose 0.1 but I did, can anyone say where Ive gone wrong ??

 

[pushrod]

That is correct, if you are correct that the fuse is disconnecting the fault in 0.1 seconds.

Usually you would have to look up the disconnection time in appendix 3 as J.mawer said above. Sometimes they give you an assumed disconnection time in the question. 0.1 S is a common value to use.

 

[ezzzekiel]

someone posted an excellent worked example maybe look in the wiki it prob there.

 

[lofty84]

cheers guys, can I ask more questions ?

 

[pushrod]

Only if you hit the "thanks" button

 

[lofty84]

does anyone know why the adiabatic equation may be used in preference to the selection tables to determine the size of an earthing conductor ?

 

[pushrod]

The adiabatic equation will usually give a lower answer and so if you calculate it you may find that the protective conductor does not need upgrading when a table might suggest that it does.

 

[lofty84]

A water heater with an output of 5kw is connected to a 230v supply via a bs 60898 type b protective device. the cable is 70 oC PVC insulated thermoplastic coper conductors and the length of cable is 18m. the ambient temperature is 35oC. The cable runs for 300 mm through thermal insualtion. it runs together with one other cable for most of its length and is clipped direct to the wall. Determine the following and show all calcs and refs to tables.

1. Design Current

Ib = 5000/230 = 21.73 A

No diversity according to the on site guide (page 97)

Minimum protective device rateing

In is greater then or equal to Ib = 25 A mcb BS 60898

Correction Factors

It ? = In/caxcgxcixcc

ca=table 6a OSG/ cg = table 6c OSG/ ci = i think table 6b OSG/ cc = 1 i think page 124 OSG ??

It = 25 / 0.94 x 0.80 x 0.51 x 1 = 65.18 A

not sure what do do or say for the minimum cable rating ??

Minimum cable size

= 10mm2

Maximum allowable voltage drop

= 4.4 according to table 6D on page 130 OSG

Actual Voltage Drop

= (mv/a/m) x ib x l / 1000

= 4.4 x 21.73 x 18 / 1000

= 1.72 A

is it 5 % of 230 (11.5) so yes that means it doesnt need a bigger cable ?

 

[pushrod]

A water heater with an output of 5kw is connected to a 230v supply via a bs 60898 type b protective device. the cable is 70 oC PVC insulated thermoplastic coper conductors and the length of cable is 18m. the ambient temperature is 35oC. The cable runs for 300 mm through thermal insualtion. it runs together with one other cable for most of its length and is clipped direct to the wall. Determine the following and show all calcs and refs to tables.

1. Design Current

Ib = 5000/230 = 21.73 A

No diversity according to the on site guide (page 97)

correct
Minimum protective device rateing

In is greater then or equal to Ib = 25 A mcb BS 60898
correct
Correction Factors

It ? = In/caxcgxcixcc

ca=table 6a OSG/ cg = table 6c OSG/ ci = i think table 6b OSG/ cc = 1 i think page 124 OSG ??
correct selection of factors and use of tables

It = 25 / 0.94 x 0.80 x 0.51 x 1 = 65.18 A
correct maths but see below
not sure what do do or say for the minimum cable rating ??

Minimum cable size

= 10mm2 According to table 6F (osg) and your result you would need 16mm

Maximum allowable voltage drop

= 4.4 according to table 6D on page 130 OSG

Actual Voltage Drop

= (mv/a/m) x ib x l / 1000

= 4.4 x 21.73 x 18 / 1000

= 1.72 A Volts not amps

is it 5 % of 230 (11.5) so yes that means it doesnt need a bigger cable ?

What you have done is pretty good but not necessarily completely correct
because argueably the nature of the load and the rating factors need to be simultaneously occurring - give me a bit of time and i'll explain.

 

[pushrod]

Because the heater is purely resistive and not subject to an overload, like a motor, when you calc It instead of using the In figure of 25A you can use the Ib figure of 21.73A (your tutor might not want you to but that is how i was taught)

Also the factors have to be simultaneously occurring - from the question it looks to me as if they are not and so you just go with the worst case scenario of the Ci (insulation factor) which would bring you right down to an It=42.6 then in table 6F, 6mm can go up to 47 so 6mm would be ok.

Then your Vd factor would be 7.3mV/A/m. which would now , i think, give a Vd of 2.86V - still ok.
hope that helps, good on you for doing it yourself - that is the way to learn.

 

[lofty84]

Thanks a million mate, its really decent of you to take the time to go through that for me.

If anyone can help me out here again it would be appreciated

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

 

[pushrod]

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

Now i don't like stuff like that lol, but i'm guessing it's do with presence, right size and connection of things like earthing conductor, protective bonding conductors, cpcs and may be to do with Size of Zs and nature and type of protective device. But not certain by any means!

Edit - identification of earthing system would also be important

 

[gamblor6]

see where lofty says max vd allowable 4.4? surely thats the (mv a m) he.s quoting not the max vd.

 

[pushrod]

see where lofty says max vd allowable 4.4? surely thats the (mv a m) he.s quoting not the max vd.

He is just quoting where he got the 4.4 figure from, which is good practice in the open book exams. He goes onto work out the Vd

 

[Deleted member 26818]

A couple of problems with the calculation.
Why have you chosen 0.1s, and why have you chosen 115 for k?
By choosing 0.1s you are limiting the fuse rating to a maximum of 16A, whereas in reality as these types of fuses are generally used for distribution circuits, 5s can be used, which allows a maximum fuse rating of 40A.
The 115 value of k is for multicore cables, whereas the 143 value is for single core cables.
I'd expect your calculation to work out to about 2.67mm².

 

[lofty84]

Im still trying to learn how to do it mate so thanks for picking that up for me, Ill have another look at it after some more revision.

 

[lofty84]

can anyone describe the process of measureing Ze on a TNCS system

 

[pushrod]

Turn off installation/open main switch and disconnect earthing conductor and measure with EFLI tester - should be less than 0.35 ohms for tncs. TBH you can get all this and more from your OSG chr 10. If you haven't got one your next question should be who is selling the OSG cheap

 

[lofty84]

nice one I know no where to look

 

[lofty84]

pushrod what is a EFLI tester

 

[pushrod]

earth fault loop impedance tester

 

[tony mc]

Thanks a million mate, its really decent of you to take the time to go through that for me.

If anyone can help me out here again it would be appreciated

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

Have a look at the Schedule of inspected items and supply characteristics Earthing & bonding arrangements.

TN systems & TT systems have different disconection times have a look .