Minimum CPC sizes

[revjames]

Another question. Cracking on with the assignment. A question asks me for minimum size of cpc for given circuits. I'll give an example:

4.2KW cooker circuit - already done the calcs and Cg etc the formula for size is:

S = SQ root I(SQ) x T / K

Might make more sense if I say it: S equals the square root of I squared times T divided by K

where I = Fault current Uo/Zs in this case 1444A

K factor = 115 from table 54.3

T is the disconnection time and is (somehow) gained by looking at the charts in appendix 3 in this case page 250 as I am using RCBOs with type C overcurrent characteristics

Its a 20A device. The chart goes straight up for about 40% of the vertical axis before the curve starts. I am confused. I used a figure of 0.1S that was in an example question and the size came out at 3.97mm ie 4mm but the cable is only 2.5mm after my usual calculations.

I know its sunday afternoon but please if anyone can put my mind at rest that'd be great!

 

[Marc]

From the numbers you've used I get 3.97 as well.

Will dig my BRB out and have a gander.

Edit: So you're getting 2.5? Are you remembering to square the 1444?

 

[revjames]

In my preliminary calcs I decided upon 4mm so thats OK. I was confusing it with this circuit:

I did the same calc on a shorter run feeding a 2.4kw load which is 1329A so works out at 3.65mm

I think its because Ze is only 0.08 and Zdb only 0,028 , R1+Rs 0.059 so Zs only 0.173

Low Z means more fault current can flow I suppose?

 

[Marc]

.08+.028+.059= .167, no?

Low Z = lower resistance = increased fault current.

 

[revjames]

.08+.028+.059= .167, no?Low Z = lower resistance = increased fault current.

Quite right slip of the finger but only a minute difference in the actual outcome. It menas a circuit that is only 4M long supplying 2.4KW from a 16A protective device will have to be in 4mm!

 

[Marc]

Quite right slip of the finger but only a minute difference in the actual outcome. It menas a circuit that is only 4M long supplying 2.4KW from a 16A protective device will have to be in 4mm!

Tiny difference can have a big effect!

You only learnt the adibiatic equation this week as well then?

 

[septiclecky]

Another question. Cracking on with the assignment. A question asks me for minimum size of cpc for given circuits. I'll give an example:4.2KW cooker circuit - already done the calcs and Cg etc the formula for size is:

S = SQ root I(SQ) x T / K

Might make more sense if I say it: S equals the square root of I squared times T divided by K

where I = Fault current Uo/Zs in this case 1444A

K factor = 115 from table 54.3

T is the disconnection time and is (somehow) gained by looking at the charts in appendix 3 in this case page 250 as I am using RCBOs with type C overcurrent characteristics

Its a 20A device. The chart goes straight up for about 40% of the vertical axis before the curve starts. I am confused. I used a figure of 0.1S that was in an example question and the size came out at 3.97mm ie 4mm but the cable is only 2.5mm after my usual calculations.

I know its sunday afternoon but please if anyone can put my mind at rest that'd be great!

Haven't got book handy but think you will find it is the Sq root of I squared X T only not the whole formula

 

[revjames]

Tiny difference can have a big effect!You only learnt the adibiatic equation this week as well then?

Actually did the course material in November at Connaught Rotherham, sat exam in Dec and now doing the project. We did do the equation on the course but now that seems such a long time ago!

Its a vicious circle really, I did some caable calcs, decided to put an extra DB in to keep the circuit lengths down. Then had to use a big SWA for the sub main to allow for VD and keep the overall VD within the 5 and 3% limits. That coupled with a very low 0.08 Ze makes for some interesting results!

 

[Marc]

Haven't got book handy but think you will find it is the Sq root of I squared X T only not the whole formula

√(I

 

[revjames]

√(I

 

[Marc]

Thats what it says in BRB I think Sep tought I was sq rooting the whole equation but in fact I am only sq rooting the top bit.

Yep, only the top bit. I presume this is for 2391/2? How are you finding it?

 

[revjames]

Yep, only the top bit. I presume this is for 2391/2? How are you finding it?

yes 2391-20

Found it very interesting. I only really did it because I got a grant from the Welsh assembly to do a few courses, I did a project management course, building regs 2393 and the 2391-10. I still had loads left so on advice of the tutor did this as well.

I guess I could end up better equipped for pricing jobs, designing installations etc. looks good on the CV in any case. How about you?

 

[Marc]

Is the 2391 as hard as they say it is?

Definitely looks good, how did you go about getting this grant? We don't get anything in England (or not that I know of anyway!). Are you in the industry?

Final year of my 2330, contemplating a BEng, but at 21 it's too long to be doing Uni full-time.

 

[revjames]

the 2391 -10 isnt 'hard' as such a lot to take in but ask lots of questions dont let the teacher plough on if your niot sure. Do lots of practice exam questions and prepare for the practical and you'll be fine.

The 2391-20 is a step further and builds on the test and inspection 2391-10. A requirement is the 2391-10 practical. There is an exam - 40% pass mark - easy and then this project.

I live in a economically depressed area and there are grants available for those made redundant like me from our gov't. You should ask in CAB if you think you should get a grant.

Back to the q. I notice table 9B in OSG gives temp correctiob factors but only up to 25C my ambient temp is 30C ????

 

[septiclecky]

Thats what it says in BRB I think Sep tought I was sq rooting the whole equation but in fact I am only sq rooting the top bit.

Cheers, that what I said

 

[revjames]

amazing I actually have completed one of 10 assignment questions!

 

[Marc]

amazing I actually have completed one of 10 assignment questions!

Only 9 more to go. Guinness

 

[revjames]

Actually have most of the info to hand for over half the work and some rough drafts done for the answers. Not due in til end of next month but if I get it sent off early perhaps I'll get a chance to correct some mistakes.