Another 2391 Question,

[tex431]

Hi,
This is my first post so Hello to everyone and what a great forum this is.
Just reading the practical guide to Inspection, Testing and Certification of Electrical installations by Christopher Kitcher. Seems a good book as I am going for my 2391 after Christmas so Iam just reading up ready. On page 174 there are some exercises to do and I am struggling to get the same answer as in the back. Heres the Question-

You have installed a new ring circuit consisting of eight socket outlets on the ground floor of a detached house for Mr Hawth who resides in Crawley, Hampshire

There is no documentation available for the existing installation, but it appears to be in good condition. There is space available in the existing consumer’s unit for a new circuit.

The supply is a TNCS 230V 50Hz single phase, with an 80amp supply fuse to BS1361. Measured value of PSCC and Ze are 1350A and 0.28Ώ, respectively. On completion of the additional work the maximum demand is 78 amps.

Meter tails are 16mm² copper, the earthing conductor is 10mm² copper, and the main equipotential bonding conductors to the oil and water supplies are 10mm² (correctly connected).

The new circuit is wired in 63metres of 2.5/1.5mm² thermoplastic PVC twin and earth cable. Protection is by a 30A BS 3036 semi enclosed rewirable fuse which has a maximum Zs value of 1.09Ώ. The consumer’s unit, situated in a cupboard in a hall, has a 100A BS EN 61008-1 RCD as a main switch with an I▲n rating of 30mA. It has a tripping of 56ms at its rating and 24ms at five times its rating. The insulation resistance value is >200MΏ

A visual inspection of the new circuit shows no defects,and the test instrument used is a Megger multifunction instrument, serial number CJK1047.

1. Complete the correct paper work.

A few weeks after the circuit has been installed, Mr Hawth requests that you install an additional twin socket outlet in the kitchen; this is to be spurred from the new ring circuit. The socket is 10 metres away from the nearest existing outlet.
2. Complete the paper work

The spur is to be 2.5/1.5mm² twin and earth PVC. The resistance of the 2.5mm² is 7.41mΏ per metre and the resistance of the 1.5mm² is 12.1mΏ per metre


The books answer for R1+R2 on the spur is 0.28 but I cant seem to get that. The question seems a bit vage on correction factors etc.

Any help would be much appreciated,
Thanks

 

[Sparky Ninja]

Okay, I am unfortunate enough to own these Kitcher books.

If you look at the previous certificate (for the ring final) it has a measured R1&R2 of 0.08Ω

You calculate the additional resistance with the data given.

So..

The resistance of the 2.5mm² is 7.41mΏ per metre and the resistance of the 1.5mm² is 12.1mΏ per metre

will result in... ((7.41 + 12.1) x 10) / 1000

= (19.51 x 10) / 1000

= 195.1 / 1000 = 0.1951 (which is rounded up to 0.20Ω in this case).




So the additional resistance of 0.20Ω is added to the ring final original of 0.08Ω. This gives you the 0.28Ω figure on the certificates in the book.





The best thing you can do next with your Kitcher books is set fire to them.

 

[tex431]

Yes, I can see what your saying. How did R1+R2 get to 0.8 i have got it by

R1 =63x7.41/1000=0.46
R2= 63x12.1/1000=0.7623
R1+R2 = 0.46+.7623 = 1.68

Many Thanks

 

[DaveyD]

The best thing you can do next with your Kitcher books is set fire to them.

Really, was going to get them? Is it because outdated? Do you recommend another Wid?

 

[tex431]

You can have mine! lol

 

[tony mc]

I think you will find quite a few mistakes in the book you are talking about but that will be a good thing IMO as it makes you look a little deeper into the calculations and answers in the book..

So you have a r1 0.46 & a r2 0.76 so added a total of 1.22 ohms / 4.

So a( R1+R2) of 0.30 ohms for a ring final circuit!

Best of luck with the studying !!

 

[tex431]

Thanks Tony, why dived by 4?

 

[tony mc]

Basically you are increasing the csa of the circuit on a final ring circuit so reducing the resistance of the circuit.

So after measuring the end to end values you add them together and dived by 4 for a ring.

 

[MarkieSparkie]

I think you will find quite a few mistakes in the book you are talking about but that will be a good thing IMO as it makes you look a little deeper into the calculations and answers in the book.

Your spot on there Tony, mistakes on approximately 10% of the pages in the book at my last count, most of them typographic ... but if the reader is experienced enough to be doing 2391, then most of the mistakes should be obvious.