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Topic Title: ring final with different CSA in a 'leg'
Topic Summary: i wish i was better at math
Created On: 25 April 2014 04:05 PM
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 25 April 2014 04:05 PM
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psychicwarrior

Posts: 271
Joined: 18 October 2010

good afternoon to all, hope Im not making a fool of myself ...again.....

I came across a ring final with a leg wired from DB in 6mm2 for about 8m up to a joint where 2.5mm2 continued and for the rest of it. I decided not to 'worry' about it.

It was a socket circuit so unknown loadings, but then for some reason I did get theorising with known loads (to try help me) and seeing what a different CSA may cause regarding current sharing in a leg or section of etc

It all got too complicated with my drawing and i went into a ring (loop) in my head , burnt out my one nerve (that it was then getting on) and my undersized protective device failed to operate.

Have i understood this correctly please: thinking kirchhoff, if the CSA was uniform then Ive always just ignored the impedance in the equation for working out the currents, but if the CSA is not uniform then it cant be and there will be some effect?

(initially thinking of one load at end of equal length parallel conductors where one is less resistance than the other, then more current will go down the least resistive path)

And if the above point is correct, how does one factor in the different CSA issue.......? Im presuming it is just a case of reintroducing the impedances in each section of the RFC when current summing etc so perhaps Ive answered my own question) ?

I must admit, it can get quite complicated the more I find out what I dont really know and then realise my math is not up to it :-)

Of course, I could be making too much out of this ! Any pointers, or should I just forget it hehe
 25 April 2014 04:21 PM
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daveparry1

Posts: 6382
Joined: 04 July 2007

You're worrying unnecessarily, there's no problem there.
 25 April 2014 05:11 PM
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psychicwarrior

Posts: 271
Joined: 18 October 2010

yes, thanks. i do worry....and in this case i did not fuss..................but i did go on to just be interested in the theoretical issue and wondered on the principle thats all.

cheers.....perhaps ive been on too much chocolate
 25 April 2014 05:32 PM
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daveparry1

Posts: 6382
Joined: 04 July 2007

No doubt one of the "regs. book anoraks" will be along later to say i'm wrong!
(please take this as a joke chaps)
 25 April 2014 05:47 PM
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Zs

Posts: 3042
Joined: 20 July 2006

Too much and chocolate do not go together in the same sentence. Most important that. Especially the one with the little shots of dried strawberry in it.

Great question psychic . Assuming a 32A ring final? Then your 8m of 6mm is at least oversized. I've seen similar only in bell wire.

Did the 6mm have a 2.5mm CPC or was it bigger?

Was that 6mm the very first leg of the ring and in the same breaker as the 2.5mm leg going the other way?

I see exactly where you are coming from. I get your curiosity but you're wrong about the chocolate. You didn't by any chance put it under significant load and clamp the legs did you?

I've something on my mind on the subject of fault currents/carrying capacity/ lowering impedance/ lack of balance/ temperature, but am not capable of going in to it on here. Also, given that it is a socket circuit and not a furnace..... I shall wait a bit and see if one of the brains comes forward.

I'd have crunched the numbers too, and might yet do that.

Zs

Edit: oh, I see, was typing while you posted your last Dave. Check out my designer anorak
 25 April 2014 06:18 PM
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OMS

Posts: 19900
Joined: 23 March 2004

I have several anoraks Dave - but not one of them in BS 7671 Green

Kirchoff was mentioned above and that's how you work it out.

Quickest way would be to substitute the 8m of 6.0mm for an equivalent length of 2.5mm ie make that section shorter to match the impedance and then you can work out the current flows as normal.

The effect in practice is to make the larger cable section appear shorter and thus it would have lower impedance and may (depending on the ring arrangement), carry proportionatly more current - but as it's bigger CSA that's not a problem unless it imposes that tiny bit of extra current onto the "jointed" bit of 2.5mm to the next socket

You'll need the load disposition (in theory) to work it out, but effectively all it's doing is "shortening" that section of the ring and thus potentially altering the current flows around the ring noting that the current magnitudes are the same, they just appear in slightly different places. So in practice, it could shift the theoretical point at which the greatest voltage drop occurs.

Taking my anorak off now - it's not likley to be a real issue, just an academic exercise

If you want to do sums on this Zs then either you need more strawberry chocolate until that feeling goes away or you need to do hair and go out more

Regards

OMS

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Failure is always an option
 25 April 2014 07:50 PM
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Zs

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Joined: 20 July 2006

A brain came forward psychic. There you go, it doesn't matter. But it is interesting.

Errr, excuse me OMS but...Who was it that sat me down and drew me a diagram of resistances and current flow on a ring final socket circuit???

Oh, that'll be you then

Cheeky bugger. I shall have a word with Kirchoff about you.

Not going out. My pals are all racing motorbikes in Spain. Have mad wild hair for the evening but I am very glad that you grasped the concept of going out hair. You hear as well as listen. The gospel according to Zs for the rest of you, is that going out hair needs energy consuming Halogen lighting and ordinary work hair is ok done under LED.

Zs
 25 April 2014 07:55 PM
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Grumpy

Posts: 461
Joined: 09 January 2009

Originally posted by: OMS



If you want to do sums on this Zs then either you need more strawberry chocolate until that feeling goes away or you need to do hair and go out more



Regards



OMS


OMG OMS "eat chocolate", "do your hair"!!
My goodness I hope the suffragettes aren't reading your post. The internet trolls'll be after you and you'd be a dead man.
You didn't tell Dave Parry to get his hair done (though I bet it needs doing).
PS Psychic it's maths we're not American!
 25 April 2014 07:59 PM
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GeoffBlackwell

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Grumpy its mathematics - we are British and often pedantic .

Regards

Geoff Blackwell
 25 April 2014 08:07 PM
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OMS

Posts: 19900
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Errr, excuse me OMS but...Who was it that sat me down and drew me a diagram of resistances and current flow on a ring final socket circuit???

Oh, that'll be you then

Cheeky bugger. I shall have a word with Kirchoff about you.


LoL - I didn't know I was letting the Genie out of the bottle though did I

You hear as well as listen


Yep - suprisingly !!

OMG OMS "eat chocolate", "do your hair"!!


'SsOK - Zs is cool about it -

You didn't tell Dave Parry to get his hair done (though I bet it needs doing).


Dave wasn't threatening a Kirchhoff analysis though

PS Psychic it's maths we're not American!


LoL - confuses the hell out of them when you tell the yanks there is more than one kind of math

Regards

OMS

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Failure is always an option
 25 April 2014 09:04 PM
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Zs

Posts: 3042
Joined: 20 July 2006

Thank you Grumpy.

It is all cool because ending up here wasn't on purpose and probably because I'm quite old and the first album I ever bought was the Original Soundtrack by 10CC. Even though I may not behave that way and I keep meaning to, but then I forget on the way upstairs. OMS makes tea back so as he says 'ssssOk.' I thank you though.

As an aside, not a deliberate hijack or anything but stimulated by your kind comments Grumpy; That tiny article in the training section of ECN that arrived this week, on the subject of special courses by Scolmore on wireless RF Technology for the NIC just for girl electricians made me mad as hell. I'd love to know about that stuff but shall not be attending. D'you know what? I genuinely wish for one of you, one day, to come forward and sue the NICEIC for sexism against men. Shocking behaviour. Jobs for the Girls Academy. WTFluke? It is not alright to do these kind of training courses and I apologise that you have to see this kind of thing in our industry. Rant over.

Mwah Grumpy

Zs
 27 April 2014 06:10 PM
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davebarman

Posts: 285
Joined: 27 February 2006

Sugar me, anything out of the ordinary and panic sets in. I'm glad I ain't starting out.

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Never knock on Death's door. Ring the doorbell and run like hell, he hates that!
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