Rfc Lenghth As Per Resistance Value

[BES]

As i said have told customer what is required didnt think i need to go as far as issuing a danger notice. If it was down to me id install a 20a type b rcbo to air on the safe side but you cant make people do thing . Thanks for your input Tech. Cheers.  

Tech the pfc and pefc are the same due to tncs supply at origin 230/.32=720amps and that approx the readin im getting.

 

[Steve3948]

Errr....no. I agree with 

Your total length of individual conductors are:-

r1 = 191.6m 

r2 = 192.6m  if @ 1.5mm

rn = 190.3m 

average length 191.5m

but

19.51mΩ/m (0.01951Ω) is the R1+R2 value/metre for a length of T+E 2.5/1.5.

and

14.82mΩ/m (0.01482Ω) is the R1+R2 value/metre for a length of T+E 2.5/2.5.

So 

0.98Ω/0.01951=50.23

So roughly

50.23X4=200.92

which is roughly in keeping with the values you worked out using the individual r1,r2 and rn

I'm not nit picking. Just that if someone who doesn't know reads this they may take those figures as read rather than check the correct table and come up with the wrong result. 

 Sorry SL but I agree with this.  :innocent   :innocent

 

[SPECIAL LOCATION]

19.51mΩ/m (0.01951Ω) is the R1+R2 value/metre for a length of T+E 2.5/1.5.

and

14.82mΩ/m (0.01482Ω) is the R1+R2 value/metre for a length of T+E 2.5/2.5.

So 

0.98Ω/0.01951=50.23

So roughly

50.23X4=200.92

So who is actually interested in where the quarter point is around the circuit???

R1+R2 should be the reading at the furthest point of the circuit from the protective device... 

Not a quarter way around a ring??

Consider a single radial circuit of 2.5mm T&E 100m in length...

This will have an R1+R2 of (19.51miliohms per meter x 100m) = 1.95ohms at the furthest piont.

{The R1 leg will be 0.74ohms and the R2 leg will be 1.21ohms   1.21+0.74=1.95}

If we made this 100m radial into a 200m ring by putting another 100M of 2.5mm T&E back to the CU...

the R1 leg will have a parallel path of equal resistance thus will be halved

(0.74/2) = 0.37ohms

and the R2 leg will also have a parallel path of equal resistance thus will be halved

{1.21/2) = 0.60ohms

The combined R1+R2 of this new ring which is 200m and 2x100m parallel paths at the furthest point  

will be 0.37 + 0.60 = 0.97ohms   {near enough to BES's 0.98ohms R1+R2 }

==============================================

A single radial 2.5mm with a R1+R2 of 0.98ohms would have a length of 50.2m... 

But no idea what relevance this has to our 200m ring circuit??

 

[Steve3948]

Simple r1 = 1.42,   1.42/0.007 ohm metre = 200 metre. 

 

[ProDave]

Just to be pedantic

Simple r1 = 1.42,   1.42/0.00741 ohm metre = 191.63 metre. 

R1R2 Copper Resistance.jpg

 

[Steve3948]

Okay Dave I was rounding off, I thought 3 places after decimal point. was enough for general idea.

 

[SPECIAL LOCATION]

Which is what I said in post #12...

But somewhere a figure of 50m  was being brought up..????

190m around the ring...  furthest point approx 95m..100m ish around the ring..!

Guinness

ps

I had already used the unrounded figures from pg 182...   saves on 4%+ error margin! :innocent

 

[Rutts35]

So who is actually interested in where the quarter point is around the circuit???

R1+R2 should be the reading at the furthest point of the circuit from the protective device... 

Not a quarter way around a ring??

Consider a single radial circuit of 2.5mm T&E 100m in length...

This will have an R1+R2 of (19.51miliohms per meter x 100m) = 1.95ohms at the furthest piont.

{The R1 leg will be 0.74ohms and the R2 leg will be 1.21ohms   1.21+0.74=1.95}

If we made this 100m radial into a 200m ring by putting another 100M of 2.5mm T&E back to the CU...

the R1 leg will have a parallel path of equal resistance thus will be halved

(0.74/2) = 0.37ohms

and the R2 leg will also have a parallel path of equal resistance thus will be halved

{1.21/2) = 0.60ohms

The combined R1+R2 of this new ring which is 200m and 2x100m parallel paths at the furthest point  

will be 0.37 + 0.60 = 0.97ohms   {near enough to BES's 0.98ohms R1+R2 }

==============================================

A single radial 2.5mm with a R1+R2 of 0.98ohms would have a length of 50.2m... 

But no idea what relevance this has to our 200m ring circuit??

I was just pointing out that you used the wrong figures in one of your calculations and was just using the correct figures as an example. I did point out you got the lengths correct just that your statement that 

19.51 is the miliohms per meter of a single piece of 2.5mm T&E..

if on a ring, 2 x 2.5mm are in parrallel...

so miliohms per meter would be 19.51/2 = 9.755..

19.51 is the milliohms/metre for 2.5/1.5 not 2.5/2.5 Some people, who are not sure, may use those figures that are incorrect. There was a   as I wasn't nit picking or trying to make you look stupid.

 

[BES]

Just to clarify guys R1+R2 reading was measure at approx the mid point of the ring. The problem is as its protected by a rcbo type c i was concern about the overcurrent side of rcbo not disconneting . If i work thiss out correct and r1 & rn being same 1.42 * by 2 divuide by 4 = 0.71 when measuring R1+Rn loop at sockets im getting approx 0.7 this value used to calculate the If does not disconnect the rcbo if short circuit LN. I have made customer aware of this and told them to remede the rcbo needs changing to a type b. But they told me they would get back to me matbe they think im making extra work iver done all i can by noting on cert.

 

[batty]

What actual work have you done to the ring circuit?

 

[Steve3948]

Its what they look for when you go to prison

 

[Technician]

19.51 is the miliohms per meter of a single piece of 2.5mm T&E..

if on a ring, 2 x 2.5mm are in parrallel...

so miliohms per meter would be 19.51/2 = 9.755..

No;  19.51 is the figure for (R1+R2) PER METRE of a

radial circuit with a line conductor 2.5 mm and a cpc of

1.5 mm csa respectively, in a RADIAL circuit.

(R1 + R2) calculations are done using the method in

testing of ring final circuits.

 

[BES]

What actual work have you done to the ring circuit?[/quote The socket had burnt out it . Someone had used it to spur off in swa no glands just from outside thro wall in to back of skt just bent back armour and used 3rd core as earth. To feed 8 lantern lights around patio a lampost a water feature and best of all a summer house 4 way cu which in turn feed a number of sockes and lights. Again no glands used anywhere for swa . The latern lights were interconnected buy single cables run in oval conduit the terminations were just strip connetors open to elements. Needless to say this has now been disconnected.Not customers fault only moved in for 2 wks. Have recommended a eicr thay siad they want to finish the decotating first ?!! I give up. Wish id took some pics.

 

[SPECIAL LOCATION]

I was just pointing out that you used the wrong figures in one of your calculations and was just using the correct figures as an example. I did point out you got the lengths correct just that your statement that 

19.51 is the milliohms/metre for 2.5/1.5 not 2.5/2.5 Some people, who are not sure, may use those figures that are incorrect.

The figures and the statements I used were correct...

It looks to me as though you miss read the bit where I had the "T&E" included in my statement?

dunno where on earth you are getting the idea that we have a piece of Twin & Earth with 2.5 Line & 2.5 CPC conductors???

As a said...

19.51miliohms per meter is the resistance of a single piece of Twin & Earth..   

{in the context of an R1+R2 measurement, which we were discussing}

also I didn't clarify that by saying that T&E has a 2.5mm Line & 1.5mm CPC.

as I assumed most people realised current standard T&E has a 1.5mm CPC..

and from the figures calculated from the OP it clearly is a ring with a 1.5mm CPC...

So to mention a 2.5mm CPC is a bit daft and will only cause more confusion...

:C

What actual work have you done to the ring circuit?[/quoteThe socket had burnt out it . Someone had used it to spur off in swa no glands just from outside thro wall in to back of skt just bent back armour and used 3rd core as earth. To feed 8 lantern lights around patio a lampost a water feature and best of all a summer house 4 way cu which in turn feed a number of sockes and lights. Again no glands used anywhere for swa . The latern lights were interconnected buy single cables run in oval conduit the terminations were just strip connetors open to elements. Needless to say this has now been disconnected.Not customers fault only moved in for 2 wks. Have recommended a eicr thay siad they want to finish the decotating first ?!! I give up. Wish id took some pics.

So this..  "rather long".. ring..

also had a SWA spur branched off it supplying a summer house?????

Voltdrop & Zs must be way out down in the summerhouse surely???

 

[Rutts35]

dunno where on earth you are getting the idea that we have a piece of Twin & Earth with 2.5 Line & 2.5 CPC conductors???

I didn't. I know what CSA's are in a standard piece of T+E. So there's no real need for the condescending nature of your replies.   

So to mention a 2.5mm CPC is a bit daft and will only cause more confusion...

That was a typo by me sticking T+E in front of the milliohm/metre value for 2.5/2.5. I'd copied and pasted the line before and not removed the T+E.  :Blushing

You wrote

19.51 is the miliohms per meter of a single piece of 2.5mm T&E..

if on a ring, 2 x 2.5mm are in parrallel...

so miliohms per meter would be 19.51/2 = 9.755..

so

BES's  (0.98 / 9.755)x1000 = 100.46m from CU to furthest point ..

You've said 2x2.5 then quoted the figure for 2.5/1.5. Perhaps I'm reading it wrong?

I fully understand about the calculations, agree with the lengths from r1, r2 and rn.

 

[BES]

Special location

i very much doubt the person who install the spur part had no clue what voltdrop or zs is. Really wish id took some pics its was one off the worst installs ive ever seen. Im hopin it was done by previous owners thierself and no by somebody who called thierself an electrician.

 

[Rutts35]

TBH all you can do is recommend. The 'your clearly touting for more work or making it up look' is very common amongst customers.Changing the socket isn't really an awful lot of work to lose. Someone will end up changing it whilst ignoring what you found. The socket will probably burn out again. At the end if the day you've told em why.