Rfc Lenghth As Per Resistance Value

[BES]

Hello people
checking a rfc circuit today 2.5/1.5 t&e r1=1.42 rn=1.41 r2=2.33
R1+R2=0.98
my concern is cable length1.42/7.41=191m or am i not calculating it correctly.

Ta

 

[Steve3948]

51.57M ?? X 4

 

[BES]

51.57M ??

cheers for reply how do you get to that?

 

[Steve3948]

Well I may be wrong but 2.5/1.5 is 0.019 per metre and your R1+R2 is 0.98.

You mus have a rough idea of the circuit length, what does it seem like.

7.41/1000 is for just 2.5

 

[BES]

i was working out length by the end to end of r1 or rn being 1.42 then dividing by resistance of 2.5 being 7.41 then multiply by 1000 or am i doing it wrong. it is a big detached property and it could be a lengthy run but didnt think 191m also concern about voltdrop if the length is correct. 

 

[Steve3948]

Your forgetting the 1.5mm  @ 12.10 also divide not multiply

 

[BES]

so the R1+R2=0.98 / 19.51  x 1000 = 50.2m  at furtest point not end to end length sorry rfc cals always confuse me  

 

[Steve3948]

Well I divide 19.51 by 1000 which gives 0.019 ohms per metre, 19.51 mohm metre

Then R1+R2 / 0.019

 

[Rutts35]

Do you mean 19.51 milliohms not meg?

 

[BES]

As this a ring it is basicly a radial circuit wired in 5mm2 cable so what is the maximum length from cu to furthest point   

 

[Steve3948]

Do you mean 19.51 milliohms not meg?

Oops, 1000th of 1 Ohm

 

[SPECIAL LOCATION]

Hello people

checking a rfc circuit today 2.5/1.5 t&e r1=1.42 rn=1.41 r2=2.33

R1+R2=0.98

my concern is cable length1.42/7.41=191m or am i not calculating it correctly.

Ta

Your calculations appear correct to me...

Assumed lengths of individual conductors from your values of readings should be within a gnats whisker of each other, as they are all in the same cables...

Your total length of individual conductors are:-

r1 = 191.6m 

r2 = 192.6m  if @ 1.5mm

rn = 190.3m 

average length 191.5m

So assuming you got the best part of 200m of cable making up this ring...

What floor area is it covering??

I would guess its a bit bigger than the rule of thumb max 100m2 as per appending 14?

R1+R2 0.98...  is that a measured value?

it's not far off the calculated value   1.42+2.33/4 = 0.9375

Whats your Ze? and what type of MCB is on the circuit?

32A B  Max Zs is 1.44 @ 80% is 1.15..

1.152 - your 0.98  only leaves 0.172ohms worth of Ze till you are getting high on your Zs.

Well I divide 19.51 by 1000 which gives 0.019 ohms per metre, 19.51 mohm metre

Then R1+R2 / 0.019

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 ..

which is in keeping with his 190+m worth of cable used.. 

My gut feeling this circuit is feeding a bit to large an area.

 

[Rutts35]

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. 

 

[Technician]

Have had a run through these;

190 Metres of cable,  using the calculation on page 94 of the OSG

(R1 + R2)  will be 0.94 ohm.

Bes got the reads right;  r2 divided by r1 gives about 1.6.

SL is quite right;  RFC is too long. If you multiply that R1  + R2 value

by 1.2 you get 1.128 which would be the corrected value at 70 Cel.

Now check out the allowable Zs as a live read in the OSG and see

how bad it is.  And I have not even assumed what Ze is.

I am making the assumption, of course, that a B32 circuit breaker

might be in use.

 

[BES]

Tks for replys. The it is a 32a type c rcbo. Ze is 0.31 tncs. dont know why type c. Zs is way bhigh 3.14 but i guessing that due to resistance thro rcbo as i quickly check two other circuits protected by same type rcbos and ive come across this quite a few times either a rcd or rcbo pushing up zs but not to concerned due to being a rcbo ie 1667 and all that. it is a big detached property and the cables within cu are br/bl and at sockets r/bl so it looks like its bein extended when an extention was built.

 

[Technician]

Bes if it is a type C it is even worse.  Is it a domestic install?

Are there any inductive loads?

 

[BES]

yeh domestic no inductive loads it a 3 phase board being being used as a single.ive been back this morning to see customer advise them of problems and told them at least to have the type c replaced with type b they said thy will think about it what more can i do 

 

[Technician]

You may need to think about telling them that a

smaller type B would be more suitable.  If you go

back you may need to assess the loading of the

RFC.

 

[BES]

i checked the l -n loop at a couple of socket i was getting around 0.70 if i use 230/.72 the fault current l-n = 328a is that correct mind gone blank

Its just got the normal tvs etc nothing to hea.vy the kitchen is on seperate circuit . it was done by a niceic company as ther is a circuit id at db but no test results.

 

[Technician]

That PFC, I think, is too low.  The dis. time would be excessive.

If there are no kitchen loads, you could reduce the rating of the

circuit protective device.

Can you not measure the Earth Fault Prospective Current?