question about ring calculation...

[dingodeano]

just a quick question while i was doing some calculation a ring main on a tns system
Wired in 2.5mm twin and earth ( r1+r2 value 19.51 m.ohm/M )
Approx length of run 35 Metres
Type B circuit breaker max E.F.L. 1.16ohm (on site guide)
Zs = 0.8 + (0.68 ) = 1.48ohm (exceeding max value for breaker)
is it the fact that circuit will have r.c.d. protection or because its a 'ring' mean this circuit is ok even though the e.f.l is high? or is it not ok and ive missed something?

 

[GaryMo]

35M 2.5 twin and earth wired as a ring final:

r1 = (35x7.41)/1000 = 0.26ohms
r2 = (35x12.10)/1000 = 0.42ohms

R1+R2 = (r1+r2)/4

R1+R2 = 0.17ohms

Zs = Ze + (R1+R2)

Taking Ze to be maximum of 0.8ohms for a TN-S

Zs = 0.97ohms (maximum calculated at design stage without knowing true Ze)

 

[dingodeano]

r1 = (35x7.41)/1000 = 0.26ohms
r2 = (35x12.10)/1000 = 0.42ohms

Where did you get these figures from garymo?

 

[ban-all-sheds]

Probably the same place you got your figure for r1+r2....

 

[GaryMo]

GN3, Table A.1, page 104
OSG, Table 9A, page 166

 

[GaryMo]

Probably the same place you got your figure for r1+r2....

 

[dingodeano]

ahhh .............!!!!!!! found it

 

[GaryMo]

ahhh .............!!!!!!! found it

..and the difference between r1 and R1?

 

[ban-all-sheds]

<ignore>

 

[GaryMo]

<ignore>

 

[dingodeano]

rings have two wires at any outlet so infact an increased csa..... greater csa lower resistance ... r1+r2 /4
r1+r2 = ring loop
R1+R2 = Actual loop.

cheeers
Garymo

<ignore>

whats ban up to?

 

[ban-all-sheds]

But why ÷ 4?

 

[GaryMo]

But why ÷ 4?

Are you asking why R1+R2 = (r1+r2)/4?

 

[ban-all-sheds]

Yes, but I'm not asking you....

 

[dingodeano]

Because the fault will not cover the entire ring length as it stands as a measured value end end. the true earth fault path is the R1+R2 value.