Maths help

[juniorspark]

bit of help on maths needed, pretty basic question

if reading end to end reses of ring and a cpc-cpc reading of 0.43 ohms is read what is the actual ohm value (have trouble understanding this part)

is it half of one ohm (nearish)

 

[Megaohm7]

Near enough but remember to null your test leads first and whatever your meter is telling you, you must use this figure on your calcs. You say it as you see it..

 

[juniorspark]

so on ring

r1 0.43

rn 0.49

r2 0.79

R1+R2 0.39

Am i right in saying that these values for ring are ok as they are (acording to my previous question) very small values and if there was a ring problem i would expect to see values of around at least half an ohm (or more?) difference

 

[Ash]

0.43+0.79/4=0.305

r1 and r2 are near enough the same

it's your r1+r2 value on each socket you should be worried about, it shouldn't really be more than 0.05Ω difference on each socket.

r2 should always be higher than r1 and rn because the cpc is 1.5mm2

 

[steptoe]

with a cable of 2.5mm L&N and a CPC of 1.5 then r2 should be roughly r1 X 1.66

 

[PC Electrics]

0.43+0.79/4=0.305r1 and r2 (should read r1 and rn) are near enough the same

it's your r1+r2 (should read R1+R2) value on each socket you should be worried about, it shouldn't really be more than 0.05Ω difference on each socket.

r2 should always be higher than r1 and rn because the cpc is 1.5mm2

My corrections in red above

 

[juniorspark]

if 0.50 is half an ohm then a length of cable on a standard sort of circuit must have very little resistance

 

[steptoe]

correct,

there should be a list of cable resistances in the download section I think,

if not I will try and do one this weekend.

---------- Post Auto-Merged at 02:06 ---------- Previous post was made at 02:03 ----------

if 0.50 is half an ohm then a length of cable on a standard sort of circuit must have very little resistance

a Zs on a lighting circuit of a normal house would be < 1.5 ohm under normal circumstances and maybe even < 1.0 ohms , this is only a rough figure by the way, and perhaps a Zs for a ring final/sockets may be .57 ohm .

so you see we are not looking at massive figures.

 

[juniorspark]

thanks very much,

that is why cooker circuits have an even lower reis typically 0.05 etc due to the larger csa

 

[safedepth]

That's right.

As csa goes up resistance goes down.

As length goes up resistance goes up.