test results help

sima24 · Jun 9, 2012

Hi guys,
Im currently training as an electrician and have been practicing my testimg at home. Was wondering if anyone can take a look at my results and see if they look "average" as its nagging me as i think it seems too low (zs too low)

Ring final figure of 8 test R1 + R2 = 0.31/4 = 0.07. Ze = 0.24 - zs = 0.31 ohms.

IR on the same circuit = 124 Mohms

Iv recorded pretty opposite results between downstairs and upstairs circuits

Upstairs lights IR = 116Mohms
Upstairs sockets IR = 124.7 Mohms

Downstairs lights = 6.38 Mohms
Downstairs sockets = 6.86 Mohms

Upstairs lights continuity 0.32 ohms
Downstairs lights continuity 1.38 ohms

Could anyone please shed any light on why these results differ so much? Im a bit confused as how to interpret these results, any help would be appreciated, thanks.

Plonker 3 · Jun 9, 2012

Different circuits will give different results with varying reason.

Why do you think the results are too low?

JUD · Jun 9, 2012

sima24 said:
Hi guys,
Im currently training as an electrician and have been practicing my testimg at home. Was wondering if anyone can take a look at my results and see if they look "average" as its nagging me as i think it seems too low (zs too low)

Ring final figure of 8 test R1 + R2 = 0.31/4 = 0.07. Ze = 0.24 - zs = 0.31 ohms.

IR on the same circuit = 124 Mohms

Iv recorded pretty opposite results between downstairs and upstairs circuits

Upstairs lights IR = 116Mohms
Upstairs sockets IR = 124.7 Mohms

Downstairs lights = 6.38 Mohms
Downstairs sockets = 6.86 Mohms

Upstairs lights continuity 0.32 ohms
Downstairs lights continuity 1.38 ohms

Could anyone please shed any light on why these results differ so much? Im a bit confused as how to interpret these results, any help would be appreciated, thanks.

What did you get for your RFC end to end measurements (r1, rn & r2)?

If the 0.31 in your calculation is the result of r1+r2 (which it should be) then r1 & rn should be around 0.11Ω and r2 should be around 0.2Ω making the length of the ring only 14m (very short for a ring circuit).

However, if the 0.31 is the result you get at each socket (which seems more likely) then there's no need to divide it by 4 and this will in fact be the value that should be recorded as R1+R2. This would mean that your r1 & rn measurements should be around 0.47Ω and r2 should be around 0.78Ω making the length of the ring around 63m

trev · Jun 9, 2012

sima24 said:
Ring final figure of 8 test R1 + R2 = 0.31/4 = 0.07. Ze = 0.24 - zs = 0.31 ohms.

If that 0.31 ohms is a measured value why are you dividing it by 4 mate?
After you have carried out your r1, r2, rn and linked L and N in a figure 8 you do the same with the L and cpc the highest measured value is your (R1+R2) figure.
I'm thinking you may have missed something or done something wrong so if you want to describe your RFC test procedure we can put you right. Better to make a mistake which can be put right now than wait till a practical exam and cock it up and fail it

sima24 · Jun 10, 2012

Hi, thanks for the replies.
Think i may be doing something wrong as iv got it stuck in my head that with a figure of 8 test i take my highest reading at the sockets and divide this by 4 to get my R1 + R2?.

Sorry i wasnt very clear, my end to end results were
Upstairs sockets
rn 0.3
r1 0.32
r2 0.51

Downstairs sockets
rn 0.66
r1 0.65
r2 1.07

Im confused with the rfc? I understand its the fault current, voltage / ze, how is this worked out for each circuit? I presume it will be covered in level 3 but im too eager if i dont understand something lol, thanks in advance for helping.

sima24 · Jun 10, 2012

Sorry guys forgot to mention yes the 0.31 was my highest reading at the socket

telectrix · Jun 10, 2012

yes you have gotr a bit confused. if you measure you end/end, then add r1 to r2, divide by 4 to get and idea for R1+R2. then when you measure R1+R2 having cross connected, the result should be about the same.

sima24 · Jun 10, 2012

Ah i get it now, we have done this so i should have known, thanks a lot.

So how would i work out the fault current on the ring? Thanks

JUD · Jun 10, 2012

sima24 said:
Hi, thanks for the replies.
Think i may be doing something wrong as iv got it stuck in my head that with a figure of 8 test i take my highest reading at the sockets and divide this by 4 to get my R1 + R2?.

Sorry i wasnt very clear, my end to end results were
Upstairs sockets
rn 0.3
r1 0.32
r2 0.5

With these readings you would expect the reading at each socket to be around 0.21Ω [(0.32+0.5) ÷ 4] ± 0.05Ω unless you have 1 or more spurs which would give you higher readings

Downstairs sockets
rn 0.66
r1 0.65
r2 1.07

Here you should be getting around 0.43Ω ± 0.05Ω at each socket, again unless you have spurs.

Im confused with the rfc? I understand its the fault current, voltage / ze, how is this worked out for each circuit? I presume it will be covered in level 3 but im too eager if i dont understand something lol, thanks in advance for helping.

sima24 said:
Ah i get it now, we have done this so i should have known, thanks a lot.

So how would i work out the fault current on the ring? Thanks

I[SUB]f[/SUB] = Uo ÷ Zs [I[SUB]f[/SUB] = Fault Current, Uo = nominal voltage to earth (230V), Zs = Earth Fault Loop Impedance of the ring circuit (Ze + ((R1+R2) x 1.2)]

I[SUB]f [/SUB]= 230 ÷ (0.24 + (0.31 x 1.2)) = 375.1A