Zs and PFC

[fattony]

Ive been doing some testing and was confused at some of the result, Im hoping someone can help me out here.

I tested the ring in my house (just practising as I dont do much testing) and readings are below:

Continuity CPC 1.5
Continuity P 0.8 These are Straight forward
Continuity N 0.6

Now my query came when testing Zs and PFC, I tested at each socket with nothing connected and thought on a ring the readings would be fairly similar, but mine are not...

Zs PFC
0.82 292
0.78 308
0.65 369
0.61 390
0.54 437
0.59 401
0.73 324
0.97 242
0.99 240
0.94 252
0.90 266

So you see the readings are all different, can someone clarify if the readings should be similar or is this the norm?

Thanks

 

[ricicle]

Did you do the correct ring final circuit test?

Your readings suggest a problem with the ring.Could be -

Incorrectly wired ring

Loose connection

Bad connection between socket and tester if using the plug in lead.

 

[fattony]

Yep I thought something was wrong, I did use the plug in lead, I know I have a ring from the continuity test, I'll do some more practising in a few days time,

cheers for the help

 

[ricicle]

I know I have a ring from the continuity test

Did you do the proper ring final circuit test???

 

[scott6471]

I would also look at your continuity readings!

The r1 and rN readings should be almost identical, 0.2 ohms difference seems quite substantial.

How are you carrying out your ring final tests/what tester are you using/is the circuit on an RCD (using trip lock on the tester)?

I'm yet to find a decent all in one tester that accurately tests the Loop Imp on and RCD cct with trip lock (Anyone know of one, i'd love to know if there is one out there?)

Scott

 

[Spark123]

You'll see varying measured Zs around a healthy ring circuit owing to its composition, the maximum being at the half way point on a true ring where both legs (parallel resistances) are equal. Either way from the centre point and the resistances of the cables in parallel will cause the resistance to drop. If a spur has been installed it may have a higher resistance.
Like has been said if you do the continuity check as described in the OSG or GN3, this should reveal more. The end to end L and N shouldn't be more than 0.05ohms difference.

 

[fattony]

cheers I'll check the OSG and maybe re test continuity

 

[viewer]

Suggestion; plot the figures against position of sockets (There is an obvious curve) and look for bad connections (etc) on neutral in the high resistance areas.

 

[Dippy]

Assuming the ring uses 2.5/1.5 twin and earth, not only is the 0.2 ohm difference a problem, but also the ratio to cpc resistance is a problem too. I'd suggest that you map all points and estimate the total cable run length and this will give you an idea if the cpc resistance is high or the phase/netural resistance low. I think the typical resistance of 2.5 mm copper is about 12 milliohms per meter but I'll check.

 

[ebee]

On a ring the highest Zs is at the centre for all points actualy on the ring, spurs add to a higher reading.
Have you measured Ze to calculate Zs = Ze + R1 + R2 ?
This should give you calculated Zs at mid point.
Your r1 & rN (P & N as you call them) readings should be the same , yours are too different and not 1/1.67 times r2 (CPC) as would be the case with 2.5/1.5 T & E.
I suspect that you 0.6 on the neutral might be pretty correct, 0.8 on the phase could be showing poor connections and 1.5 on cpc might be because 0.6 x 2.5 = 1.5 therefore a 1.0 cpc instead of 1.5.
Hope that helps

 

[fattony]

thanks for all the advice guys, I'll hopefully check this out further at the weekend if I have some spare time.