excessive earth loop impedance on final circuit

[AmeliePoulain]

Hi. This seems like a newbie question but please help.
Received the test results with an 'excessive earth loop impedance' on one final circuit. Circuit originates with a 32A MCB and serves 10 socket points. Wiring is 2x2.5mm live, 2x2.5mm cpc. Max Zs = 0.72 Ohm. r1 (line) = 1.68, rn (neutral) = 1.67, r2 (cpc) = 0.44, R1+R2 = 0.74, maximum measured earth loop fault impedance = 0.89.
Checked wiring and connections - seems ok. Made some silly measurements on one of the sockets: With disconnected MCB measured current of 30mA between Neutral and Earth with 2.09V potential and 16M Ohm resistance. N - L 0A 0.07V, E - L 0A 2.02V.
Should I replace the sockets, rewire the whole lot or what else could it be?
Thanks in advance!
Michael

 

[telectrix]

is it a type C MCB?

 

[AmeliePoulain]

yes, MCB MerlinGerin multi9 C60HC C32

 

[telectrix]

both r1 and rn are high compared with r2. looks like poor connections in socket/s to me.

 

[SimpleSimon]

You say you've checked connections, two things could be

1. Loose connection somewhere out of sight. Hidden JB maybe

2. Although connections look good some may be screwed into the insulation as oppose to the bare copper. I've done that a few times.

 

[pushrod]

What is the Ze? Not necessarily a lot wrong with that figure. r2 looks small -parallel path? Or are r1 and rN high for the length of the circuit?
What is on the sockets can you not just put a type B mcb on and that would be 1.44Ω

 

[WDMDL]

If you have an idea of the order of sockets on the ring final, initially just link one line and cpc like you would for a radial, leaving everything else open.
Let's assume your measured r2 was a valid result, so unless you have some parallel paths, your measured r1 should also be in the order of 0.44 Ohms.

So, treating this as a radial you would expect to measure no more than (0.44+0.44) = 0.88 Ohms at any socket.
Looking at your measured r1, it is about 1.2 ohms higher than expected.

So unless there are multiple loose connections, I'd go round until your (R1+R2) suddenly jumps up by over an Ohm.
If you're really lucky, the loose Neutral will be at the same place.
Otherwise, repeat the same test substituting neutral for line.

Then do the ring final test to prove everything once you've located and rectified the fault.

Simon.

 

[IQ Electrical]

If the r1 and rn figures are within 0.01 Ohms then I'd say the r2 reading is benefiting from a parallel earth path.

What is the approximate length of the ring?

I haven't got time to do the calc but you must be looking at about 150M for those readings?

Your 80% maximum value for comparison is actually 0.57 Ohms

 

[Deleted member 9648]

As IQ said....r1 and rn are virtually identical which suggests poor conections are unlikely, and this is just an excessively long ring with parallel paths bringing down the r2. Check the legnth of the circuit and calculate an approximate r1 and rn to compare.
A type B may get over it.....or an RCD makes the Zs almost irrelevant. But if this is a new circuit it may have been poorly designed and need improving.