High earth continuity ready. then low.

Hi Kev,
I know you have checked all connections and tested one socket, but i would go to every one and test mate. Also, are you using a good connection at the board when connecting your meter?

 

Are you using a plug at the sockets or probes on the terminals.. I find plugging into the socket has a huge effect on the reading, probes on the rear terminals are far more consistent

 

yeah i guess its because you can get a nice firm connection with the test probe although one could argue a true reading is at the socket as thats where the appliance will connect.
Our college stance on it was test via the socket first and bear in mind readings will be different when using the probes on the connectors. Plus the sockets had been taken apart/unplugged millions of times!

 

I usually push the test plug in and out of the socket a few times when I get higher than expected readings, also flick the switches on/off a few times too, that often cures the problem. (cleans the contacts)

 

When doing End to End continuity tests at the CU (Which i understand is what the original post was about) a high result on the CPC compared to the L/N results could be due to a 1mm2 CPC size (or imperial equiv) + some connection resistance at the various terminals of the circuit.The L/N size (if imperial sized cable) could be larger than todays 2.5mm2 (metric) cable hence the big difference between the results.The result on the CPC might indicate a lose connection - somewhere? it could be at an FCU on the ring rather than a socket - possibly in the kitchen or possibly the boiler cupboard. - The low result tested at the socket is due to a parallel earth path or the CPCs have been bridged somewhere? (if so, usually at a ring extension in the kitchen)this produces two rings on the cpc so when you test at a socket you are only measuring one side of one ring of what is now effectively two rings on the CPC - hence the low result -(confused?) you might have to see this done to understand but i hope this may help anyone who comes across this problem.- I've had this loads of times in kitchens - they extend the ring with lovely crimped terminations but bunch all four ends of the cpc extension in a connector block in the back of a socket which produces two CPC rings - the ZS results will be fine and low but the end to ends will be well confusing.

 

Hi Shocker,
I thought i understood what you meant mate when you said double ring, but then a bit confused when you said all the CPC's bunched in a connector block.

 

Thats ok mate - to explain - the live & neutral are extended via a crimp or connector block at a socket (for example to add additional points) keeping the two ends on the Line & Neutral a ring (one ring that is, which is correct) - but as a short cut, sometimes (as I've seen loads of times) the four ends of the CPC are put all together in a block for quickness instead of crimping or joining the ends together - i can understand the reasoning, their all earths bang em together! The problem is this produces two rings one on the extension side and then one on the original side of the CPC ring. The live & neutral side is ok and correct - but when a resistance test is done at the CU on the CPCs you will only measure the original CPC ring not the extension. If you test at a socket where the extension has been carried out you will only test the extension side of the ring - hence you will get two very different results usually one will be high(the original part of circuit) the other will be low (the extension)

 

Just to add - the 1.86 + 0.36 ohms = 2.22 ohms / 4 = 0.55 (R1&R2) + Ze 0.35 max (if tncs system) = zs of 0.90 ohms well within regs so i would not be too worried by those results with an RCD protected circuit.