Discuss Very high Zs reading in the Electrical Forum area at ElectriciansForums.net

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M

mspms

Good evening all. I wonder if anyone could give me a steer as to why I am getting very high Zs readings. I have been testin a circuit that has a R1+R2 of 0.23 ohms but I seem to be getting Zs reading on L-E of + 40ohms. L-N is coming in at 0.2ohms. The incoming fuse is a 63amp BS1361 an RCD main switch protecting all circuits. All other tests are fine. I am wondering whether it could be my meter not reading correctly on the Zs test. Any ideas?

Thanks all.
 

telectrix

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try taking as reading on the L busbar. if that is high, then it's due to internal resistance of RCD and/or MCB. flick them on and off a couple of times. if that reading is OK, try at the terminals of the socket, not the front. try flicking the socket switch/es on/off.
 
OP
R

Rauer

try taking as reading on the L busbar. if that is high, then it's due to internal resistance of RCD and/or MCB. flick them on and off a couple of times. if that reading is OK, try at the terminals of the socket, not the front. try flicking the socket switch/es on/off.
Never considered that but will remember it for future reference!!
 
OP
M

mspms

Thanks for your suggestion I will give it a go. Just to add more pain, I thought I would try a reading from a socket in my own home and again I'm getting +40ohms. Just coincedence?
 

Strima

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What are you using to test the socket? Have you tried testing at the terminals on the back?
 
OP
M

mspms

Sorry teletrix. It was a long day. I gave up and came home. The house is empty at the moment so no chance of any mishaps!
 
OP
M

mspms

My Ze was really low at 0.09ohms. My meter has just been calibrated and it is a TN-C-S.

- - - Updated - - -

I'm using my 3pin plug and leads.
 
OP
M

mspms

Brand new RCD main switch today. I even tried with the old 2 pole main switch. still high.
 

stef

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Batteries going down? Bad contacts on the three pin plug? Dodgy terminations, any load on the circuit? (mainly an inductive might cause a false reading) What make of RCD is it?
 
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M

mspms

Just a quick note I have just tried a socket in my place and I;m getting 48.1ohms with a pefc of ony 4.78amps.
 

Richard Burns

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Do check your settings on the meter and the battery level just in case.
Then try testing in your house as close to the origin as possible, without any switches, just the supply and see what reading you get, if it is still +40ohms then sounds like a leads or meter problem.
What reading do you get on testing resistance on the leads?
 
OP
R

Rauer

Try it with your probe leads and not your plug top lead
 
OP
M

mspms

I have spent the last 2 days going through the entire house checking and testing and putting right some very sorry wiring expecially on the R2 side of things. I am happy with all of the wiring and the results that I have had on the continuity and resistance side of things. it is this very high Zs I can't seem to fathom out. The RCD is a Hager.
 

stef

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Switch off your meter, remove all batteries, wait then put them back and try again. I am running out of ideas.
 
OP
R

Rauer

Plug top lead sound a bit suspect to
Me at the mo
 
OP
M

mspms

I have just tested at the terminals of the same socket with my 3 test leads and am now getting 0.01 ohms and 23.00 Ka.
 

Richard Burns

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Change your plug tester unit!
But try a few contiuity tests through the unit first and see that there is not dirt in the connections or bent pins from dragging around in your tool bag.

Edit
No hang on 0.01ohms sounds a bit low for a standard value, could be, but 23kA is too high for a domestic CU.
 
OP
M

mspms

Gentlemen, thankyou for your time and patience. I will try again with just my probes and see what I get. Just one other question; why does a main switch RCD trip whwn carrying out Zs testing? i had one the other day, another empty house and need to go back to sort it.
Kind regards.
 

Strima

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Post #10... :lol:

Have you got a 'no trip' setting on your MFT?
 
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