R1+R2 high but IR low

[Bradley6969]

Testing SWA 16mm sub main to a garage, getting reading of 66 ohms between R1+R2 and a IR resistance of between 4-5 M Ohms anyone explain how? The RCD is all fine and not tripping just testing
L
All I can think of is loose connection one end, high resistance joint, or SWA corroded

 

[telectrix]

is the armour used as cpc or is there a separate conductor in the cable?

 

[Bradley6969]

is the armour used as cpc or is there a separate conductor in the cable?

Yes sorry 2 core SWA used as armouring
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Glands either end all good look new no corrosion

 

[telectrix]

combination of damp and corrosion oerhaps. open up the glands and check them.

 

[Julie.]

What is the R1 + Rn?

If that matches the resistance one calculates due to length, then it's almost certainly a bad joint armour to gland or gland through to your test point.

So short one end and test between L and armour at the far end, then swap round end to end etc to narrow it down

 

[Bradley6969]

What is the R1 + Rn?

If that matches the resistance one calculates due to length, then it's almost certainly a bad joint armour to gland or gland through to your test point

I’ll test that, and I’ve re tested and it’s still 50-60 ohms R1+R2 but I’m getting nearly a dead short between L-N 0.5 M Ohms and L-E which is just higher than 1 M Ohms

 

[Strima]

New or existing cable? Can you inspect along the run for damage?

 

[Bradley6969]

New or existing cable? Can you inspect along the run for damage?

Existing cable old colours, and runs about 30 metres all underground

 

[Strima]

Existing cable old colours, and runs about 30 metres all underground

Sounds like someones stuck a fork through the cable.

 

[Bradley6969]

Sounds like someones stuck a fork through the cable.

My thoughts too, it’s RCD protected and customers says it’s not tripped or anything, RCD tested all fine, just merely testing the house, no doubt there will be problems when I’ve left hahahaha

 

[telectrix]

time to hire a mini digger???

 

[Strima]

If this is an EICR then I would FI and move on.

High R1+R2 could well be stopping the RCD from tipping.

 

[Lucien Nunes]

nearly a dead short between L-N 0.5 M Ohms and L-E which is just higher than 1 M Ohms

Megohms? A dead short is typically a reading of an ohm or two, due to conductors touching together. Your reading is a million times higher than that, so it's just low insulation. Still high enough to work fine (creating less than half a milliamp of leakage) but not as high as you would expect or want to see and therefore a possible pointer to a damp or damaged cable.

The term 'short-circuit' is oft abused, but it specifically means that the insulation is completely breached by effectively zero resistance, so only the conductor resistance is present. Even if an insulation test reads 0.00MΩ it's not a cast-iron guarantee that a short is present, as it could still be 10kΩ which represents a small load (and still too high to trip an RCD by itself.)

 

[telectrix]

my procedure would be to undo the glands at both ends, separate the cable from the glands and test the cable itself to find or eliminate the fault/s.