Continuity and IR Test questions

[PrenticeBoyofDerry]

Continuity test on a lighting circuit:
Two core 1.00mm line and CPC looped in at switch points.
Downstairs highest reading is R1+R2 0.67 ohms

Then the readings are as follows for upstairs rooms and landing:
Bedroom 1: 0.73
Bedroom 2: 0.87
Bedroom 3: 1.48
Landing: 2.50
Bathroom: This reading then flucuates between 3.10-4.8 omhs.

It's a standard three bedroom layout with the footprint of the house being about 5x6 metres.

So the first question being what would give me these sort of readings?
I have checked all accessible points for continuity and tightness.

Second question: what would be considered an unsafe continuity reading on a CPC of this type.

Third question, if an an IR reading was giving something like 40 Mega ohms, on an old installation.
What would be considered the cables life expectancy?

 

[ricicle]

I take it your leads and batteries are ok and that these readings are consistent ? If so I would be looking for a couple of bad connections on the upstairs circuit.
As for cable life expectancy - if this is for the whole installation then it isn't bad. If it is an individual circuit then although still acceptable some investigation might be necessary.

 

[bernardgreen]

One possible reason for the strange readings is a current flow along one or more of the wires being measured for resistance. A current NOT from the ohm meter will creates a voltage difference along the conductor which will be comined with the voltage created by the test current from the meter. The result of this combination of voltage is then likely to "confuse" the ohm meter and produce a false reading.

 

[PrenticeBoyofDerry]

Thank's for the replies,
The down/upstairs lighting circuit is the same circuit.
The MFT is working fine, no battery or lead problems, had it on a job today with no bogus readings.
I'm sure when I did the r1+r2 test on the lighting circuit the whole installation was dead. I might have had the RFC on, but only that circuit.
The loft has been partly boarded out and the circuit does disappear underneath these boards and a structure has also been erected on this part of the loft. Making it impossible, without dismantling the structure to lift the boards and see what lies beneath!
The IR test was just on this one circuit.
Two readings came in at plus 70 and the other was 40ish. There are no previous test sheets to compare with, the system is of an age but the other circuits on the system all clear 299+

 

[1550]

Well the uncorrected max zs for a 3036 5 amp is 9.58 ohms and a b type mcb is 6 amp is 7.67 for 0.4 disconnection at normal working temp. 4 sec on other cpd's are higher.

If you have a long cable run to the upstairs in say the most convenient long route it will substantially increase your cable resistance , but as these are progressive it's what you would expect from a long radial of one circuit covering both floors. As has been said ,I would like to see more than 40MEG on one circuit , but if it's old and a global reading it's not bad.

If you are testing continuity through the switches you may have a dodgy one or two .Make them and break them or link them out , see what happens.

 

[1john]

As mentioned, i'd be shorting the switch out with a connector block on the light you are testing, 40 Mohms is loads, I wouldn't be worried about it.

 

[SgtTrojan]

Althought conducting continuity of cpc is required it's value need not be verified. Rather than scratching my head I would conduct an earth loop impedance test on the furthest point and verify that value. If it fails that then investigate. Test results and maths don't usually sing from the same hymn sheet if you know what I mean?