Lighting Circuit producing strange R1 + R2 readings

[Hefferl]

I've been looking at a domestic lighting circuit today. Singles within steel conduit.

With line and CPC connected in connector strip at CU, I measured R1 + R2 at a switch. At live side of switch got reading of about 0.7 ohms. At dead side of switch got around 140 ohms!? So I tried various things to locate problem but without success. However I did notice that when I close a switch in adjacent room then at the original switch at the dead side I then get a reading off the scale as expected!?

R2 at all points in the circuit is acceptable. Took R1 + R2 readings at all points - roughly half where under 1 ohm and the other half around about that 140 ohm figure again.

Has anyone experienced anything like this before? and is this the result of the switches some how having been connected in series? headbang

 

[Ali]

Can you explain a bit more?

I'm not sure what you mean by a reading off the scale. Do you mean a very high reading or a very low one?

If I'm reading your post correctly it sounds like the swith is open when you are testing. Is that right?

I'm a bit of a newbie to all this but I'm sure a grown up will be around shortly to explain everything

 

[Hefferl]

Can you explain a bit more?I'm not sure what you mean by a reading off the scale. Do you mean a very high reading or a very low one?

If I'm reading your post correctly it sounds like the swith is open when you are testing. Is that right?

I'm a bit of a newbie to all this but I'm sure a grown up will be around shortly to explain everything

Off the scale meaning very high reading (open circuit) as you would expect with the open switch. But with the switch in the adjacent room closed the 140 ohm reading appears at the open side of the original switch suggesting there is a path via some other route introduced when the adjacent switch is closed.

 

[Doc Hudson]

Most likely through a light fitting back to neutral at the CU, thus the suppliers earth path, then joined back to the conduit via a parallel path. What is the reading between your earth and neutral bars at the CU? have either or both of these got a continuity back to the steel conduit?

Doc H.

 

[kme]

Sounds like you`re reading light bulbs, TBH, as Doc H suggests above. If you take `em all out (as if doing the IR test), the anomalous reading should disappear.

KME

 

[sellers]

This may be something very simple, it seems simple to me but I'm not on the job so to speak.

Your measuring r1+r2 on the feed to a swictch and get .7ohms

then you measure at the switched live side and get 140ohms, this is because your switch is off and you are going up the switch live through the lamp, down the neutral to the next lamp through that lamp to the live back to the board to your link.

do your r1+r2 test with all switches switched on. I think you'll find thats where the problem lies.