IR Test Line to CPC same as Neutral to CPC

[PeatBog]

I was Insulation Resistance testing a lighting circuit yesterday. Because of ELV transformers in the circuit I only tested between Line to CPC and Neutral to CPC. I was surprised to get the same value 259Mohms for both. I know that it is quite a respectable value (for an oldish installation) but was surprised that both lines to earth were the same. It kind of indicates a common path to earth? For the insulation in both lines to have degraded at exactly the same rate would be surprising? Or perhaps these are the values "as manufactured"???

Any one have any thoughts? Pete.

 

[spinlondon]

What was the reading when you tested L+N to E?

 

[Apache]

Isn't that what you would expect? Both live conductors are same CSA and same length and same age.

Am I missing something here?

 

[septiclecky]

Isn't that what you would expect? Both live conductors are same CSA and same length and same age.Am I missing something here?

And don't forget being connected together via the winding of the transformer possibly

 

[Andy™]

I was Insulation Resistance testing a lighting circuit yesterday. Because of ELV transformers in the circuit I only tested between Line to CPC and Neutral to CPC. I was surprised to get the same value 259Mohms for both. I know that it is quite a respectable value (for an oldish installation) but was surprised that both lines to earth were the same. It kind of indicates a common path to earth? For the insulation in both lines to have degraded at exactly the same rate would be surprising? Or perhaps these are the values "as manufactured"???Any one have any thoughts? Pete.

you do realise that if you dont short L&N together, and there is high IR on what you test, but low IR on the other (i.e L-E + 299, N-E 0.00), then you may aswell test L-N and risk blowing up everything

also, from the test you done, its hardly surpising that IR is the same. infact, i would be more worried if they were different. you will get some test current flowing through the transformer & other connected loads, so in effect, your measuring the same thing twice

 

[PeatBog]

What was the reading when you tested L+N to E?

I was being lazy as I couldn't easily get both L+N in the croc clip together. I think I was lucky not to have blown-up any of the (electronic) transformers!!

Thanks for your input, it's obvious really then you think about it.

I = 240/259000= 1milliAmp (ish)

So was the current flowing likely to be flowing through the loads (Tungsten bulbs/Electronic transformers) or a real leak through the insulation?

 

[Andy™]

I = 240/259000= 1milliAmp (ish)

not quite... 259M ohms is 259000000 ohms.

so 230/259000000 = 0.0008mA

 

[ianmacd]

microamp more like as you are a power of 3 out there.

 

[a1spark]

microamp more like as you are a power of 3 out there.

Agree with Ian [on that one ] microamp ,approx.

a1spark