help with testing

[jasonmizen]

hi guys! im doing testing in work with my boss in preperation for my am2. when i do insulation resistance test on my lighting circuit it comes back with a test result of greater than 500 megohms but when i reconnect the circuit into the mcb and earth and neutral into the terminals i then tested it again and had a reading of 179.3 megohms and the reading slowly went up to greater than 500 megohms!!!. when i asked my boss he said this reading is ok because it gets to 500 megohms any other better diagnosis? its on a new build house and i've taken all the equipment out that would interfere with testing eg lamps. any help is much appriceated.

 

[Adam W]

There could well be some parallel paths somewhere, even wet plaster.
179.3MΩ is well in excess of the 2MΩ required though so I wouldn't worry about it too much.

 

[jasonmizen]

ok thanks novus that helps i understand now. i think im used to seeing greater than 500megohms pop straight up all the time when i do get a reading of 179.3 i panic a bit lol i guess it comes with experience

 

[antimatter]

I would say wet plaster may be causing these low readings, especially if it gets >500 Megohms eventually.

Plaster can take some days to dry, and with it being a new build house, can take some time for all the water to come out of the house, which can affect the readings of an Insulation Resistance test.

 

[jasonmizen]

these houses were plastered a good 6-7 weeks ago. i have a 2g switch in the hall which has strappers from upstairs and strappers with switch live from the hallway light do you think it would give me a low reading if the switch live was connected in with the hallway strappers. as these are on 2 different circuits could this create the parallel paths?

 

[martysparky]

A new build can take several months to dry out even a year. You shouldn't even wallpaper in the first year because of that reason.

 

[jasonmizen]

but why is it when i disconnect the circuit from the mcb and the earth and neutral bar i get greater than 500 megohms but as soon as it is connected back to the earth and neutral bar it gives me a reading 0f 179 megohms and rises slowly to 500megohms? i tested the other circuits but they all came back 500 megohms. sorry to be a pain but i cant understand it lol

 

[antimatter]

have you tested all other circuits N - E separately with an IR test (not globally), see if you can find if it is another circuit goving you the strange readings?

 

[jimes]

would bare cables be in the plaster then?