insulation resistance on a domestic ring circuit

[mk1]

how would you calculate the insulation resistance on domestic ring circuit when measuring with both live and neutral conductors connected together to the circuit protective conductor, is there any way of calculating the approximate value before using a insulation resistance instrument. cheers

 

[davesparks]

I am quite certain that there must be calculations that are employed by the cable manufactures to determine the effects of parallel paths and insulation resistance over a given length.
cheers

There is a calculation for 'parallel paths' as you call them. We all learned it at school, and then went over it again when training to be electricians. Does 1/R = 1/R1 + 1/R2 + 1/R3 ....... + 1/Rn ring any bells with you?

 

[PEG]

my question was that instead of testing live to earth and neutral to earth separately, connect both conductors to one lead of the tester and other lead to earth, the reason I am asking this question is because recently I was carrying out a test on a ring circuit run in twin and earth, when testing live to earth and neutral to earth separately I obtained values of resistance greater than >9000 when both live and neutral are connected together on one lead of the instrument and the second lead to earth my insulation resistance was less than half on a short test and would increase on a 30 second test, is this just due to the increase in parallel paths.

Just out of interest,when you say "half",do you mean half of >9000 or half a M Ohm? Also,there is a difference between parallel paths and resistances in parallel. The difference between your two IR readings,if known for sure,would help explaining what may be occurring.Thanks

 

[davesparks]

What is the difference in parallel paths and resistances in paralell?

 

[Knobhead]

Why not just measure IR like normal?

Trev is right why complicate things...... lifes hard enough!!!! :banghead:

Quite simple, the IET’s one size fits all doesn’t fit in the real world.

When you’re faced with 500yards of 0.3²” 3c PILC SWA that’s over 50 years old, are you going to replace it? 0.3 is about 240mm.
A large motor (100’s of HP not some piddling little thing) are you going to replace it?

You work out the minimum acceptable resistance and then use your experience to say if it’s a goer or not.
Before I condemned anything I made damned sure I was right.

Regarding the 50 year old cables, they were that old before I diverted them for reuse 25 years ago. 75 years old and still in service to this day.

Believe it or not dazza, we had electric long before the 17[SUP]th[/SUP] edition.