View the thread, titled "Help needed with understanding IR" which is posted in Electrical Wiring, Theories and Regulations on Electricians Forums.

the problem question I have states:

"the following line to earth values were recorded for six circuits in an installation. What would be the expected overall value when measured at the origin and what action should be taken, if any?"

I understand that an IR reading higher 1 ohm is a pass (though you'd want 20+ and it to be in the 100s for a new circuit). But what I am stumped on is how I work out the origin?

do I add the resistance values together and divide it by the 500v from the IR meter?

Any help would be greatly appriated
 
the problem question I have states:

"the following line to earth values were recorded for six circuits in an installation. What would be the expected overall value when measured at the origin and what action should be taken, if any?"

I understand that an IR reading higher 1 ohm is a pass (though you'd want 20+ and it to be in the 100s for a new circuit). But what I am stumped on is how I work out the origin?

do I add the resistance values together and divide it by the 500v from the IR meter?

Any help would be greatly appriated
 
Or...
product over sum method
10x20x30x40/10+20+30+40 =240000/100=2400
then (reciprocal) 1/2400 = 4.166666667 =4.8 ohms reduced to the most significant number. My preferred method btw.
 
I always thought that the Product Over Sum method only worked for resistances taken two at a time.

For resistances 10 20 30 and 40 ohms in parallel.

For the first pair 10 x 20 / (10+20) =6.666 ohms

For the second pair 6.666 x 30 / (6.666+30) = 5.454 ohms

For the third and final pair 5.454 x 40 / (5.454 +40) = 4.8 ohms

Total resistance is 4.8 ohms
 
I understand that an IR reading higher 1 ohm is a pass … But what I am stumped on is how I work out the origin?
Firstly I presume you meant one Megohm (one million Ohms) as opposed to one Ohm.

Secondly the minimum values in the Regulations are for the entire installation (i.e. after you have done your calculation if you don't measure the whole installation in one go). (For a larger installation it can be subdivided.)
 
As above you can't accept a lower value than 1mohm on any part of the installation whether combined or not. If you measure 20 circuits and get 0.1mohm combined that could be a fault on one circuit.
However if you break it down and they are all 1.5mohm then that's classed as ok.
 
brill pete. that method works. on my calc. i use x to -1 as it don't have 1/x
wtf is elecnewt giving me a dumb. i tried that method pete linked to and it gave me 4.8 ohms in about 30 seconds, as opposed to doing all that lowest common denominator what i vaguely remember from secondary school in the early 60's.
 
I always thought that the Product Over Sum method only worked for resistances taken two at a time.

For resistances 10 20 30 and 40 ohms in parallel.

For the first pair 10 x 20 / (10+20) =6.666 ohms

For the second pair 6.666 x 30 / (6.666+30) = 5.454 ohms

For the third and final pair 5.454 x 40 / (5.454 +40) = 4.8 ohms

Total resistance is 4.8 ohms

correct, but too long winded. would need a whole beermat to work it out that way.
 

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