It would appear that some taking the 2391 exam are forgetting basic principles when it comes to insulation resistance.
Resistance in series, the formula is Rt = R1+R2+R3+R.... (Resistance value will be higher)
Resistance in parallel the formula is Rt = 1/(1/R1+1/R2+1/R3+1/R....) (Resistance value will be lower).
Where multiple circuits are to be tested from the same distribution board or consumer unit then the overall insulation resistance can be calculated using the parallel formula.
So for example, tests of 5, 20, 10, 20, 2 are obtained from individual circuits insulation tests on a consumer unit. What is the overall insulation resistance for the installation?
So we have Rt = 1/(1/5 + 1/20 + 1/10 + 1/20 +1/2) (Mohms)
thus Rt = 1/(0.2+0.o5+0.1+0.05+0.5) (Mohms)
Rt= 1/0.9 or 1.11111 (Mohms)
Notice that I have shown all the working out here. This is so that if I had made a simple error in the calculation I may still get marks for using the correct formula!!!!!
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