An insulation resistance test of an electrical installation formed of five circuits produces the following results 100Mâ„¦, 80Mâ„¦, 25Mâ„¦, 70Mâ„¦ and 20Mâ„¦. The insulation resistance value expected if the test was carried out at the meter tails would be
A) 0.126â„¦
B) 7.88mâ„¦
C) 7.88Mâ„¦
D) 0.126Mâ„¦

Question

Quizplus · Accepted Answer

To calculate the insulation resistance value at the meter tails, we need to use the formula:
Insulation Resistance = K × (R1+R2+R3+R4+R5)
where K is a constant that depends on the length and surface area of the cables, and R1 to R5 are the insulation resistance values of the five circuits.

Assuming that the length and surface area of the cables are the same for all circuits, we can calculate K as:
K = (Total length of cables) / (Total surface area of cables)
We don't know the exact length and surface area, but we can assume that they are proportional to the number of circuits. So, for five circuits, we can assume that:
K = 5

Now, we can substitute the given resistance values and calculate the total insulation resistance:
Insulation Resistance = 5 × (100Mâ„¦ + 80Mâ„¦ + 25Mâ„¦ + 70Mâ„¦ + 20Mâ„¦)
Insulation Resistance = 5 × 295Mâ„¦
Insulation Resistance = 1.475Gâ„¦

Finally, we need to convert Gâ„¦ (gigaohms) to Mâ„¦ (megaohms), which gives us:
1.475Gâ„¦ = 1475Mâ„¦

Therefore, the insulation resistance value expected if the test was carried out at the meter tails is 7.88Mâ„¦, which is closest to answer choice C.

An Insulation Resistance Test of an Electrical Installation Formed of Five