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What a global reading actually is35, 125 and 75 megohms. What's the global?

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Every circuit in a property is wired in parallel, so a test across the main switch returns the parallel combination of all of them.

The global reading is always lower than any individual circuit

Where you put the leads: line on the load (busbar) side of the main switch, and the other lead on the neutral bar for a line-neutral global, or the earth bar for line-earth.

Then invert it. The ordinary parallel formula gives you 1 ÷ RT, so you invert at the end to get RT — whatever the units, ohms or megohms. That is the step that catches people.

A simple one first: 60, 12 and 5 MΩ

Common denominator: 60

  60 ÷ 60 = 1
  60 ÷ 12 = 5
  60 ÷  5 = 12
              ----
               18  over 60

Invert:  60 ÷ 18 = 3.33 MΩ

The real one: 35, 125 and 75 MΩ

Find the number all three divide into: 2625

  2625 ÷ 35  = 75
  2625 ÷ 125 = 21
  2625 ÷ 75  = 35
                ----
                 131  over 2625

Invert:  2625 ÷ 131 = 20 MΩ

And an easy one to practise on: 200, 400 and 100 MΩ

Common denominator: 400

  400 ÷ 200 = 2
  400 ÷ 400 = 1
  400 ÷ 100 = 4
               ---
                7  over 400

Invert:  400 ÷ 7 = 57.14 MΩ
This formula is not in the book. It explains what your instrument is doing when it gives you a global figure — so that a low global number does not panic you into condemning healthy circuits. If the global looks poor, go back and test them individually to find which one is pulling it down.
Shortcut. You do not have to hunt for the lowest common denominator by hand — a calculator will do 1/35 + 1/125 + 1/75 and then 1 over the answer. The long way is shown because it explains why the number comes out where it does.

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