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Insulation resistance, and the test voltagesWhich instrument does the insulation test?

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Insulation resistance: what it is, and what you test with

Continuity asked for the lowest possible resistance — an unbroken path for fault current. Insulation resistance asks for the opposite.

ΩContinuity
Very low resistance
Low resistance ohmmeter
MΩInsulation resistance
Very high resistance — millions of ohms
Insulation resistance tester
✋Get the instrument names right. If the exam asks what instrument carries out an insulation resistance test, the answer is an insulation resistance tester — something capable of reading megohms. Not "a multifunction tester". The MFT contains both; the question is about the function.

What you're checking is that the insulation around the copper is in good enough condition that the conductors aren't touching — and aren't leaking to each other or to earth. During continuity you deliberately linked conductors together. Here you want them completely separated.

Watch the display. Switch to insulation resistance and the MΩ symbol appears — capital M, megohms. That's your confirmation you're on the right function before you apply any voltage.

GN3 · 2.6.7 · p.68 · insulation resistance ★

Test voltages, and when to drop to 250 V

Guidance Note 3 carries the table of test voltages against circuit voltage. Three rows:

CircuitTest voltageMinimum
SELV and PELV250 V DC0.5 MΩ
Up to and including 500 V
Single phase and three phase both live here		500 V DC1 MΩ
Above 500 V1000 V DC1 MΩ
The middle row is yours. Single-phase and three-phase LV installations both fall into "up to and including 500 V", so your test voltage is 500 V DC. Highlight the whole table in your book, because questions come up on all three rows.
?Where does DC come from? The installation is AC. The instrument generates the DC test voltage — from its batteries and internal electronics. Most MFTs offer 250 V, 500 V and 1000 V; some offer 750 V.

When you drop to 250 V

Some things will not survive 500 V. Where a circuit contains:

  • USB socket outlets
  • Neon indicators in switches or sockets
  • Dimmer switches
  • An SPD or an AFDD anywhere in the circuit
OptionWhat you do
Drop the voltageTest at 250 V between the live conductors (joined) and earth, and note it in the remarks column
Bypass itDrop the plate, disconnect the cables into a connector, and test the wiring at 500 V
Do it earlyTest the cable before you terminate it. Nothing is connected, so 500 V is safe
Which test voltage, and the one that is yours Guidance Note 3 carries the table — three bands of circuit voltage SELV and PELV 250 V DC test · 0.5 MΩ minimum up to and including 500 V 500 V DC test voltage 1 MΩ minimum single phase and three phase both live in this band — this is the row you use above 500 V 1000 V DC test · 1 MΩ minimum drop to 250 V DC where sensitive equipment cannot be removed — and write down that you did, and why
Three bands, and the middle one is almost always yours: an ordinary LV installation, single or three phase, is tested at 500 V DC against a 1 MΩ minimum. The outer bands: SELV and PELV at 250 V DC (0.5 MΩ minimum), and above 500 V at 1000 V DC (1 MΩ minimum).
§The book says it plainly: during initial verification, a test of the installed wiring at 500 V DC prior to connection of equipment is required to be carried out between live conductors, and between the live conductors connected together and the protective conductor.
The sensible order. Run the cable, test it at 500 V before terminating, then wire it up and test again at 250 V, live conductors to earth, if the accessories need it. Best of both.
✋Don't attempt a 500 V insulation test on an ordinary multimeter. And don't hold the metal of a clip while the other lead is on a live test point — 500 V will give you a belt. Very little current behind it, so you shouldn't come to harm, but connect the leads before you press test.

GN3 · 2.6.7 · p.68 · insulation resistance test voltages ★

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