Insulation Tester (Megger) / Earth Tester
Test Instrument
Applies 250–1000 V DC to measure resistance in megaohms — proving that insulation which reads 'open' on a multimeter actually stays open at working voltage. Plus its sibling, the earth tester that measures how good your ground really is.
What it looks like
Insulation Tester (Megger) / Earth Tester
A typical test instrument — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Test voltage
V (DC)250 V for electronics-adjacent wiring, 500 V for 230/415 V installations (the standard), 1000 V+ for HV gear. Test at roughly 2× working voltage.
Resistance range
MΩ / GΩReads up to 2–100 GΩ. Rule-of-thumb minimum: 1 MΩ per kV of working voltage, but healthy new installs read hundreds of MΩ.
PI / DAR ratios
—Polarisation Index = 10-min/1-min reading (>2 is healthy); DAR = 1-min/30-sec. Trends reveal moisture and ageing better than one number.
Earth tester range
ΩEarth electrode targets: <1 Ω substations, <5 Ω commercial, <8 Ω domestic (per common Indian practice).
Laws that govern it
- Ohm's lawV = I × R
It applies 500 V or 1 kV and measures the tiny leakage current, computing R = V/I to give gigohms — a normal multimeter's 3 V cannot.
- Laws of resistanceR = ρ × L / A
Insulation resistance is ρL/A of the insulating material, which is why readings fall with moisture, contamination and temperature.
The reason to test at high voltage at all: a weak spot may hold off 3 V happily and break down at 500 V.
- Law of capacitance — capacitance formula Q = CVQ = C × V, i = C·dv/dt, E = ½CV²
Long cables act as capacitors that charge during the test, which is why the reading climbs for a minute and why you must discharge afterwards.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Terminals: LINE (E), EARTH (L) and GUARD (G — intercepts surface leakage). Earth testers: E/P/C terminals with two stakes and leads. Test voltage selector prominent on digital units.
Standard values
500 V/1000 V digital insulation testers (UT501/Fluke 1503 class) for electricians; 4-terminal earth testers with 20 m/40 m stake leads; clamp earth testers for built-up areas.
How to choose
1) 500 V class covers all LT installation work; add 1000 V for motors/machines. 2) A guard terminal matters for cable testing in damp conditions. 3) PI timing functions help motor maintenance. 4) Earth testing in cities: clamp-on type, since stake placement is impossible. 5) ISOLATE AND VERIFY DEAD before megger testing — and disconnect electronics (VFDs, ELCBs, LED drivers), which the test voltage destroys.
Pinout & package
Two main leads with heavy insulated crocs, guard lead, and on earth testers, current/potential stakes with long leads. Crank units generate on the handle; digital units step up from batteries.
Example circuits
- Motor test: each phase to earth and phase-to-phase at 500 V — healthy >100 MΩ, investigate <1 MΩ
- New house wiring: L+N (linked) to earth before energising
- Fall-of-potential earth test: current stake at 40 m, potential stake at 62% distance
- Tracking a transformer's PI annually to catch moisture ingress early
Common failures
Testing live circuits (destroys meter, endangers tester), frying connected electronics with test voltage, lead insulation breakdown, and misreading surface leakage as bad insulation (use the guard, clean the bushings).
How to test
Open leads should read near-infinite; shorted leads ~0. Verify against a known high-value resistor (e.g. 10 MΩ). Earth testers: check stake lead continuity and test on a known electrode. Remember readings vary with humidity and temperature — trend them.
Substitutes
Nothing safe substitutes: a multimeter's ohms range tests at ~0.5 V and will pass insulation that arcs at 230 V. For earth loop impedance specifically, an earth-loop tester (working on the live supply) complements the stake method.