Quick Answer
Insulation resistance tester (megger) calibration verifies the DC output voltage accuracy (500 V, 1 kV, 2.5 kV, 5 kV, 10 kV) and insulation resistance measurement accuracy using NABL-calibrated high-voltage precision resistors as reference standards.
Key Takeaways
- Megger calibration verifies both output DC test voltage and resistance measurement accuracy.
- Reference standards: NABL-calibrated high-voltage precision resistors (1 MΩ to 1 GΩ).
- Test voltage ranges: 500 V, 1 kV, 2.5 kV, 5 kV, 10 kV DC (model dependent).
- NABL calibration required for electrical safety audits, CEIG approvals, and TNEB/MSEDCL inspections.
- Prism calibrates meggers (Megger BM11, BM15, MIT330, MIT525), Fluke (1507, 1550C), and others.
Insulation Resistance Tester Calibration Procedure
DC Output Voltage Verification
Measure the megger's output voltage at each test voltage setting (500 V, 1 kV, 2.5 kV, 5 kV, 10 kV) using a NABL-calibrated high-voltage probe and precision voltmeter. Acceptable error: ±5% of nominal voltage.
High Resistance Measurement
Connect NABL-calibrated precision resistors (1 MΩ, 10 MΩ, 100 MΩ, 1 GΩ) across the megger terminals. Record megger reading vs. certified resistance value.
Continuity Test Verification
Verify the continuity beeper and low-resistance measurement (0–10 Ω range) accuracy using precision low-value resistors.
Timer/DAR/PI Verification
For meggers with timed functions (dielectric absorption ratio, polarisation index), verify timing accuracy against a calibrated timer.
Certificate Issue
Issue NABL certificate with voltage accuracy, resistance measurement accuracy at each test point, and expanded uncertainty.
Applications of Insulation Resistance Testing
- Motor insulation testing (IE2, IE3 efficiency class motors)
- Cable insulation testing (MV/HV cable commissioning and maintenance)
- Transformer insulation testing (winding-to-winding, winding-to-earth)
- Switchgear and busbar insulation verification
- Generator and alternator insulation maintenance
- Electrical installation testing (IS 3043, IS 5578)
Frequently Asked Questions
Why does a megger need NABL calibration if it just tests insulation?
A megger that outputs only 400 V instead of 500 V will show higher resistance readings than actual — cables and motors that have failed insulation may appear to pass. NABL calibration verifies both the test voltage and resistance measurement accuracy.
How often should meggers be calibrated?
Annual NABL calibration is the standard. Meggers used for safety-critical electrical testing (switchgear, transformers, generator maintenance) should be calibrated every 6 months.
Can Prism calibrate 10 kV high-voltage meggers?
Yes. Prism calibrates insulation resistance testers up to 10 kV output using NABL-calibrated high-voltage dividers and precision resistors suitable for the test voltage.
Is NABL megger calibration required for electrical contractor licensing?
CEIG (Chief Electrical Inspector to Government) and state electrical inspectorates increasingly require contractors to demonstrate calibrated test instruments. NABL calibration is the accepted form of compliance.
What brands of meggers does Prism calibrate?
Prism calibrates all major brands: Megger (BM11, BM15, MIT series), Fluke (1507, 1550C, 1555), Kyoritsu (KEW 3122A), Hioki, RISHABH, MECO, and Yokogawa.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
Hours: Mon–Sat, 9:00 AM – 7:00 PM