Megger ®  BM21, BM25 Series

5 kV Analog/Digital Insulation Testers

Megger BM21 and BM25 Insulation Testers
  • BM21
    • Simple, push-button operation is user-friendly and easy to use.
    • Analog/digital LCD enables direct reading in all ranges.
    • Lock-down TEST button and integral timer are useful for extended tests such as polarization index.
    • Rechargeable, battery-powered instrument is useful anywhere because of its portability.
    • Automatic discharge of capacitive test sample charge promotes user safety.
    • Guard terminal eliminates surface leakage current from the measurement.
  • BM25
    • Insulation test to >5 TΩ tracks insulation resistance on highly specified high-voltage equipment.
    • Automated testing allows diagnostic tests to be applied to routine maintenance, saves time and avoids operator error.
    • Polarization index tests track insulation condition on many different types of equipment with no need for temperature correction.
    • Step voltage tests detect localized insulation problems
    • Dielectric discharge tests diagnose failed layers in multilayer insulation.
    • Optically isolated RS-232 interface allows download of test results for storage on a PC.
    • Integral timer eases test operation.
    • Specifications are well-suited for accuracies and environmental performance.
    • Capacitance measurement at the end of a test ensures confidence in test piece identification.
    • Automatic charging control allows easy charging from 95 V to 265 V and prevents overcharging.
    • Durable, field-portable, weatherproof, tester gives long service life
Product Description:

The Megger BM21 5-kV Insulation Resistance Testers are portable, battery-powered, high-voltage instruments that use the latest electronic design and technology. The instruments have user-friendly keypads with push-buttons to select test voltages of 500, 1000, 2500, and 5000 V dc. In addition to the standard test voltages, a variable voltage feature on the BM21 selects test voltages between 25 and 5000 V dc in 25-V steps. An integral timer automatically controls the duration test and stops the output voltage when the preset time has expired.

Analog/digital displays offer practicality and precision. The analog/digital LCD displays readings of insulation resistance to 5 Tohms leakage current to below 1 nA, and capacitance to 10 μF. The analog scale rapidly identifies insulation condition and monitors variable measurements. The digital display complements the analog scale by simultaneously displaying an easy-to-read, precise measurement. As a safety feature, these testers act as an ac/dc safety voltmeter, indicating the level of applied voltage before a test and monitoring discharge voltage after a test. For added safety, an alarm warns the operator if the circuit under test has not been de-energized. Both testers incorporate a guard terminal to eliminate the effects of surface leakage in the item under test, thus ensuring accurate measurements.

The Megger BM25 is a compact automated 5 kV insulation tester. It can be used to conduct polarization index, step voltage, dielectric discharge diagnostic and standard insulation tests up to 5 Tohms. Results and test options are displayed on a large, clear analog/digital readout for both practicality and precision.

The BM25 provides an optically isolated RS-232 port for direct download of test results. This enables the user to conduct cost-effective predictive maintenance programs. The instrument displays leakage current down to <1 nA and the capacitance of the test piece up to 10 μF. A guard terminal allows surface leakage to be removed. Standard test voltages are preset at 500 V, 1000 V, 2500 V and 5000 V and combined with an output voltage setting that is variable in 25 V steps. The integral timer starts automatically at the beginning of a test and can be used to control test duration. The BM25 is powered by rechargeable lead-acid batteries. The built-in charger operates from 95 V to 265 V power supplies without switch selection. The instrument is supplied complete with test leads, RS-232 interface lead and file transfer utility software.

  • The BM21 series test the insulation resistance of:
    • High-voltage power cables
    • Large motor/generator windings
    • Line and substation transformers
    • High-voltage buses
  • Both instruments perform spot tests, step-voltage tests, and dielectric absorption tests for the following applications:
    • Acceptance testing at installation to check conformance to specifications.
    • Routine preventative/predictive maintenance testing after installation.
    • Quality assurance testing as part of the manufacturing process.
    • Diagnostic testing to isolate faulty components for repair.
  • Detect high- or low-resistance grounds and short circuits in apparatus, cables, wiring, caused by moisture, oil, dirt, corrosion, damage to insulation or natural deterioration. The units determine the presence of moisture, solvents and semiconducting foreign materials in wires, cables and other conductors, and in built-up insulation systems such as those found in motor windings.
  • The BM25 is designed for testing the insulation of highvoltage electrical equipment. Its wide voltage range also allows applications to low-voltage equipment. Generators, motors, transformers, cables and switchgear all require effective maintenance. The test techniques on the BM25 provide valuable diagnostic information. Insulation suffers from gradual, steady decline as well as occasional sudden damage. The effects of dirt, grease, moisture, vibration and chemical attack can be tracked by recording polarization index tests which are not limited by the temperature-dependence of raw insulation resistance measurements.
  • Polarization index (PI) is a time-resistance dielectric absorption test. PI tests are also helpful when full test records have not been maintained. To find more localized insulation problems, the BM25 includes both step voltage and dielectric discharge tests. Step voltage identifies local weak spots which respond differently as electrical stress is increased. The dielectric discharge test can show a single bad layer in multilayer insulation.
  • The key to predictive maintenance is the trending of diagnostic tests. The BM25 can download to a PC to store test results.
Insulation Ranges BM21 BM25
Test Voltages: 500, 1000, 2500, 5000 V; plus 25 to 5000 V in 25 V steps 500, 1000, 2500, 5000 V; plus 25 equal steps to 5000 V in 25 V steps
Insulation Resistance Range: Digital 10 kΩ to 500 GΩ @ 500 V10 kΩ to 1 TΩ @ 1000 V 10 kΩ to 2,5 TΩ @ 2500 V 10 kΩ to 5 TΩ @ 5 kV Analog 100 kΩ to 1TΩ @ all voltages
Accuracy ±5% on 100 MΩ load ±25V for test voltages <500V
Short Circuit Current 1,8 mA nominal, 2 mA max.
Basic Accuracy ±5% of reading 1MΩ to 1 TΩ@ 5 kV (0 to 30°C) (32 to 86°F)
Voltage Range 50 to 1000 V d.c. or a.c. (0 to 5000 V d.c. when testing)
Accuracy: ±2% ±1 V
General BM21 BM25
Display : analog/digital (3 digits)
InterferenceRejection: 1 mA r.m.s. per kV to a maximum 2 mA
CapacitorDischarge Time: < 2 s per μF to discharge from 5000 V to 50 V
Leakage currentmeasurement 0,01 nA to 999 μA –Accuracy ±5% ±0,2 nA
Capacitancemeasurement: 0,01 – 10,0 μF (displayed at end of test)Accuracy ±15% ±0,03 μF
Operating Temperature : Operating -20to +50°C (-4 to 122°F)Storage -25°C to +65°C (-13 to 149°F)
Storage Temperature: -25°C to +65°C, (-13 to 149°F)
TemperatureCoeffecient: (Applies over range 0 to 30°C)0,2% per °C for test currents >100 nA (0.1%/°F) 0,1% per °C for test voltage (0.05%/°F)
Humidity Range 90% RH @ 40°C max. (104°F)
Safety Safety IEC348 IEC1010-1 (1995), EN61010 (1995) to installation category III, 300 V, phase to earth, 500 V phase to phase
pdf_pic Downloads: BM21 General Specifications
BM25 General Specifications
space space

Power Supply: Rechargeable sealed lead-acid batteries (12 V, 4 Ah).

Battery life: typically 8 hrs continuous testing. Built in charger operates from 95 V to 265 V a.c.,

Recharge time: 8 hrs to 90%, 16 hrs to 100% d.c. emergency charge socket from 12 V d.c.

Dimensions: 344 x 245 x 158 mm (13 1⁄2 in x 9 5⁄8 in )

Weight: 5,6 kg (12 1⁄2 lb approx.)

EMC: In accordance with IEC61326 incl. amendment No.1 Altitude to 2000 m to retain full accuracy.

index web