Metrel MI 3201 and MI 3210 Insulation Testers: 5, 10 kV

Short answer

The Metrel MI 3201 TeraOhm 5 kV Plus and MI 3210 TeraOhm XA 10 kV are portable high-voltage DC insulation testers. The first reaches 5 kV and 10 TΩ; the second, 10 kV and 20 TΩ. Besides insulation resistance they calculate polarization index, DAR and dielectric discharge, and apply step or ramp voltage to reveal defects that a single reading does not show.

What a high-voltage insulation tester is

An insulation tester applies a stable DC voltage to an insulation system and measures the current flowing through it; the ratio between the two is the insulation resistance. Metrel’s 5 and 10 kV models, the MI 3201 TeraOhm 5 kV Plus and the MI 3210 TeraOhm XA 10 kV, are designed for medium-, high- and extra-high-voltage equipment. On such equipment, a 500 or 1000 V tester does not stress the insulation enough to show its defects.

According to Metrel’s data sheets, the MI 3201 applies 250 V to 5000 V in 25 V steps and measures up to 10 TΩ. The MI 3210 applies 50 V to 10 kV, in 50 V steps up to 1 kV and 100 V steps above that. It measures up to 20 TΩ with a declared accuracy of ±5% of reading plus three digits, widening to ±15% above 10 TΩ. Metrel relates the MI 3201 to IEC 61557-2, the part of the series that defines the requirements for insulation resistance measuring equipment, and both models declare CAT IV protection.

How the measurement works

The current delivered by the DC source is not a single current. When the voltage is applied, three components appear:

Capacitive charging current, large at the start and decaying within seconds.
Absorption current, from the polarization of the dielectrics, decaying over minutes.
Conduction or leakage current, which is constant and is the one that actually reports the condition of the insulation.

That is why a reading at 10 seconds and one at 10 minutes can differ widely on a transformer or a long cable. That is also why these instruments include a 5 mA charging source (the MI 3201 declares it for quickly charging capacitive loads). They also include a programmable timer: the MI 3201 allows 1 s to 10 minutes per step.

Time-based diagnostics: PI, DAR and dielectric discharge

Besides the instantaneous resistance, both models automatically calculate:

Polarization index (PI): the ratio of the resistance at 10 minutes to that at 1 minute. Dry, clean insulation keeps improving with time; contaminated or wet insulation levels off quickly and the ratio approaches 1.
Dielectric absorption ratio (DAR): the same concept with short times, normally 60 s versus 30 s, useful when the test cannot be held for 10 minutes.
Dielectric discharge (DD): the relaxation current is measured after the insulation is discharged and normalized with the applied voltage and the capacitance. It can reveal a defective layer inside multilayer insulation that the PI cannot distinguish.

Step voltage, ramp and withstand voltage test

The MI 3201 offers a step voltage test with five proportional steps and a timer per step, and a ramp from 250 V to 5 kV with a programmable threshold current. Both also include a DC withstand voltage test. In both cases the goal is to check that the resistance does not degrade as the electric field rises, and to stop the test if the current exceeds the threshold.

The instrument also plots R(t) and measures capacitance, voltage and frequency. On the MI 3210, the voltage measurement goes up to 550 V.

Accessories and variants

The standard MI 3201 kit comes with 10 kV shielded leads with probe and clip, a green guard lead, a mains cable and a bag; the instrument weighs about 3 kg and runs on rechargeable NiMH batteries. The MI 3210 is more rugged (IP65 protection, about 6.5 kg, 12 V lead battery) and communicates over RS232, USB and Bluetooth.

The guard terminal is the accessory that matters most in the field: it diverts the surface currents of a bushing or a dirty terminal so they do not add to the reading of the volume insulation. MI 3201 results are downloaded to a PC over USB or RS232 with the manufacturer’s HVLink PRO software.

Where it is used

Power transformers: windings to each other and to ground; Metrel explicitly mentions this application. See insulation resistance and polarization index test. See also Bender ISOMETER insulation monitoring in IT systems.
Medium- and high-voltage cables: insulation of long cables, where the capacitance calls for a strong charging source.
Motors and generators: medium-voltage motor testing.
Current and voltage transformers, bushings, surge arresters and switchgear.

Each piece of equipment requires its own test voltage according to its rated voltage and the applicable practice: IEEE 43 for rotating machines, IEEE C57.152 for transformers and the ANSI/NETA tables for other equipment.

How to read the result

A single value says little. What helps is comparison:

With the factory or commissioning value of the same equipment, corrected to the same temperature.
Between phases or between windings of identical equipment: one phase well below the others points to the problem.
With the trend: a sustained drop between maintenance cycles weighs more than any absolute value.
With the PI and the DAR: a PI lower than expected with high resistance can indicate very good new insulation, not bad insulation; that is why they are read together.

The specific numerical criteria depend on the equipment and the standard, not on the instrument, and there is no single minimum for transformers. A low resistance does not locate the defect: it says you must continue with tan delta, frequency-domain dielectric response (for example, with the DIRANA) or inspection.

Precautions that affect the reading

Temperature: resistance drops sharply as temperature rises; it is recorded and corrected.
Humidity and surfaces: a damp or dirty bushing diverts current; clean it and use the guard.
Discharge before and after: the equipment must be disconnected, grounded long enough before measuring and discharged afterwards, with a safe procedure because of the stored energy.
Interference: in energized yards, induction from neighboring lines affects teraohm readings; the manufacturer highlights the RF immunity of the MI 3210.
Safety: 5 or 10 kV call for a marked-off area, verified grounds and a work procedure.

Insulation testing on request

Do you need this test? We carry it out for you: depending on your case, we recommend the right test and equipment and include it in the quotation. That way you get the test your case really needs, without paying for extra tests. Field work is done in Colombia, or elsewhere on request.

See the scope in power transformer testing and request your quotation through the form.

References

Metrel, data sheets and product page for the MI 3201 TeraOhm 5 kV Plus and the MI 3210 TeraOhm XA 10 kV (metrel.si).
IEC 61557-2, Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. – Equipment for testing, measuring or monitoring of protective measures – Part 2: Insulation resistance.
IEEE Std 43-2013, Recommended Practice for Testing Insulation Resistance of Electric Machinery.
IEEE Std C57.152-2025, Guide for Diagnostic Field Testing of Liquid-Filled Power Transformers, Regulators, and Reactors.

Frequently asked questions

What is the difference between the Metrel MI 3201 and the MI 3210?

The maximum voltage and the range. The MI 3201 applies 250 V to 5 kV and measures up to 10 TΩ; the MI 3210 applies 50 V to 10 kV and measures up to 20 TΩ, according to the manufacturer's data sheets. Both perform insulation resistance, polarization index and DAR tests. Metrel announced that the MI 3210 has been discontinued and recommends the MI 3211 TeraOhmHP as its replacement, so its documentation remains useful for installed units.

What are step voltage and ramp tests for?

To detect defects that depend on voltage. In sound insulation the resistance stays almost constant as the voltage rises; if it drops sharply in the upper steps, there is moisture, contamination or a crack that only conducts at a high electric field. The ramp with a programmable threshold current also lets you stop the test before damaging the insulation.

Can a 5 kV insulation tester be used on 500 V equipment?

The instrument can apply low voltages, but the test voltage is defined by the equipment, not the instrument: excessive voltage on low-voltage insulation can damage it. The test voltage is selected according to the equipment's rated voltage and the applicable standard or manufacturer's instructions.