What is a megaohmmeter and how to use it

Megaohmmeters are convenient and functional instruments for measuring insulation resistance, they allow not only accurate measurements, but also to verify the integrity of the insulating material. Insulation resistance meters are mainly used by professional electricians and specialists servicing high-voltage electrical equipment, due to the features of such a device. The device allows you to measure large values ​​in the resistance of circuits, insulation materials, motors, telecommunications installations and other types of equipment, and the main purpose is to determine the safety of operation of the tested objects.

Megaohmmeter: what is it, scope and principle of operation

Megaohmmeter is a special meter by which measurements of high resistance indicators are performed. The main difference from traditional ohmmeters is presented in that the measurements are carried out at a significant voltage level, independently generated by insulation meters.

The functioning of the insulating resistance meters is explained by Ohm's law, which is valid in the area of ​​the electrical circuit: I = U / R. The main components installed inside the case are represented by a voltage source having a constant and calibrated value, as well as a current meter and terminal outputs.

The connecting wires are fixed on the terminals using ordinary “crocodile” clamps, and the current values ​​of the electrical circuit are measured by the ammeter present. Some models are characterized by a scale with two kinds of values ​​or numbers displayed on the screen.

The megaohmmeter operation principle

Megaohmmeters are used in measurements of insulating resistance, as well as with the aim of determining the coefficient of insulating absorption of electrical equipment that does not abide under operating voltage conditions. Insulation resistance meters are classified depending on the typical features of the circuit and indication method.

Digital models are cheaper devices, and analog devices have a high cost, but are characterized by high accuracy measurements.The main scope is currently represented by production and distribution systems of electrical energy, control systems for the operation of electrical equipment in industry, laboratories and in the field. In everyday life, such devices are not too much in demand.

How is the device

Different models of meters are distinguished by their design features. Inside the old devices there are manual dynamos, and the new devices are supplied with external and internal sources.

The diagram shows the elements of a megaohmmeter

• “L” - clamp “Line”;
• “E” - clamp “Screen”.
• "Z" - clamp "Earth";

The output power of devices designed to test the insulation of industrial high-voltage equipment can be several times higher than the characteristics of models designed for use in domestic wiring

The design feature of the measuring head is the frame interaction, and the switch toggle switch is responsible for switching support. The reliable and durable dielectric casing is equipped with a portable handle, a folding folding portable generator-handle, a switch and special output terminal elements.

Features of the operation of the device

Any measuring measures in electrical installations are carried out exclusively by working, surely tested and fully tested electrical devices or devices with strict observance of all the rules of measurements.

Before proceeding with measurements, make sure that the megohmmeter is working

Megaohmmeters are selected in order to check the insulating properties and measure the dielectric resistance indices according to the established indicators.

Influence of induced voltage

Electricity, which is carried by wires of electric transmission lines, creates a large magnetic field that can be changed according to a sinusoidal law. This feature provokes guidance in the conductors of metal the appearance of an electromotive secondary force and current indicators of significant magnitude.

Electricity transmitted by power lines, a powerful magnetic field is formed.

This feature has a tangible effect on the level of accuracy of all measurements performed, and the resulting sum of a pair of unknown current values ​​can make the metrological task very problematic. It is for this reason that measuring the insulation resistance of a network insulation under voltage conditions is an absolutely hopeless event.

Residual Voltage Action

The generation of voltage parameters by the generator, which enters the measured electrical network, contributes to the appearance of a potential difference between the grounding circuit and the wires, which is accompanied by capacitive formation with a certain charge.

Before connecting to make measurements, make sure there is no residual voltage.

Immediately after disconnecting the measuring conductor, a quick circuit break occurs, which helps to partially preserve the potential by creating a capacitive charge inside the bus or wire system. If you accidentally or deliberately touch this area, there is a risk of electrical injury when a current discharge passes through the body. Injury prevention is ensured by the use of a mobile earthing system with a handle provided with high-quality insulation.

Before connecting to perform insulation measurements, it is important to ensure that there is no residual charge or voltage inside the circuit under test. For this purpose, specialized indicator devices or voltmeters having the corresponding nominal values ​​are used. For fast and absolutely safe operation, you will need to connect one end of the grounding conductor to the ground loop. The other end of the conductor is in contact with the insulation rod, which allows grounding to eliminate residual charge.

How to use the device

When the handle of a hand-held device is rotated or as a result of pressing a button of electronic devices, high voltage indicators are applied to the terminal outputs, which are transmitted via wires to the measured electrical circuit or to electrical equipment. For measurements on a scale or screen, resistance values ​​are displayed.

Table: megohmmeter parameters for measurements

Safety Instructions for Using the Instrument

Modern megaohmmeters generate a voltage level within 2500 V, therefore only employees who have completed a full course of special training and are familiar with safety regulations can perform work on such a device. Only fully serviceable and trusted measuring instruments can be used in the work. Measurements on loose wires show the value of insulation resistance.

On measuring instruments of resistance indicators of an older sample, this value is equal to "infinity".

Be sure to read the safety rules when working with the device

When operating an electronic device equipped with a modern digital display, the readings are always fixed.

• During the measurement of insulation resistance, any touch to the output terminals of the measuring device and contact with the exposed parts of the connecting wires in the form of the ends of the probe are strictly prohibited. Do not touch bare metal parts of the measured electrical circuit in equipment that is under high voltage.
• It is strictly forbidden to measure the insulation resistance without checking the absence of voltage if measures are planned with conductors of an electric cable or with any live parts of electrical installations. Checking for the presence or absence of voltage in the wires and installations is carried out using an indicator, a special tester or voltage indicator.
• Measurement measures in the presence of a residual charge on electrical equipment are prohibited. To remove the residual charge, an insulating type rod or grounding with a short-term connection to live parts of the device should be used. The residual charge is eliminated after all measurements.

The use of a validated and standard test megaohmmeter is possible only after its operability is confirmed. Ensure the correct operation of such a measuring device is necessary immediately before conducting measurements of insulation resistance. To this end, the connecting wires are connected to the output terminals, after which wire shorting is performed, which allows you to start the measurement. It should be remembered that in conditions of shorted wires, the resistance indicators should be zero, and shorted connecting wires allow us to verify their integrity.

Is there an alternative to a megaohmmeter

To date, a huge number of multimeters are being implemented with resistance level measurements in the range up to 100 MΩ. Despite the solid operating range, such testers cannot be a worthy replacement for a megohmmeter, which simultaneously checks the electrical insulation strength and ensures work with a measuring voltage of 250, 500, 1000 V and even more.

The principle of measuring insulation resistance with a megohmmeter

Currently, the most common measuring instruments include megohmmeters M-4100, ESO202 / 2G and MIC-1000, as well as MIC-2500.

Certified megaohmmeters: manufacturers overview

The main, most significant technical characteristics and parameters of megaohmmeters include:

• resistance - within 0–49 900 MΩ;
• voltage - 100-5000 V;
• working temperature ranges - from -20 to + 40 ° С.

Megaohmmeters, which undergo a periodic check of their performance in METROLOGY and are included in the Register of Measuring Instruments of Russia, are produced by many manufacturers, but safe and reliable models of the measuring instrument have proven themselves to be the best.

Table: list of devices with characteristics

Less popular among consumers, but well-established models of digital and analog megaohmmeters.

Table: characteristics of digital and analog megaohmmeters

A megaohmmeter is, of course, one of the most necessary devices in working with high-voltage equipment. The choice of model and, most importantly, the safety rules for its use should be treated with maximum responsibility.