Insulation resistor, soul torture

Insulation resistance tester, commonly known as digital shake meter and digital megohmmeter, has been more and more accepted by the majority of electrical people.

Many engineers who have upgraded from the old shake watch, or who have little contact with the new recruits who use the shake watch, often have a lot of confusion when using it.

Let's take stock of these soul tortures one by one:

How to wire the insulation resistor?

First of all, the system under test must be powered off, and the insulation test voltage end and COM end of the insulation meter must be connected to the DUT respectively.

For example, if you measure a wire with an insulating layer, one end is connected to the insulating rubber* and the other end is connected to the metal wire, so that the tester-pen-insulating rubber-metal wire-pen-tester becomes a loop.

[Tips: * When actually using the watch pen to contact the insulating rubber, it is recommended to wrap the insulating rubber with aluminum foil first, so as not to miss the fault point because the contact surface of the watch pen is too small]

Why does the output voltage always runout?

The basic principle of insulation resistance can be compared to a typical Ohm's law diagram: a meter with a known internal resistance outputs a constant test voltage while detecting current. In order to ensure personal safety and equipment safety, the insulated meter is equipped with current limiting protection. If the resistance of the insulator under test is low, or if a local breakdown occurs, or if the distributed capacitance is large, the current-limiting protection function will come into play, and a fluctuation in voltage will be observed. The insulation meter calculates the insulation resistance based on the actual voltage and current, and the measurement results remain accurate.

[Dry question 1: If the actual voltage cannot go up, it means that the external insulation resistance value is too small.] You should choose a lower output voltage to measure correctly. ]

Is High Voltage Measurement Safe? The meter is not grounded, how to discharge?

Fluke insulated meters, all of which are equipped with discharge circuits as standard, recycle the electrical energy after the measurement to the inside of the meter and consume it through the scientifically designed internal load. Guarantee the safety of users.

How to wire the insulation test of the three-phase motor?

There are 2 gauge pens or crocodile clips of the instrument, one is connected to the winding of the motor under test, and the other is connected to the casing of the motor.

Which one is connected to the casing and which one is connected to the winding resistance?

A: In fact, most of the time, the two connection methods have little impact on the test data.

Why does the insulation value reading of the insulation meter keep jumping? What exactly is a reading that is accurate?

After the insulation meter is pressed for testing, it generally takes about 1 minute to read the insulation value, which is the most accurate reading of the insulation value (the actual situation is related to the measured material, temperature and humidity environment).

This is because, after the tester energizes the insulator into the test state, there will be 3 additional currents generated by the insulator, which will affect the measured value, and most of the additional currents will decrease over time. Specifically:

Capacitive Current:

The capacitive effect, especially the insulation of multiple layers + different materials, will be more obvious. In the initial stage, the 'capacitor' is charged, so that the capacitive current is larger, and as the charge is filled, the capacitor current gradually tends to zero. It usually lasts from a few seconds to a dozen seconds. When the insulation resistance is high, the swing of the test wire can also show obvious capacitive charge and discharge current

Current Absorption:

The insulating material itself is a molecule that is affected by the test electrical energy, and the rearrangement will absorb the electrical energy and thus produce an absorbed current. It usually lasts 1 minute or more to tend to 0.

Leakage Current:

The more significant influence mostly exists in high insulation value + high test voltage (such as 2500V or above), taking F1535 2500V insulation meter as an example, when 2500V is working, this part of the current is captured through the G port supplied with the meter, which can make the test value more accurate.

Therefore, the Fluke 1000V Insulation Meter 1508/1503 is not equipped with a leakage current function. The G port of the F1535 is in the 1000V range and below, so it generally does not need to be used

Why is it that when testing the same insulator, some tables fluctuate very slowly and inaccurately, and some meters can enter the stable reading faster and it is convenient to judge the test value?

A: After understanding the second question, it is not difficult to answer: one of the key book factors is whether the maximum short-circuit current is large enough. This is because the resistor can only obtain a stable value if it can output the energy to the capacitive current/absorbed current effect more quickly. In addition, of course, there is a comprehensive design of the overall circuit, transformer design, and algorithms to ensure the entire measurement process and the final result.

Insulation resistor, soul torture

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