The cable insulation test is also called a continuity test. In order to do it, you need to perform the following series of steps
- First, you need to turn off the power. First, you can turn off the breaker, or in the case where you are testing the wiring which is placed in equipment, you can unplug it now you need to find the connection point of the wire or the cable you are trying to test.
- In order to gain access to the exposed conductors, you need to clamp the alligator clip on the megger probes present to the wire or cable.
- Then touch the other probe to the expose conductor and push the start button.
- By doing so there will be a generation of current between the probes by the megger and the megger will start recording a resistance of the jacket to the passing of current.
- You need to keep the contact for 30 to 60 seconds for getting an adequate reading.
- You need to start looking for the resistance reading which is present between 35 and a hundred ohms. The insulation resistance which is acceptable for Electrical cable is 1 ohm per thousand volts of electricity.
in order for tracking insulation performance over time, you can take readings from time to time and then place them on a graph if there is a downward line of a curve it will show the declining and slope of the curve will give you the idea of how long the insulation is supposed to remain
What Is Megger?
Megger is a company that is leading in Electrical tests and measurement and has been in existence for the past 130 years. it covers many of the products from the electrical supply industry their products are categorized into 7 core application segments which are given as under
- Cable test and diagnostics.
- Protection relays and systems.
- Circuit breakers.
- Transformer test and diagnostics.
- Low voltage installations.
- General electrical testing.
- Motor and generator testing.
Megger has its manufacturing sites in many locations which are present in Germany USA UK so they have their headquarters which is based and in the UK and have their sales offices around the world.
How To Use Insulation Tester
Megger Insulation tester is quite useful when you are doing some electrical work at your home and to ensure that you didn't have any insulation disorder ordered a mistake while wiring. If your installation is damaged it can cause a circuit breaker to trip or blow off. It can also result in electrical shocks for the people who come in contact with the poorly insulated wire or touch it. The use of megger test insulation can cause such dangerous to prevent from happening. A detailed answer is given on How To Use Insulation Tester.
the first thing you can do it to switch off the circuits you are trying to test and if you are really confused about the fuses or Breakers of the circuits you can switch the main breaker if you need to make sure that the breaker panel doesn't get switched on as you are working on the circuits. You need to keep the switch closed if it is possible for you.
Now you need to set a table through which you can keep a track of the insulation values that you have evaluated from your tests. If there are the 110-volt circuits you need to leave spaces to record the test results in between the black wire and white wire and the insulation between each wire and ground. If you are working on 220-volt circuits you need to leave spaces for the three wires and the other two and between each other three wires and ground. If you are working on equipment which could be a circuit breaker or any other appliance you need to leave space for the insulation values between the east terminal and also each terminal and ground.
Now you need to keep the positive and negative probes of the megger on the conductors or terminals which you are trying to test the insulation resistance and if you are conducting test insulation resistance to the ground you need to place the positive probe on the ground wire and the negative probe on the conductor or terminal. Know you need to empower megger for 1 minute then record the value of the resistance at the end of the minute and mark the value in your table then continue with testing procedures until or unless you acquire all of the values for the spaces of your table.
Now we need to evaluate the values in your table according to the national electrical code values that are noted must not be over 25 Megohm and if one of the value is different from all others you need to check your connections and then repeat the test and if the value lies below 25 megohms you need to evaluate your circuit for the cause of bad insulation resistance value.
Megger Clamp Meter
Test equipment depot defines the features of Megger Clamp Meter Model DCM340-EN
- Measures AC and DC current up to 600 A
- Carries out AC and DC voltage measurements up to 600 V
- Measures resistance up to 400 ohms and frequency up to 400 Hz
- Performs continuity checks
- 3 1/2 digit, 4000-count LCD display with backlight
- High-resolution digital bar graph for viewing trends
- Peak, min/max and data-hold functions
- Zero-offset relative mode (REL) enables comparative measurements
- Safety rated to IEC 61010-1 Cat III 600 V
- Installing, maintaining, fault-finding, or monitoring electrical systems and equipment
- Motors and heaters
- Domestic power generation from solar panels and wind turbines
- Battery monitoring
- Electric vehicle servicing such as fork-lift trucks
- Elevator maintenance
- UPS commissioning, servicing, and maintenance
- Electro-plating plants and welding equipment servicing
Cable Megger Test Values
Cable megger test values have been defined by the electrical engineering portal in the following table:
The Resistance of Transformer Coil Insulation
|Transformer Coil Voltage
|Min.IR Value Liquid Filled T/C
|Min.IR Value Dry Type T/C
|0 – 600 V
|600 V To 5KV
|5KV To 15KV
|15KV To 69KV