Instruments & Electrical Measurements Lecture 4

History of Megger The device is being used since 1889, popularity raised during 1920s, since long back devise is same in its uses and purpose of testing, few real improvement appeared in recent years with its design and quality of tester. Now high quality option are available which are easy to use and quite safe.

What is Megger? Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture and dust particles. It also get impacted negatively due to the presence of electrical and mechanical stress, so it’s become very necessary to check the IR (Insulation resistance) of equipment at a constant regular interval to avoid any measure fatal or electrical shock.

Insulation Resistance – Importance And Causes of Decrease  Insulation resistance (I.R) is a critical parameter as it’s directly related to personal safety, the safety of machinery and power reliability. The I.R value of electric devise changes with ageing, mechanical and electrical stresses, temperature, contamination, atmosphere, humidity etc. It is therefore important for the engineers and electrical officers to identify this detection to avoid any accidents on board ships because of electric shock.

Another common cause of the decrease in the value of insulation resistance is the ingress of water. If the electrical equipment is wet due to freshwater, it can directly be dried out for checking the IR values. However, in the case of seawater ingress, the first step is to wash it with fresh water to remove the salt deposits which will cause corrosion of metal parts and insulating surface. Remove oil and grease from such equipment using a suitable solvent.

Uses of Megger The device enable us to measure electrical leakage in wire, results are very reliable as we shall be passing electric current through device while we are testing. The equipment basically use for verifying the electrical insulation level of any device such as motor, cable, generator winding, etc. This is a very poplar test being carried out since very long back. Not necessary it shows us exact area of electrical puncture but shows the amount of leakage current and level of moisture within electrical equipment/winding/system.

Types of Megger This can be separated into mainly two categories:- 1. Electronic Type (Battery Operated) 2. Manual Type (Hand Operated) But there are another types of megger which is motor operated type which does not use battery to produce voltage it requires external source to rotate a electrical motor which in turn rotates the generator of the megger.

Electronic Type Megger Important parts:- 1. Digital Display :- A digital display to show IR value in digital form. 2. Wire Leads :- Two nos of wire leads for connecting megger with electrical external system to be tested. 3. Selection Switches :- Switches use to select electrical parameters ranges. 4. Indicators :- To indicates various parameters status i.e. On-Off. For Example Power, hold, Warning, etc. Note: - Above construction is not similar for every megger, it difference appears manufacture to manufacture but basic construction and operation are same for all.

Advantages of Electronic Type Megger  Level of accuracy is very high.  IR value is digital type, easy to read.  One person can operate very easily.  Works perfectly even at very congested space.  Very handy and safe to use. Disadvantages of Electronic Type Megger  Require an external source of energy to energies i.e. Dry cell.  Costlier in market.

Hand Operated Megger Important parts:- Analog display:- Analog display provided on front face of tester for IR value recording. Hand Crank:- Hand crank used to rotate helps to achieve desired RPM required generate voltage which runs through electrical system. Wire Leads:- Used same as in electronic tester i.e. For connecting tester with electrical system.

Advantages of Hand Operated Megger 1. Still keeps important in such high-tech world as it’s an oldest method for IR value determination. 2. No external source required to operate. 3. Cheaper available in market.

Disadvantages of Hand Operated Megger 1. At least 2 person required to operate i.e. one for rotation of crank other to connect megger with electrical system to be tested. 2. Accuracy is not up to the level as it’s varies with rotation of crank. 3. Require very stable placement for operation which is a little hard to find at working sites. 4. Unstable placement of tester may impact the result of tester. 5. Provides an analog display result. 6. Require very high care and safety during use of the same.

What Things to Record After a Megger Test? When performing a megger test on machinery or equipment, following things to be recorded: Name and location of the equipment/ wiring Date on which the test is performed The Insulation Resistance values of test results along with time Range, voltage, and serial number of the Megger instrument used The temperature of the apparatus during the time of IR test When doing IR test of bigger machines such as alternator, transformer etc. wet and dry bulb temperatures and dew-point determinations to be noted Insulation resistance measurement corrected for temperature

Always remember to disconnect the machinery and equipment being tested for insulation resistance as there is a possibility of voltages being induced in apparatus under test or lines to which it is connected (because of proximity to energized high voltage equipment). Use required PPEs such as rubber gloves etc. when connecting the wire leads to test the apparatus for performing the insulation resistance test. Some megger may be provided with a voltage scale to ensure the line to be tested does not have any voltage for insulation testing.

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