The most important factor for the staff working onboard is personal safety and the safety features incorporated in machinery and systems. Electrical components and machinery systems present in the engine room are maintained by the electrical officer. The most important routine maintenance for electrical machinery involves checking of insulation resistance which is done by an instrument called “Megger” or “ohmmeter”.
Insulation resistance (I.R) is a critical parameter as it’s directly related to personal safety, safety of machinery and power reliability. The I.R value of an electric devise changes with aging, mechanical and electrical stresses, temperature, contamination, atmosphere, humidity etc. It is therefore important for the engineers to identify this detection to avoid any accidents on board due to electric shock.
Megger or Ohmmeter
Megger is a portable instrument which is used to measure insulation resistance of the electrical machinery or system. It can be battery operated or mechanically operated (hand crank dc generator) and gives direct reading in ohms. For this reason it is also called as ohm meter. Onboard ship, different systems are present with large voltage ratings and therefore Megger comes in the range of 100V to 5000V.
The important construction features of Megger consist of following parts:
- Control and Deflecting coil: They are normally mounted at right angle to each other and connected parallel to the generator. The polarities are such that the torque produced by them is in opposite direction.
- Permanent Magnet: Permanent magnet with north and south poles to produce magnetic effect for deflection of pointer.
- Pointer and scale: A pointer is attached to the coils and end of the pointer floats on a scale which is in the range from “zero” to “infinity”. The unit for this is “ohms”.
- D.C generator or battery connection: Testing voltage is supplied by hand operated D.C generator for manual operated Megger and a battery and electronic voltage charger for automatic type Megger.
- Pressure coil and current coil: Provided for preventing damage to the instrument in case of low external source resistance.
The voltage for testing is supplied by a hand generator incorporated in the instrument or by battery or electronic voltage charger. It is usually 250V or 500V and is smaller in size.
- A test volt of 500V D.C is suitable for testing ship’s equipment operating at 440V A.C. Test voltage of 1000V to 5000V is used onboard for high voltage system onboard.
- The current carrying coil (deflecting coil) is connected in series and carries the current taken by the circuit under test. The pressure coil (control coil) is connected across the circuit.
- Current limiting resistor – CCR and PCR are connected in series with pressure and current coil to prevent damage in case of low resistance in external source.
- In hand generator, the armature is moving in the field of permanent magnet or vice versa, to generate a test voltage by electromagnetic induction effect.
- With an increase of potential voltage across the external circuit, the deflection of the pointer increases; and with an increase of current, the deflection of pointer decrease so the resultant torque on the movement is directly proportional to the potential difference and inversely proportional to the resistance.
- When the external circuit is open, torque due to voltage coil will be maximum and the pointer will read “infinity”. When there is short circuit the pointer will read “0”.
Reference: Book By B.L Thareja