QUE-ANS: 10-30 words

[1]        What happens to insulation at high temperatures?

Insulation at high temperature starts to break down, looses it’s properties of insulation and becomes burnt out

[2]        What is the Normal Reading on an Insulation Meter on MSB?

Infinity is a normal reading, but readings should be kept above 5 Meg-ohms.

[3]        What would you consider a minimum Insulation Reading Resistance?

Insulation Resistance must be kept above at least 1MΩ, the higher the Insulation Resistance the better.  Companies have their own Regulations, but most Companies only allow readings above 5 MOhms.

[4]        What is understood by the term ‘insulation resistance’?

Insulation resistance is a measure of opposition offered to the current by the insulating materials

[5]         What is ‘Megger’?

The instrument used for measuring insulation resistance in ‘mega ohms’

[6]        What is understood by the term ‘permissible temperature rise’ in case of insulation?

Permissible temperature rise is the difference between the ambient temperature and maximum permitted temperature for the class of insulation in use.

[7]        What is the reason for using insulated neutral system in ships?

The priority requirement on board ship is to maintain continuity of the electrical supply to Equipment in the event of a single earth fault occurring.

[8]        How do you class the electrical insulation?

By the temperature stability of the manufacturing material of the insulation. (like class a or class b etc)

  30-60 words

[1]        What is the relationship between the insulation and the temperature?

The insulation resistance falls with temperature rise. Life of insulation will be halved by every 10 deg centigrade above the permitted temperature. For example a machine designed for continuous Operation at 70 deg C will have its useful life cut in half when operating at 80 deg C.

[2]        Why The insulation tests are carried out on electrical equipments?

A measurement of the insulation resistance gives the best guide to the state of health of the Electrical equipment. The marine environment is particularly arduous for electrical equipment, Due to damp salt laden atmospheres, extreme temperatures and vibrations. The continuous Operation of equipment on board ship demands high operating efficiency and optimum economy In order to keep down costs.

[3]   What happens with one earth fault in an earthed distribution system?

With one earth fault in an earthed distribution system, it would be an equivalent to a short circuit fault across the load via the ship’s hull. The resulting large earth fault causes blowing the fuses in the line conductors.

[4]        Describe three insulating materials suitable for permanent electric wiring.

Three insulating materials suitable for permanent electric wiring are;

PVC; which is a very good insulator and commonly used.

Mica; this is another good insulator and is able to with stand high temperatures.

Rubber; which is another good insulator, it can with stand moderate temperatures.

Silicon rubber; this is also a very good insulator and can with stand high temperatures.

[5]        How would you go about finding an Earth Fault in the System?

Finding an Earth Fault would be by the process of elimination i.e. circuit breakers would be opened and closed until earth fault disappeared, taking care which breakers were being opened and closed as they maybe supply essential loads at the time.  It would be good practice to start with places such as the Galley and Laundry where faults are common.

[6]        Why do you require Earth Lamps on 220V Distribution Board when you have them on the Main Switchboard?

You require Earth Lamps on the 220V Distribution Board as well as the Main Board, due to the air gap in step down transformer, i.e. 440V Earth Lamps can not detect Earth on 220V systems due to the gap.

[7]        What is the most preferred condition for measuring insulation resistance?

Insulation resistance becomes leakier at high temperatures. So insulation test should be done while the machine is hot just after it has stopped. This will give the realistic reading at near working temperature.

[8]        What should be done if the windings resistance is less than 0.5MΩ?

If winding resistance is less than 0.5 M ohms, they should be given a thorough cleaning and dried out, if the value has recovered to a reasonable value which has become steady during the drying out period, its windings should then be coated with high quality air drying insulating varnish. Should the IR values remain low, this could mean the machine needs to be rewound.

[9]        What is the sign of an earth fault using earth lamps?

The sign of an earth fault using earth lamps is as follows; if the system is healthy (no earth faults); the earth lamps all glow at equal half brilliance. If an earth fault occurs on one line, the lamp connected to that line is dim or extinguished and the other two glow.

[10]      What consideration is used to choose the value of an earthing resistor?

In choosing an earthing resistor, consideration has to be taken; the ohmic value of each earthing resistor is usually chosen so as to limit the maximum earth fault current to not more than the generator full load current.

[11]      What should switchgear insulator be checked for?

A switch gear insulator should be checked for signs of tracking and blistering in the vicinity of exposed live metal. If the material is of the bonded laminated type, the laminate should be checked for signs of spills along it

[12]      What indicators does an earth fault instrument give?

See sketch of earth lamps;

The indicators of an earth fault instrument gives is a visual and audible indication in the event of an earth fault. If healthy, insulation resistance is high and greater than 1 M ohm.

  60-100 words

[1]        How the insulation resistance of alternator is checked?

Disconnect any electronic circuit components which may be damaged by a 500v insulation test. Consult the wiring diagrams and manufacturer’s instructions before testing. Measure the Insulation of the stator winding and the rotor windings to earth, and between stator phases. A minimurn value is 1 Mohm, but a lower value may be acceptable to surveyor based on 1Kohm/volt. E.g. 450 Kohms for a 450 volt generator.

[2]        How the insulation of pedestal bearing checked?

1. The pedestal insulation of a dismantled generator may be tested with a megger. Insulation of 20 Kohms and above is considered satisfactory
2. Condition of insulation can also be checked while machine is running by measuring mllivolts between the shaft and the bedplate. Two readings are taken, one with the jumper connecting shaft to pedestal and one without the jumper. If insulation is good, both readings will be alike. If the insulation is defective, the reading in (1) will be higher than in (2).

[3]        How the earth faults are treated?

Earth faults should be eliminated when located. Damaged conductor insulation must be repaired. The method of repairing depends on the cause of earth fault and this is determined by visual Examination. A lamp fitting that is damaged must be replaced. Dampness or moisture in Insulation must be dried out by gentle heat and then precautions taken to prevent future ingress of moisture. Insulation that has been mechanically damaged or damaged by overheating must be made good again. If surface dirt is the cause, a thorough cleaning with electro cleaner solvent will probably cure the fault.

If insulation is faulty, insulation resistance is low and less than 5M ohms.

  100-150 WORDS

[1]        What are the common locations of earth faults?

Earth faults are found at lamp fittings and terminals etc., where moisture has found its way in’

1. Lamp fittings on open deck, exposed to rough weather are prone to ingress of moisture.
2. In accommodation, washing machines drenched with water area another source of earthly faults.
3. In galley, the ovens, hot plates etc., are also exposed to earth faults.
4. Dripping water over electrical machinery.
5. Motors standing idle for long time – their insulation get affected by condensed moisture.
6. Overheated insulations of motor and cables cause drop in insulation resistance.
7. Dirty electrical apparatus with surface tracking (leaking current)
8. Old electrical apparatus whose insulation has aged has become brittle.
9. Terminal wire which has become loose from the terminal and is touching the metal casing of electrical machinery.

 

[2]        What are the factors that affect insulation resistance?

Insulation is affected by many factors such as:

1. Moisture – results in a decrease in the measured value of insulation resistance.
2. Dust and dirt deposits – reduces insulation resistance and failure of insulation due to oxidation.
3. Oil and grease – prevents heat dissipation and cause dust and dirt to settle on the insulation, Thereby reduces the insulation resistance and leads to failure.

4, Ageing – over a period of time the insulation deteriorates due to it getting affected by Temperature variation, mechanical stresses, vibrations, moisture, dirt deposits, chemical Contacts etc some varnishes tend to become hard with age and crack during the operation.

5. Temperature – excessive temperature dehydrates and oxidizes the insulation, making it Brittle and disintegrate under vibration and shock.