Showing posts with label drying. Show all posts
Showing posts with label drying. Show all posts


Measuring insulation resistance on ships. Factors affecting Insulation Resistance

A part from the fact that good insulation resistance is an essential condition for maintaining service, the regular recording of insulation resistance values is undoubtedly the best method of detecting deterioration and of indicating when remedial action is desirable, or perhaps essential, in order to prevent complete failure.

Measuring insulation resistance on ships

Insulation resistance (IR) should accordingly be measured and recorded at regular intervals, the recording being preferably on a separate log-sheet for each important machine or circuit, so that each fresh reading can be compared with previous values and any downward trend immediately observed. It cannot be too strongly emphasised that, subject of course to reasonable minimum values being maintained, trends are more important than actual values; a single value is of comparatively little significance. The intelligent interpretation of results is therefore of great importance and a general appreciation of the factors involved is essential.


An in-Situ Method of Drying out Electrical Machinery

It is interesting to note that a quiz for junior engineers asked the question: "What is the reason for the most frequent cause of breakdowns in electrical equipment?" The answer was: "The failure of insulation due to salt and moisture ingress".

The ingress of moisture to electrical equipment at certain locations on board ship is almost inevitable.

It follows, therefore, that any piece of electrical repair equipment which can, in the first instance, be used preventively, that is to ensure t.hat motors and cables are kept free of moisture, and, in the second role, can repair equipment which has been subjected to total flooding, should be a wise investment.

Preventive maintenance. The special equipment is designed to resist dampness in electrical equipment by keeping the installation at a temperature a few degrees above its surroundings. This means that any motor present will be removed without damaging the equipment.

The 2 kVA, 400 mm x 400 mm x 510 mm transformer is normally placed at the main switchboard. It has 8 secondary voltage tappings from 2 to 60 V and current varies from 33 to 220 A. A rectifier supplies d-c for the megger test.

An outlet terminal board has connection for cables from 1.5 to 10 mm2 and a common outlet for larger cables. All these are thermostatically controlled to prevent overheating of insulation. Meters indicate current, resistance and temperature.

For routine checking, resistances in the cold conditions are compared with the reading when a machine is heated up.

The conventional method of testing by a "megger" is done when the motor is at the same temperature as its surroundings. The megger reading may be well within the prescribed parameters for the motor to be started.

This, however, could lead to motor failure as very often considerable difference between resistance at working temperatures compared with the resistance of idle machinery at ambient temperature.

Damage Repair. However much preventive care is taken, accidents will occur, such as flooding, and the machinery becomes totally inoperative.

In such cases it is now almost always possible to repair the damaged motors or cables in-situ.

As a case in point, the Norwegian rig had two of its main pillars flooded with seawater. As a result, there was extensive damage to the steering motors, ballast and seawater pumps, and all associated cables. The pillars were cleared of seawater and all machinery, including cable ends, was washed down with fresh water.

In the initial stages of repair, two such special machines were used to treat the cables and motors for the ballast and seawater pumps. After 24 h, the first seawater pump was dried out and back in full operation and it was decided to increase the number of these machines to five. After 23 days, all ballast and seawater pumps had been satisfactorily treated, as well as over 100 cable courses with diameters ranging from 1.5 mm2 to 240 mm2.

The alternative method of repair would of course, have meant removing all motors ashore and no doubt the re-installation of some cabling.