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 that 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 mmX400 mmX510 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 mm>2 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.