Many ships, particularly small tugs and supply boats have used a neutral grounded distribution system for 120 volt single phase hotel and domestic loads. This system is identical to that used in most North American 120 volt land based installations. As the major benefit of using a grounded single phase system was generally the cost saving in circuit breakers and panels, compliance with IEEE 45 1998 begs the question “why ground?”. Ungrounded installations offer other benefits and perhaps the area of marine system grounding should once again be reviewed.
IEEE 45 1998 clause 13.1 introduced the requirement that “Circuit breakers in grounded neutral distribution panels should include a pole or switch for the neutral”. This requirement therefore removes the possibility of using single pole circuit breakers for the 120 volt services. When two pole circuit breakers are used, the standard single phase 3 wire distribution panel cannot be used without modification and three phase panels may be required configured for single phase loads.
The major factor which determines the grounding method is safety – primarily safety of people and secondly safety of equipment. On marine installations, safety of equipment may also contribute to safety of personnel particularly if loss of vital equipment places the ship in danger.The majority of faults on a marine system are ground faults. A ground fault occurs when the insulation between live conductors and the “ground” whatever that is defined to be, no longer maintains an effective resistance between the live conductor and ground. On a “grounded” system, the fault will be removed by the automatic opening of the circuit breaker. On an “ungrounded” system, the fault is detected but not removed automatically. This enables the service to be retained until such time as it may be shutdown safely. For vital services this is obviously advantageous.