There is a variety of rules and regulations regarding the electrical circuitry in, on and around a ship. While the requirements of the NFPA 70E are not used to govern the electrical connections on a vessel many of the safety requirements found in this guide for electrical safety can be applied to the electrical awareness onboard a vessel with 500 gross tonnage or greater.
When discussing shipboard electrical matters it is important to understand there is one major difference about shipboard electrical priorities as opposed to land based electrical priorities. While both are designed to provide the greatest amount of safety for human life possible, the priority of the land based electrical circuit design is to isolate the possibility of electrical shock or electrocution to humans as quickly as possible. This priority means that in a land based circuit the neutral is tied to ground so as to isolate any fault from the human or machine nearby. In ships and commercial facilities were are not talking about your typical house current or voltages we are talking in the 13.KV range and higher for most large ships. (Hezlet, A 1975)
While this is great on land the electrical objective and priority is a bit different. On the open sea it is essential that the machinery and equipment continue to work at all times. And since the vessel is actually sitting in water the concept of an earth ground is moot. Since a ship needs to keep all of the machinery and safety and operational features of the ship functioning to avoid a collision or an accidental grounding then the electrical system on the ship uses what is called an “insulated neutral” system. (Patel M.R. 2012), In a standard three phase electrical configuration this neutral configuration allows for continuous powers to the systems electrical grid even in the event of an accidental single earth or grounding fault occurs. This electrical distribution system ensures that functions such as navigation and fire control are always operational with power.
We are all familiar with how in our house if the insulation on a wire comes in contact with a piece of metal causing a “short circuit” just as the sparks start to fly the circuit breaker in the house or the protective relay in the piece of equipment trips and isolates the equipment and the person from any further current. If this type of situation were to occur on a ship and say the steering control equipment was deactivated and isolated the moving ship could run into another vessel or the ground (shore) and either capsize or do un-repairable damage to the hull of the ship. Now on a ship which uses the “insulated neutral distribution system” this cannot happen because it is designed to not isolate power to systems should a single phase ground fault occur. Theoretically, this can present more danger to the human elements on the ship however when a fault does involve shorting to the ship’s hull the hulls are designed in such a way that it will actually act like a capacitor and absorb much of the current until it, the hull, reaches the same electrical potential as the source of the current which will cause the current to stop flowing. Functionally and most critically, this process allows the equipment and machinery to still operate, and key systems to keep functioning. Especially in the military theater where you can’t afford to have you guns or missile fire controls shut down because of a simple short circuit or ground fault this system is the best for all parties concerned. (McCoy 2012)
Because of this facet of the ships electrical system it is essential that insulation resistance testing be done on a regular basis on the transformers, relays and circuit breakers and motor’s that are operating on the ship. These insulation resistance tests will allow for proper maintenance before a situation occurs and assure that all of the equipment has the recommended level of insulation to be operated in a safe manner. Just as a matter of record, the ship is not without protection because if there are two faults which would then effectively involve the ship’s hull then the breakers, protective relays and fuses involving that circuitry would operate and thus isolate the equipment and associated human involved. This system is just designed to not let a “single” fault on a single line trip the breakers and this is why it is used in the maritime world.
- Patel. M. R. (2012), “Shipboard Electrical Power Systems” ISBN: 1439828164
- US Government (1976),NAVAL EDUCATION AND TRAINING SUPPORT COMMAND 76
- Naval Ships Technical Manual (NSTM) Chapter 300, Electric Plant General Contains electrical safety requirements for the U.S. Navy Training Retrieved 10-5-14 http://www.public.navy.mil/navsafecen/Documents/afloat/Surface/Elect
- Society of Naval Architects and Marine Engineers (2013) Retrieved 10-6-14 http://www.sname.org/Membership1/AboutSNAME
- Dr. Timothy J. McCoy and Dr. John V. Amy Jr.,(2012) “The State-of-the-Art of Integrated Electric Power and Propulsion Systems and Technologies on Ships”. American Society of Naval Engineers. Retrieved 10-5-14. https://www.navalengineers.org/SiteCollectionDocuments/2009%20Proceedings%20Documents/ESDS%202009/Papers/McCoy_Amy.pdf
- Hezlet, A (1975) Vice-Admiral Sir Arthur, “Electronics and Sea Power” (New York: Stein and Day)
- DNV.GL (Online Article “Marine Electrical Systems”, Retrieved 10-5-14