Are designers missing the bus?
Prescriptive regulations currently prohibit installation of power bus distribution systems onboard ships, but a risk-based approach to ship design offers the prospect of change.
has become commonplace onshore, but doing the same onboard ship challenges existing regulations. One project within the risk-based
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‘Safedor’ project recently completed under management of Germanischer Lloyd envisaged how a power bus distribution system might look if installed on a ro-pax vessel. Part of the thinking underlying Safedor
is emerging at the International Maritime Organization as part of proposals submitted by the Danish Administration to the Maritime Safety Committee. In short, the Danish proposal looks to develop a ‘preliminary approval’ for a new aspect of ship design, based on equivalence to an existing safety regulation, where the ‘approval authority’ confirms that the design or system will – by fulfilling certain conditions – be deemed feasible for full scale approval at a later stage of development. A conventional power distribution
system features a ‘radial structure’, where the main switchboard is close to the machinery space. Here, the switchboard is split into two independent sections, with monitoring and control via a main switchboard or/and automation. Consumers are directly linked to the main switchboard or via sub-distribution boards, with connections made via cables from the main switchboard. Starter, switches and consumers mostly reach IP22 or IP44 standard, in being protected against contact and water drips/splash. In contrast, a primary power bus
system adopts a ‘tree structure’, where the distribution grid is equivalent to the switchboard. Featuring a backbone and branches, consumers are monitored and controlled via the bus system and are directly linked to the bus bars. Cable connections are only required for the ‘last metres’. Again, start, switches and consumers mostly reach IP22 or IP44 standard.
The Naval Architect July/August 2009 Conventional power distribution system versus power bus distribution. Basic requirements under SOLAS
demand that electrical installations onboard ship must observe the following criteria:
• All electrical auxiliary services necessary for maintaining the ship
• Electrical services essential for safety will be ensured under various
in normal operational and habitable conditions will be ensured without recourse to emergency source of electrical power
• The safety of passengers, crew and ship from electrical hazards will be
emergency conditions
ensured. As far as the main switchboard
is concerned, this means that there must be indicat ion and alarm circuits supplied from the emergency switchboard, while each steering gear needs to be served by at least two exclusive circuits fed directly from the main switchboard. For the emergency switchboard, cables connecting to fire pumps must be fire resistant where they pass through high fire risk areas. The electrical installation must also feature IP56 protection in wet spaces. The study related to applying bus
bar power distribution within Safedor was undertaken using input from SAM Electronics, which suggested that such an approach was very promising. According to SAM, its ‘fault tree analysis’
showed “the tendency of the PPB to have higher reliability with respect to normal operation”. SAM said that connections between single
bus bars may have an increased impact on reliability if long PPB systems were installed. However, it also said that the generic PPB design contained redundancies for some consumers (steering gear, bow thrusters) leading to a higher supply reliability. Tese redundancies were not considered in the reference design for a ro-pax vessel used in the study. Furthermore, SAM said that the distribution of consumers in the PPB approach was “more homogenous”. Tis is expected to be advantageous in cases of failures of the distribution grid, as well as of single consumers. Before designers rewrite the rule book,
however, “the results of the analysis for the systems in accidental conditions demonstrate that a further elaboration of the design is required before a final evaluation is possible”. In applying the risk-based approval
process to the primary power bus, the Safedor project concluded: “The quantative investigations are
focused on the reliability of power supply for a representative selection of consumers in normal operation (sea mode, manoeuvring) and in accidental conditions For accidental boundary conditions no
satisfactory evaluation is possible for the present stage of development.” NA
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eplacing cumbersome and space-hungry cabling with bus bar technology for power distribution
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