Requirements with rationale and quantitative rules for EMC on future ships

Abstract

To ensure the performance on equipment and subsystem level in a naval environment, the conventional approach has been to strictly require national and international military standards on equipment and installations. This approach made Electromagnetic Compatibility (EMC) a cost driver in naval shipbuilding. Many standards lack a clear rationale and scientific reference, appear to be written for old technology and contain more qualitative than quantitative rules. An alternative is a risk based approach, that replaces the strict and extensive acceptance procedures that come with the military standards. Technological developments and diminishing funding dictate the use of Commercial off the Shelf (COTS) equipment below deck. Civil development produces reliable new technology in large series at a high pace, that is uncommon in the defence industry. The short economic life cycle of COTS equipment forbids electromagnetic hardening of individual equipment that is not designed for a military environment. Building adequate electromagnetic environments for this equipment is the only affordable alternative. A protected environment below deck is created by zoning and taking the appropriate protective measures. These measures are based on best practices, which are established techniques. The research in this thesis has aimed at requirements with clear rationale, put in todays perspective, and installation guidelines with quantitative rules. Crosstalk between cables is one of the oldest types of interference. Cable separation rules have been in use for over five decades and were derived in an era where equipment did not meet legal or contractual requirements, where signals in the cables where analogue and knowledge on EMC was still in development. Different equipment that is designed for the intended use in the same environment, e.g. residential or office use, will be compatible and therefore the risk of crosstalk between cables from these equipment is low. Calculations and measurements have shown that commonly used high quality cables can be put close together for most of the systems on a ship, provided that these cables are properly installed. EMC is achieved by the decoupling of Common Mode (CM) current loops at the inside and outside of current boundaries. This can even be realised by cable terminations instead of shielding walls, to create a barrier for these currents. A numerical analysis including measurements of the magnetic decoupling between these loops has shown the importance of a low bonding resistance. Systems and cables on naval ships act as antennas and are susceptible to the external electromagnetic environment, which may cause Electromagnetic Interference (EMI). Signals, radiated from above deck cables, may also be of a concern for a possible increase of the noise floor of on-board receivers, as well as leaking information or detection by third parties. A quantitative investigation of the susceptibility of exposed cables has shown that risks can be kept low by a small exposure length or by placing cables close to a ground plane. All equipment on the market today is strictly limited in unintentional radiated emission to prevent the interference to radio reception in general. Specific maritime requirements prohibit the use of COTS equipment to insure the availability of the maritime VHF radio to make a distress call. An analysis of the limit setting rationale has led to a practical approach to avoid interference.