EMC planning

07.09.2021

EMC planning

The following describes how to build up an EMC plan for a cus­toms patrol vessel, with a complete nautical and navigation package as well as a hydraulically driven bow thruster. 

1.  General project information

This ship is a modern high speed patrol and rescue vessel with a semi-displacement hull for coastal and offshore services. The patrol craft is built with a steel hull and an aluminium superstructure. 

The propulsion system consists of two electronically controlled com­mon-rail diesel engines driving controleble-pitch propellers. The electric installation 400/230V 50Hz 3 phase 4-wire neutral earthed is powered by two elec­tronically controlled common-rail diesel generator sets. The bowthruster is hydraulically driven. All engines are electrically started from batteries. Emergency power is also from batteries.

2.  Definition of EMC

Electromagnetic Compatibility (EMC) is the ability of equipment and/or combinations of equipment to function properly together as well as within the ship's environ­ment. Type-tested electronic and electric equipment is tested by a certified testing laboratory in order to be certain that it will function properly in the expected ship's environment.

Requirements for type-tests can be found on the web sites of the clas­sification societies as well as Inter­national standard IEC 600945 and IEC 600533. Parts of these tests are related to EMC and are also related to the disturbance of low level emergency transmission signals such as VHF signals in the 156-165 MHz range. For more extensive definitions of EMC see IEC 533 electromagnetic compatibility onboard ships.

Power and control cables in a double floor

3. General arrangement plan

This plan is used to achieve the first impression how to start with EMC. It helps to derive the guidelines and recommendations for technical measures to achieve electromag­netic compatibility in ships and of ships' equipment. These preventive measures con­cern electric and electronic equip­ment and in special cases, non electric equipment. The following general measures are applicable to EMC: 

a. Decoupling 
b. Reduction of the interference level at its source
c. Increase of the susceptibility level of the affected equipment or system.

 4.  Decoupling

Space is limited in ships, especially in small ships. The installation of equipment in an other space or at sufficient distance from each other to prevent interfer­ence, is difficult. To find the best compromise for the location of radio and navigation aerials, a listing of the aerials in se­quence of importance is made and then a suitable position is found. Aerials do interfere when fitted close to each other.

In order to ensure proper television reception, it is advisable to install the omni-directional television aeri­al above the everyday working VHF aerials.

5.  Reduction of interference level at its source

After having established the loca­tion of the different aerials, the ef­fect on the equipment onboard has to be determined.

Then the distance to the other equipment has to be considered and the measures defined.

The first source of interference is the outside environment, such as other ships or shore-based ship guidance systems.

All equipment located in the above deck zone must be suitable for an EMC environment according to IEC 801-3 frequency range 27 MHz-500 MHz field strength level 10 V/m.

Near ship's aerials these levels are far exceeded, for example:

Consequently the antenna plan must also be reviewed for the environmental impact to on­board signals.    

 6.  First source of interference

The environment is the first source of interference with signals originating from other ships and shore systems. This environment has been defined in standards. All type-approved equipment fulfills the standard and is suitable to op­erate in the ship's environment.

Outside the ship's structure the signals are stronger than inside the metal structure.

The environment can be divided into:

Due to the large window area, the wheelhouse is considered 'above deck zone'.

Cables running in the 'above deck zone' act as aerials and transport the signals into the 'below deck zone' and to other electronic equip­ment. To avoid this, all outside cables must either be run in galvanized steel pipes or be screened. This screen has to be earthed at both ends, preferably as close as possible to the location where the cable enters the steel structure.

 7. Second source of interference

The second source of interference is the system of cables within the steel and aluminium structure, transporting all sorts of signals through the ship. The type of signal transported through a cable determines what type of cable has to be used and the group to which the cable be­longs:

(This is the basic matrix linking sig­nals to measures. Every application has to be provided in detail.)

Group 1 - indifferent

Normal non-screened cable

 -  Power circuits
 -  Lighting circuits
 -  Control circuits
 -  Analogue and digital data sig­nals
 -  Approximate signal range:10 V - 1000 V DC  50-60 Hz 400 Hz
Group 2 - sensitive

Single-screened cable, additional twisted pairs

Group 3 - extreme sensitive

Coaxial cables

Coaxial cables screened power ca­bles

Approximate signal range:10V - 1000 V broadband signals

To keep the coupling between the cables small, all lengths must be as short as possible. In order to avoid interference be­tween the cables of the different groups these must not be run close together for longer lengths and a separation distance must be used. Also, the distance between the steel or aluminium deck or bulk­head must not exceed the figures from the table below. 

Example of separation distance in cm to be maintained between ca­bles of several groups.

	Maximum distance cable to cable (mm)			Max. distance from metal surface
GROUP	1	2	3	4
1	0	5	10	10
2	5	0	5	15
3	10	5	0	20
4	10	15	20	20

Cables terminating in one piece of equipment do not require separa­tion from each other.

Screened cables

Earthing

8.  Third source of interference

The third source of interference is the power supply system. Again, the following is the basic standard which must be detailed for the specified project.

The project power system sup­plies a three-phase four-wire neu­tral earthed system with two diesel driven generators. Neutrals are earthed in the genera­tors. The generator circuit breakers have four poles. All equipment is also adapted to the "mechanical" aspects of a ship's environment with respect to tem­perature, ship's movement and vi­bration.

This supply system is very similar to onshore industrial installations. Standard industrial frequency con­verters with standard filters limit the harmonic distortion to accept­able levels as defined below. All equipment must function cor­rectly when supplied from an AC power supply system with the fol­lowing characteristics:

 9.  Increase of the susceptibility levels

 Remote control and automation systems are often distributed sys­tems, with intelligent local units, with suitable filtering and limitation circuits, to allow non-screened ca­bles for digital input and output.

The data communication between the local units and the workstations must be performed with screened cables and routed separately from power cables.

Data communication has to be in­stalled using coaxial cables or the signal has to be amplified to such a level that the susceptibility levels exceed the interference levels from the power cables. In that case no separation is required.

This solution can also be used when, during Harbour Acceptance Tests (HAT) and Sea Acceptance Trials (SAT), unexpected interfer­ence is found 

 Дистанционное управление и системы автоматизации – зачастую распределенные системы, с «умными» локальными устройствами, снабженными фильтрующими и ограничивающими цепями, чтобы можно было использовать неэкранированные кабеля для цифрового входа и выхода.

Обмен данными между локальными устройствами и рабочими станциями должен осуществляться экранированными кабелями, проложенными отдельно от силовых кабелей. Обмен данными должен проходить по коаксиальным кабелям или же сигнал должен быть усилен до такого уровня, что порог чувствительности превышает уровень помех от силовых кабелей. В этом случае не требуется сепарация.

Это решение также можно использовать, когда во время приёмных испытаний в гавани (Harbor Acceptance Tests, HAT) и в море (Sea Acceptance Trials, SAT) обнаруживаются непредвиденные помехи.

Cables directly into the structure to reduce interference

10. Communication and navigation equipment:

                                              Warning signs to be positioned near the stairs to the top deck:Danger electromagnetic radi­ation

Conning position

Communication position (GMDSS)

Nautical position 

Cables for energy generation and energy conversion:

- Navigation lights: outside cables must be screened and run in pipes with open bends, exposed length limited to 20 cm per bend

- Whistle: outside cables run in pipes with open bends

- General alarm system: non-screened cables, no sepa­ration

- Main generators: non-screened cables, no separation

- 24 V DC systems: non-screened cables and no separation, with exception sup­ply circuits into the wheelhouse area if not installed inside a steel-clad console. These cables have to be screened, but no separation is necessary

- Starters: both for power and control cir­cuits non-screened cables and no separation

-  Lighting: cables to outside light­ing must be routed through gal­vanized steel pipes with open bends. The cable length ex­posed shall be limited to 20 cm per bend. Non-screened cables and no separation necessary. For wheelhouse area, screened cables and no separation

- Cables between frequency con­verters and motors must be screened, earthed at both ends, separated from other cables and to be considered as ex­treme jamming (group 4).

Switchgear and control sys­tems.

- Switchboards/motor control centers: both for power and control circuits non-screened cables and no separation.

-  Main lighting switchboard: non­screened cables and no separa­tion, with the exception of sup­ply circuits into the wheelhouse area, if not installed directly in­side a steel-clad console. These cables have to be screened, but no separation.

-  Emergency lighting switch­board: non-screened cables and no separation, with ex­ception supply circuits into the wheelhouse area, if not in­stalled inside a steel-clad con­sole. These cables have to be screened, but no separation.

-  Lighting distribution panels non­screened cables and no separa­tion, with the exception of sup­ply circuits into the wheelhouse area, if not installed inside a steel-clad console. These cables have to be screened, but no separation.

Signal processing equipment

- Fire detection systems screened cables, no separation

- The remote control and auto­mation system can be a dis­tributed system with intelligent local units with suitable filter­ing and limitation circuits. Non­screened cables for digital input and output is sufficient, but may be executed with screened ca­bles without separation. Ana­logue input must be executed with screened cables without separation. Data communica­tion between the local units and work stations must be execut­ed with screened cables routed separately from power cables or with coaxial cables

                                                 

Non-electric outfit

-  Rigging shall be earthed.

Integrated equipment

- Voyage management system: video signals coaxial, network coaxial cables

- Enclosures of equipment in e.g. wheelhouse consoles shall not be taken off or modified without permission of the manufacturer.

Equipment located in hazard­ous areas

- Cables for intrinsically safe cir­cuits must be screened and clearly- marked, for instance, by colours and separated from other cables

- Cables for power circuits in haz­ardous areas must be screened for earth fault detection.
 

11. Mast construction and cable routing

The masts of some ships are re­movable. Therefore, junction boxes are fitted for cables to the equip­ment in the mast. These junction boxes have to be watertight and have a metal-clad cover, preferably bolted and separately earthed. The mounting plate should be metal and separately earthed. The screen of the cables has to be coupled through isolated terminals.

All cables must be routed inside the mast and/or in steel or aluminium pipes with open bends to avoid in­terference from Radars and MF/HF aerials.

Cables of group 4 Transceiver ca­bles have to be routed separately from other cables as well as sepa­rate from each other. This can be achieved by introduc­ing mounting hatches and fastening strips in two legs of the mast, or in pipes. One pipe to be used for groups 1, 2 and 3 cables and the group 3 cables should be routed separate from 1 and 2 insofar as possible.

The other pipe must be used for the transceiver cables of group 4 and as these cannot be interrupted, there is no need for a junction box. Group 4 cables, however, must also be separated from each other. When this is not possible within the space limitations inside the mast, these cables must be provided with ad­ditional screening. This then allows these cables to be routed together. This screening, however, does not fit in the plugs for the equipment. A compromise is thus, to install the additional screen only where the ca­bles run parallel for longer lengths inside the mast and wheelhouse. The screening can then be taken off near the connections at the ends and the original connectors can be used.

12. Cable routing in general

In general, cable routing, trays, deck and bulkhead penetrations must allow for separation as de­fined before. When separation distances cannot be met, as in the case of a single pipe mast, alternative measures must be taken, such as the instal­lation of an extra screen around a cable. This increases the shielding of the cable and limits the radiation to the environment.

This is applicable to all group 4 cables in this project. Additional screening has to be provided for the longer lengths and screening over the shorter lengths has to be mini­mal.

                                                                                                           Cables on deck chemical tanker
                                                                                              1. Intrinsically safe cables
                                                                                              2. Control cables
                                                                                              3. Cable tray

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