Electromagnetic compatibility


Electromagnetic compatibility

The shortest definition of EMC is that this is the capability of an electric system to neither dis­turb or be disturbed via radia­tion or transferred through the connection cables. It also includes disturbance by signals in cables not connected to the disturbed unit but signals running through cables parallel to cables of the disturbed unit.

EMC management

Determining if an installation fulfills the EMC requirements is a compli­cated and time consuming exer­cise. It starts with listing the sensi­tive equipment and verifying their acceptance limits, followed by list­ing the disturbing equipment and testing their disturbance levels. A lot of this work is done by the suppliers under the type-approval schemes.

The publication IEC 60945 defines the susceptibility and disturbing criteria for navigation and nautical equipment. The figures in that publication present the normal environment which is to be expected on the open deck and inside the wheelhouse of a normal ship. Most navigation and nautical equip­ment has been tested to be able to cope with this environment. This is simple insofar as the envi­ronment is under our control.

However, also radio and radar signals from other ships or shore based traffic guidance systems in­fluence the ship's environment.

The maintenance and develop­ment of the IEC standards is a joint exercise of industry, equipment suppliers, shipown­ers, shipbuilders, classification societies and governments and also forms the basis for the rules and regulations of all clas­sification societies.

IEC TC18 standards are pub­lished by the International Elec­tro technical Commision, Gene­va, Switzerland, as IEC 60092 series and are available at the national standards institutes. Individual references are given in the respective paragraphs.

For detailed information and pro­cedures, reference is made to IEC 60533 Electromagnetic Compat­ibility for Electric Installations on­board Ships. Navigation and nau­tical equipment has been tested in accordance with IEC 60945 and therefore, suitable for the outside maritime environment.

EMC environment

Electromagnetic immunity means equipment is capable of operating satisfactorily under the following conditions:

-  Cond low frequency inter­ference 10% under AC supply voltage 50 Hz-900 Hz

- 10%-l% under 900 Hz-10 kHz - 10% under DC supply voltage 50 Hz-10 kHz

-  Conducted radio frequency in­terference under supply of 3V rms. 10 kHz-80 MHz

-  Radiated interference 10 V/m between 80 MHz-lGHz

-  Fast transients (bursts) 2kV dif­ferential on AC power ports, IkV common mode on signal and control ports

-  Slow transients, power supply variation, power supply failure, and electrostatic discharge (the phenomenon that happens when you touch a system in winter in dry conditions), with a static discharge voltage of more than 6000 Volts are also considered.

Electromagnetic compatibility

 Cable and pipe tunnel, with power cables situated below in the tunnel and the control cables, above 

Equipment should not transmit conducted or radiated signals that disturb the correct functioning of other equipment.

Normally the conducted emission is not a problem but the radiated emission limit between 156MHz and 165MHz of only 24 dBpV/m is only slightly above the environ­mental noise level of today. This is a frequency band associated with VHF emergency communica­tion. Equipment used on board ships should not radiate any signal in this frequency.

Also frequencies of processors in programmable logic computers and other electronic control systems have to be checked against the en­vironment and tested if any possi­bility of interference exists.

Conducted radio frequency inter­ference 3V rms. 10 kHz-80 MHz Radiated interference 10 V/m 80 MHz-lGHz. These figures are for open deck ar­eas and inside the wheelhouse

EMC measures

To limit the exposed systems, the following measures are implement­ed: Cables outside the steel structure of the ship have to be screened or installed in steel pipes. The most effective means is to limit the quan­tity of cable exposed to the outside environment by installing those in­side the mast or inside a structure, only exposing them to the outside when absolutely necessary.

This also prevents incoming inter­ference.

A cable located outside will act as a receiving aerial and a transmitting aerial inside the ship if not protect­ed. The actual aerials for radio and radar reception have been designed to cope with the environment. They should not become damaged by excessive signals such as light­ning or directional radar or track antennas' signals.

The rest of the disturbing signals come from the installation itself. Disturbing signals come from radar, radio and echo-sounder and sonar transmitters. Most suppliers advise how to install their equipment, what type of cable should be used and how it should be routed in relation to other cables and equipment. These instructions are based on the equipment in their tested housing; therefore, no equipment should be dismantled to fit into a console.

Cables must be selected and routed according to the type and strength of signal they transport. Therefore, suppliers of the equip­ment have to state what signal group their cables belong to. 

Single-core cables with a current exceeding 200 Ampere per core must be routed in a three-phase triangular formation to eliminate the magnetic fields around the sin­gle cables. These magnetic fields cause distur­bance to all visual display units and cause eddy currents to flow in mag­netic materials like ordinary steel which as a result may heat up. Therefore, gland plates for single­core cables must be of a non-mag- netic material, like stainless steel.

Electromagnetic compatibility

A wheelhouse console is a collec­tion of all type and make of equip­ment. 
Most of those are tested for EMC. This equipment shall be installed in the original housing as it was test­ed to be sure the required compat­ibility is maintained.
Also earthing and type of cables shall be as used during the tests 

Electromagnetic compatibility

1. Single core cables
2. Multi Cable Transit (MCT)
3. Bulkhead
4. Dec

Electromagnetic compatibility

In this bulkhead penetration the sum of the current surrounded by the magnetic material is about zero

Back to the list