Objectives• To describe electrical hazard associated with marine environment• Top describe hazard zone system and management for marine system and space.
Hazardous Zones classification:• All vessels, in particular oil tankers, have areas or spaces, which may contain flammable or explosive gas-air mixtures. The sketch below shows the gas-dangerous spaces in an oil tanker.• In defining their hazards, hazardous zones are classed into zones, which indicate the probability of explosive gas-air mixtures being present.
Table of Zones and their classification Zone 0: In which an explosive gas-air mixture is continuously present, or present for long periods. Eg COT, CO pipes Zone 1: In which an explosive gas-air mixture is likely to occur in normal operation. Eg enclosed or semi-enclosed spaces on deck of tanker. Zone 2: In which an explosive gas-air mixture is not likely to occur in normal operation. + Other places not designated zones 0,1 or 2 may be considered SAFE or NON-HAZARDOUS.
Hazards of potentially flammable atmospheres• Where the atmosphere within a zone falls within the flammable envelope, then the hazards of fire or even explosions are real and should be avoided.• Additionally the risk of auto-ignition with certain gases may be present especially if high surface temperatures are reached during operation of the equipment.
Sources of electrical ignition• Arcing between switch contacts• Arcing between live conductor and earth• Internal arcing fault within electrical enclosure.• Overheating causing ‘hot-spots’• Electrostatic sparking• Lighting strikes.• Chemical action.
• Electrical equipment intended to be used for these hazardous atmospheres therefore need to be selected carefully consistent with the level of risk involved.• The table below gives the protection schemes and how they are suitable for different zones.
• Some equipment may be certified to more than one of these protection schemes in its construction, for example,• These designations are normally stamped on the nameplate fitted to the certified equipment. The name plate will also carry the entity, eg BASEEFA, which carries out the testing and certification.
Definition of Flameproof enclosure Exd• 1. An enclosure which will withstand, without damage, an explosion of the flammable gas within itself, 2. and that will prevent transmission of flames, which could ignite the gas in the surrounding atmospheres.• This is achieved by strengthening all parts of the enclosure to withstand the impact of an explosion and ensuring that escaping gases are sufficiently cooled before exiting from the enclosure.
Definition of Intrinsically safe equipment (Exi).• when any spark normally or accidentally caused, is incapable, under prescribed test conditions, of causing ignition of a prescribed gas or vapour.• This is normally achieved by limiting both the current and voltage in the circuits and equipment that will be used in the hazardous environment. Power for such circuits are therefore usually small.• Due to the small power available, Exi circuits are usually only used for instrumentation and control equipment. Examples are temperature monitoring, cargo tank level and alarm monitoring
Maintenance and inspection of Exi equipment and systems.• Should be performed by competent personnel• Exposed conductors – insulated and secured.• In-situ work, only for:- – Removal & replacement – Adjustment – Permitted by manufacturer.• Testing should be done using approved equipment
Electrical Testing in Hazardous Zone• Before opening up, – Ascertain that power supply is off – Take precautions to prevent accidental switching on – Make the zone gas-free• For Insulation resistance, ensure earth continuity• Use intrinsically safe tester• max values 1.2V, 0.1A, 200 uJ, 25 MW• Leads or probes – firmly connected during test• - discharge circuit for 1 min after tests• High voltage insulation test• -Make area safe or gas-free• -Remove equipment to safe area• Tools - non-sparking
Definition of pressurized equipment (Exp) • Housed in an enclosure that has been made safe by being purged with a safe atmosphere (eg air or inert gas). • The equipment is pressurized with air or inert gas thereby ensuring that the internal atmosphere within the equipment is below the Lower explosive limit (LEL). The air or gas could also assist in cooling the equipment. • The circuits could only be energized after purging has been done. The safety devices normally fitted include pressure drop and failure of electrical supply.
Definition of Increased Safety Equipment (Exe)• Designed to prevent sparking and arcing except for parts housed in a flameproof enclosure and to avoid that dangerous temperature rises be reached by any part in contact with explosive gas mixture both by normal and abnormal working conditions.• Exe equipment therefore will be enclosures that does not contain:- – Arcing or sparking (arcing and sparking will be limited in Exd enclosures) – Hot surfaces that may cause auto-ignition.
Other measures used in Exe equipment to reduce failures:-• Higher rate insulation• Increased distances between insulation• Anti-loosening devices• Special enclosures protecting against ingress of solids, liquids, mechanical damage and impact.• Control of max. Temperatures eg thermal protection.
Design features of Exe Safety Lamp• Body – non-metallic, spark-free• Glass disc – impact and scratch free• Interlock – lens ring and switch• Short circuit protection – internal cartridge fuse• Internal sparking – reed switch, sealed, magnet operated.• Battery – 4 hours or more, 2 sealed lead acid• Shoulder lanyard – for ease of carrying.
Special protection Exs• Electrical apparatus which do not comply with construction requirements of established type of protection, but which can be shown by test, to be suitable for use in hazardous areas.• This type therefore offers more flexibility in certification of specialized equipment.
• Examples of Exs equipment:- – Air driven lamps – Compressed air driven electric floodlight – Submersible pumps etc.
Special design features that may be included:-• Body – spark-free• Over-speed protection• Prime safety – pressurized and purged• Fail safe – disassembly and glass breakage• Operation – clean dry air.