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High Voltage Systems and Safe Electrical Equipment

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Delve into the intricate workings of high voltage systems on LNG carriers. From their advantages and disadvantages to the specialized equipment and safety protocols involved, uncover the crucial elements driving efficiency and safety in maritime electrical operations.

Gain insights into the intricate world of electrical arc flash management, ensuring smooth and secure operations in the high-stakes environment of LNG transportation.

Certified Safe Electrical Equipment

Reference: SIGTTOLNG Shipping Suggested Competency Standards”, Sections:

1 Know the areas where electrical equipment is required to be of a certified safe type:

  • cargo tank deck;
  • cargo tank domes;
  • cargo machinery spaces;
  • gas burning equipment spaces.

2 Know and understand the operation and maintenance of intrinsically safe systems (Ex i):

  • Zener barriers:
    • maintenance,
    • testing.

3 Know and understand the operation and maintenance of flameproof equipment (Ex d):

  • maintain flamepath;
  • cable glanding arrangements;
  • switching interlocks.

4 Know and understand the operation and maintenance of pressurised equipment (Ex p):

  • purging arrangements and interlocks.

Under the IGC Code, the following are regarded as hazardous areas:

Gas dangerous spaces or zones are zones on the open deck within 3 m of any:

  • cargo tank outlet;
  • gas or vapour outlet;
  • cargo pipe flange;
  • cargo valve;
  • entrances and ventilation openings to the cargo compressor house.

They also include:

  • the open deck over the cargo area;
  • 3 m forward and aft of the cargo area on the open deck, up to a height of 2,4 m above the weather deck;
  • a zone within 2,4 m of the outer space of the cargo containment system where such spaces are exposed to the weather.

The entire Piping System of pressure vessels on gas tankerscargo piping system and cargo tanks are also considered gas dangerous. The area around the air swept trunking, in which the gas fuel line to the engine room is situated, is not considered a gas dangerous zone under the IGC Code.

“Where electrical equipment is installed inhazardous areas… it should be selected, installed and maintained in accordance with standards not inferior to those acceptable… Equipment for hazardous areas shall be evaluated and certified or listed by an accredited testing authority or notified body recognised by the Administration.”
[IGC Code 10.2.4].
Exceptions to this requirement are when the hazardous areas have been certified gas free, e. g. during refit.

Equipment termed intrinsically safe is designed to be incapable of producing heat or spark sufficient to ignite an explosive atmosphere. Intrinsic safety (IS) is a protection technique that works by limiting the energy to the electrical equipment in the hazardous area to a level below that which will ignite given flammable mixtures.

Dangerous zones on LNGC
Fig. 1 Gas dangerous zones

Devices or wiring are not intrinsically safe alone, only when employed in a properly designed IS system. Such systems are provided with detailed instructions to ensure safe use and maintenance. A measuring instrument certified for use in a hazardous area will be designed to operate with low voltage and current, and will be designed without any large capacitors or inductors that could discharge a spark. The instrument will be connected, using approved wiring methods, back to a control panel in a non-hazardous area that contains safety barriers. Zener barriers are frequently used and the circuit connected to the voltage is limited by the zener diodes. The safety barriers ensure that no accidental contact occurs between the instrument circuit and outside power sources. No more than the approved voltage or current enters the hazardous area.

IS circuits are routine maintenance free. The barrier is usually a modular encapsulated unit, housed in the appropriate IS cabinet. Repair is based on unit replacement.

The use of intrinsically safe equipment is limited to instrumentation and control circuitry and cannot be used in power circuitry.

Note: European legislation – equipment for potentially explosive atmospheres (ATEX).

The ATEX Directive 2014/34/EU covers equipment and protective systems intended for use in potentially explosive atmospheres. The Directive defines the essential health and safety requirements and conformity assessment procedures to be applied before products are placed on the EU market and was applicable from 20 April 2016, replacing the previous Directive 94/9/EC.

EX i = intrinsically safe systems.

Flameproof equipment

A flameproof enclosure is one that can withstand the pressure developed during the internal ignition of an explosive mixture and whose design is such that any product of the ignition occurring within the enclosure would be cooled below ignition temperature before reaching the surrounding atmosphere. Therefore, flanged joints through which the gases are allowed to escape are critical. Care must be taken in assembly and maintenance to ensure that OEM maintenance procedures are not compromised in any way and that no bolts are omitted or un-tightened.

Read also: Everything you need to know about Permit-to-Work System, Manual about PTW

Care must also be taken to ensure that the designed “flame path” is not compromised, i. e. by inadvertently fitting a joint where none should be. The integrity of any electrical glands is important and if damaged during repair the sealing arrangement must be fully restored before work can be considered complete.

The switchgear on this type of equipment is usually protected by interlocks. Typically, when the switch or breaker cover is removed, or opened, the switching mechanism becomes mechanically locked, preventing operation until re-closure of the equipment.

Ex d = Flameproof equipment.

Pressurised electrical systems

Normally used for lighting circuits in hazardous areas, such as the cargo compressor room. On such systems the electrical cabling is contained in sealed conduit runs and the fittings are also sealed. The whole circuit is then pressurised with high quality compressed air, or N2 to 0,5 mbar. This low but positive pressure is control led and alarm monitored. In the event of a system leak or supply disruption of the pressurising medium, the associated electrical circuits are tripped automatically. Once the fault has been located and rectified the electrical supply cannot immediately be reinstated. The process is sequence controlled, the first phase of which involves a complete purging of the whole system, ensuring any ingress of combustible gas is expelled before the associated circuits power up. On some systems, flow of the pressuring medium is also monitored.

Maintenance involves the routine testing of the associated alarms and trips, i. e. by isolating the pressurising medium. It can be observed that the low pressure monitoring facility not only operates, it does so at the correct falling pressure, with subsequent tripping of associated electrical circuits sounding of appropriate alarms. The integrity of lighting covers, seals and gland entries is vitally important. Routine leak testing should be carried out to identify small leaks at the early stages.

EX p = Pressurised equipment.

High Voltage Systems

Reference: SIGTTOLNG Shipping Suggested Competency Standards”, Sections:

1 Know that the operation and maintenance of high voltage systems and equipment is subject to specific control procedures:

  • permit to work.

2 Know and understand the operation and maintenance of switchgear and transformers:

  • safe switching operations;
  • isolation/prove dead;
  • earthing procedures;
  • testing.

3 Know and understand the maintenance and testing of generators and motors:

  • maintenance;
  • testing.

General

Electrical systems – general. The risks of electric shock are much greater on board ship than ashore because “wetness” high humidity and high temperature (including sweating) reduces the contact resistance of the body. In those conditions, severe and even fatal shocks may be caused at voltages as low as 60 volts. It should also be borne in mind that cuts and abrasions significantly reduce skin resistance.

High voltage sign
Fig. 2 High voltage warning sign
Source: unsplash.com

A notice of instructions on the treatment of electric shock should be posted at all locations containing electrical equipment and switchgear. Immediate on the spot treatment of an unconscious patient is essential if they are to have a chance of survival. Insulated rubber matting should be fitted at switchboards and electrical panels.

Work on electrical systems – general procedures

Before any work is carried out on electrical equipment, a comprehensive risk assessment (RA) must be carried out, fuses should be removed or circuit breakers opened to ensure that all related circuits are dead. If possible, switches and circuit breakers should be locked open or, alternatively, have a “not to be closed” notice attached. Where a fuse has been removed, it should be retained by the person working on the equipment until the work is completed. A check should be made to ensure that any interlocks or other safety devices are operational. Additional precautions are necessary to ensure safety when work is to be undertaken on high voltage equipment (designed to operate at a nominal system voltage in excess of 1 kV). The work should be carried out by, or under the direct supervision of, a competent person with sufficient technical knowledge and a permit to work (PTW) system should be operated.

Some parts of certain types of equipment may remain live even when the equipment is switched off. Power should always be isolated at the mains.

Work on or near live equipment should be avoided if possible, but when it is essential for the safety of the ship or for testing purposes, the following precautions should be taken:

  • a second person, who should be competent in the treatment of electric shock, should be continually in attendance;
  • the working position adopted should be safe and secure to avoid accidental contact with the live parts. Insulated gloves should be worn where practicable;
  • contact with the deck, particularly if it is wet, should be avoided. Footwear may give inadequate insulation if it is damp or has metal studs or rivets. The use of a dry insulating mat at all times is recommended;
  • contact with bare metal should be avoided. A hand to hand shock is especially dangerous. To minimise the risk of a second contact should the working hand accidentally touch a live part, one hand should be kept in a trouser pocket whenever practicable;
  • wrist watches, metal identity bracelets and rings should be removed. They provide low resistance contacts with the skin. Metal fittings on clothing or footwear are also dangerous.

Advantages and disadvantages of using a HV system

Advantages

For a given power, higher voltage means lower current, resulting in:

reduction in size of generators, motors, cables, etc.
saving of space and weight
ease of installation
reduction in cost of installation
lower losses, more efficient utilisation of generated power
reduction in short circuit levels in the system, which decides the design and application of the electrical equipment used in the power system
Disadvantageshigher insulation requirements for cables and equipment used in the system
higher risk factor and the necessity to strict adherence to stringent safety procedures

Work with HV and special equipment

Work on high voltage equipment/installations. The demand for electrical power has increased on many ships, especially those with diesel-electric propulsion where the supply current would be far too high and not efficient or practical if a shipboard voltage supply of 440 volts was utilised to power the system. Higher voltage is needed to reduce the current. LNG ships are built with a high voltage generating plant, which means that virtually all procedures must be reworked to accommodate the increased hazard levels presented by such voltages.

In marine practice, voltages below 1 000 Vac (1 kV) are considered low voltage and high voltage is any voltage above 1 kV. Currently, typical marine high voltage system voltages are 3,3 kV, 6,6 kV and 11 kV.

When large loads are connected to LV systems, the magnitude of current flow becomes too great. Power is voltage · amps, so higher voltage means less amps.

Consider 440 kW = 440 volts · 1 000 amps, or 11 000 volts · 40 amps.

High voltage equipment

A typical high voltage installation will incorporate only high voltage rated equipment on the following:

  • generating sets;
  • high voltage switchboards with associated switchgear, protection devices and instrumentation;
  • high voltage cables;
  • high voltage/low voltage step-down transformers to service low voltage consumers;
  • high voltage/high voltage (typically 6,6 kV/2,9 kV) step-down transformers supplying propulsion converters and motors;
  • high voltage motors for propulsion, thrusters, air conditioning and compressors.

The major differences between high voltage supply and low voltage supply on board ships are:

  • high voltage systems are more extensive with complex networks and connections;
  • isolated equipment must be earthed down;
  • access to high voltage areas should be strictly limited and controlled;
  • isolation procedures are more involved for high voltage;
  • switching strategies should be formulated and recorded;
  • specific high voltage test probes and instruments must be used;
  • diagnostic insulation resistance testing is necessary;
  • high voltage systems are usually earthed neutral and use current limiting resistors;
  • special high voltage circuit breakers have to be installed.

A high voltage electrical shock is a significant danger to any person carrying out electrical work. Any simultaneous contact with a part of the body and a live conductor will probably result in a fatal electric shock. There is also a risk of severe burn injuries from arcing if conductors are accidentally short-circuited.

A high voltage electric shock will almost certainly lead to severe injury or a fatality.

Safety and risks

High voltage system safety requirements. High voltage system training is now a part of the Standards of Training Certification and Watchkeeping convention (STCW) following the 2010 Manila amendments for senior engineering staff who have responsibility for operating and maintaining electrical power plants above 1 000 volts. However, some existing officers may not have this training until their certificates are revalidated.

This training includes:

  • operational and safety requirements for high voltage systems;
  • maintenance and repair of high voltage switchgear;
  • taking appropriate action when dealing with faults in a high voltage system;
  • switching strategies for isolating components of a high voltage system;
  • using suitable apparatus for isolation and testing of high voltage equipment;
  • switching and isolation procedures on a marine high voltage system;
  • understanding safety documentation for high, voltage systems;
  • testing of insulation resistance and polarisation index on high voltage equipment.

Risk assessment. The access to high voltage switchboards and equipment must be strictly controlled by using a Risk Assessment on Liquefied Natural Gas Tankerrisk assessment and a permit to work (PTW) system. Isolation procedures must involve a safety key system and earthing down procedures.

Disconnect - Isolate - Earth
Remember this acronym: Disconnect – Isolate – Earth

To help identify high voltage system work precautions, a risk assessment must be completed by the Chief Engineer or chief electrical officer before work begins. This should consider:

  • How familiar are the personnel with the high voltage system and equipment?
  • Can the work be carried out with the equipment dead?
  • Is it necessary for someone to work on or near live high voltage equipment?
  • What precautions have been taken to avoid danger and prevent injury?
  • Is the person(s) carrying out the work competent or adequately supervised?

The company SMS should include a PTW system for electrical equipment under 1 000 volts. A similar high voltage permit should also be included in the SMS. Samples of electrical permits for low voltage and high voltage installations, and the additional procedures required, can be found in the Code of Safe Working Practices for Merchant Seaman (COSWP).

Sanction for test System

Following work on a high voltage system, it is often necessary to perform tests. Testing should only be carried out after the circuit main earth (CME) has been removed.

A sanction for test declaration should be issued in an identical manner to a PTW, and it should not be issued for any apparatus where a PTW or where another sanction for test is already in force.

Note: A sanction-for-test is NOT a permit to work (PTW).

An example of a sanction for test declaration is shown in COSWP.

Limitation of access form. When carrying out high voltage maintenance, it may be dangerous to allow anyone to work adjacent to high voltage equipment as workers may not be familiar with the risks involved. The limitation of access form states the type of work that is allowed near high voltage equipment and the safety precautions to be taken. The form is issued and signed by the Chief Engineer or chief electrical officer and countersigned by the person carrying out the work.

Earthing down

Earthing down is an important concept to understand when working with high voltage systems. It is important to ensure that any stored electrical energy in equipment insulation, after isolation, is safely discharged to earth. The higher levels of insulation resistance required on high voltage cabling leads to higher values of insulation capacitance (C) and greater stored energy (W).

This is demonstrated by the electrical formula:

Energy stored (W) Joules = (Capacitance·Voltage2)/2.

Earthing down ensures that isolated equipment remains safe.

Even if the system is isolated, you can still receive a fatal shock caused by the stored energy. The system must be earthed and proven dead before work commences.

There are two types of earthing down a high voltage switchboard:

  • circuit earthing;
  • busbar earthing.

What is circuit earthing?

Circuit earthing – an incoming or outgoing feeder cable is connected, by a heavy earth connection from earth to all three conductors, after the circuit breaker has been racked out. This is done at the circuit breaker using a special key. This key is then locked in the key safe. The circuit breaker cannot be racked in until the circuit earth has been removed.

What is busbar earthing?

Busbar earthing – when it is necessary to work on a section of the busbars, they must be completely isolated from all possible electrical sources. This will include generator incoming cables, section or bus-tie breakers and transformers on that busbar section. The busbars are connected together and earthed down using portable leads, which give visible confirmation of the earthing arrangement.

High voltage safety checklists can be found in COSWP for the following:

  • working on high voltage equipment/installation;
  • switchgear operation;
  • withdrawn apparatus not being used;
  • locking off;
  • insulation testing;
  • supply failure;
  • entry to high voltage enclosures;
  • earthing;
  • working on high voltage cables;
  • working on transformers;
  • safety signs;
  • correct personal protective equipment.

Personnel should not work on high voltage equipment unless it is dead, isolated and earthed at all high voltage disconnection points. The area should be secured, PTWs or sanction for test notices issued, access should be limited and only competent personnel should witness the testing to prove isolation.

Low voltage system PTWs are not appropriate for working with high voltage systems.

Additional precautions are necessary to ensure safety when work is to be undertaken on high voltage equipment.

Compartments and other enclosures containing high voltage apparatus shall be locked except when entry or exit is necessary. The keys that provide normal access should be accessible to authorised persons only.

Entry to compartments or other enclosures containing high voltage equipment/installations is limited to authorised persons or other persons only when accompanied by an authorised person.

Entry to compartments containing high voltage equipment/installations that are not protected by insulated covers should only be undertaken when the equipment/installations are isolated and earthed.

Electrical arc flash

The arc flash hazard is a serious electrical risk that needs to be managed in the marine environment. It is essential that personnel who operate in proximity to, or on, energized electrical equipment and cables, are aware of the risks and comply with all safety and legislative requirements. Preventing arc flash incidents or minimising their impact, requires a comprehensive safety program, involving both the shore management and ship personnel.

An arc flash is the light and heat produced from an electric arc supplied with sufficient electrical energy to cause substantial damage, harm, fire, or injury. Electrical arcs, when well controlled and fed by limited energy, produce very bright light, and are used in arc lamps for welding (welding arcs can easily turn steel into a liquid with an average of only 24 DC volts), plasma cutting, etc. However, when an uncontrolled arc forms at high voltages, arc flashes can produce deafening noises, supersonic concussive forces, super-heated shrapnel, temperatures far greater than the Sun’s surface and intense, high energy radiation capable of vaporising nearby materials. The massive energy released rapidly vaporises the metal conductors involved, blasting molten metal outward with extraordinary force. A typical arc flash incident may be inconsequential but can produce a more severe explosion. This could cause destruction of equipment, fire, and injury to personnel, including bystanders.

Many people assume that low voltage equipment is safe from arc flash, but arc flash hazard levels may be higher at low voltages due to the high fault currents. Most of the incidents that occur in low voltage systems are caused by human error, e. g. a tool slipping while working on electrical equipment.

Arc flash
Source: wikipedia.org

Individuals that work on, or near, energised conductors should be trained to understand the hazards associated with arc flash, as well as the arc flash and shock protection boundaries, the use of arc flash warning labels and the selection and use of Safe Practices and Personal Protection Equipment at Work with Liquefied Gasappropriate PPE.

Precautions to avoid arc flash:

  • De-energise equipment – eliminate the potential hazard as far as possible. Avoid working on energised electrical equipment and take extra care while testing to ensure it has been de-energized or while re-energizing it. Where possible, use remote racking technology to operate circuit breakers from outside the arc flash boundary.
  • Study the hazard – identify arc flash hazard categories for electrical equipment, and how to reduce them. Where possible use remote racking equipment and arc limiting fuses.
  • Design or re-design electrical systems and controls – determine the correct level of PPE required as per the flash hazard category and ensure that personnel are properly equipped. Design or redesign equipment and processes to maximise engineering controls that help prevent and lessen risk. Where possible substitute high risk electrical equipment with devices that reduce incident energy.
  • Safety training, risk awareness and implementation of a strict safety program – In addition to being mandated by regulatory authorities such as OSHA, safety training ensures that personnel understand the consequences of being careless. Safety procedures and protocols should be followed at all times, ensuring that only qualified and fully trained personnel equipped with the proper tools and PPE are allowed to work on electrical systems.

Arc flash boundary requirements

  • Arc Flash Boundary (AFB): is the minimum distance at which PPE must be worn to minimise burn injury in the event of an arc flash. Unqualified individuals may only cross the AFB when accompanied by a qualified worker. Any individual crossing the AFB must wear appropriate PPE.
  • Limited Approach Boundary (LAB): is the distance from a live part to which unqualified individuals may approach unaccompanied. To cross the LAB, unqualified individuals must be accompanied by a qualified individual wearing the appropriate PPE and trained on the task to be performed.
  • Restricted Approach Boundary (RAB): may only be crossed by qualified individuals with appropriate PPE and training on the task to be performed. In addition, individuals must have an approved work permit and written plan for the task. The plan should include shock prevention procedures designed to keep all portions of the individual’s body from crossing the PAB at any time.
  • Prohibited Approach Boundary (PAB): is the distance from a live part that is equivalent to direct contact. Crossing this boundary with unprotected (conductive) body parts or tools risks an electrical arc. A risk assessment must be performed and a written work plan approved before crossing the PAB.

A key feature of managing exclusion distances is the presence of a “safety buddy” stationed outside the arc flash boundary, so that they are not also incapacitated in the event of an incident.

Control measures to be consider

  • where practical, switchboard compartments should be designated as unmanned spaces;
  • where possible, remote breaker switching should be utilised;
  • metal fittings and phase conductors are to be secured to prevent any unintentional short circuit;
  • electrical equipment enclosures are to be designed and tested in accordance with international standards that direct arc flash products away from an operator;
  • fit devices that provide high speed initiation of switchgear on physical detection of an arc flash (e. g. by sensing light or pressure) and override the normal delays associated with discriminative short circuit protection;
  • only perform work on de-energised equipment that has been placed into an electrically safe condition. This should be considered the ultimate safe work practice for electrical systems, when possible;
  • unless unavoidable, such as during testing and start-up or where de-energizing a system produces an increased hazard or is infeasible, working on energised electrical equipment should be a last resort.

Arc flash protection equipment

With recent increased awareness of the dangers of arc flash, there have been many companies that offer arc flash personal protective equipment (PPE). The materials are tested for their arc rating. The arc rating is the maximum incident energy resistance demonstrated by a material prior to break open (a hole in the material) or necessary to pass through and cause a 50 % probability of second degree burns.

PPE provides protection after an arc flash incident has occurred but should be viewed as the last line of defence. Reducing the frequency and severity of incidents must be the first option.

Safe switching operations. In an emergency, operation of high voltage switching to cut off the supply may be carried out by any person competent to do so. Any message relating to the operation of a high voltage system, which has been transmitted by telephone/radio, must be repeated in full by the recipient and confirmed by the sender to ensure that the message has been accurately received. Making live or dead by signals, or pre-arranged understanding after an agreed time interval, is not permitted.

Isolation/prove dead. During failures of supply, all apparatus, equipment and conductors shall be regarded as being live until isolated and proved dead.

Protective equipment. Protective equipment associated with the high voltage equipment/installations and forming part of the system shall not be adjusted, put into or taken out of commission without the sanction of the Chief Engineer or superintendent/senior electrical engineer.

Earthing. When work is to be carried out on any connections up to a point of isolation or the windings of a transformer, all windings irrespective of voltage shall be isolated. Circuit main earths shall be applied at the points of isolation from high voltage supply. Low voltage points of isolation shall be locked open.

Testing. High voltage equipment/installations shall not be commissioned or re-commissioned (after major work) until the protective devices have been proved to be functioning correctly. All high voltage equipment/installations that are either new or have undergone substantial maintenance or alteration shall be subject to a high voltage test, in accordance with figures approved in writing by the Chief Engineer or superintendent/electrical engineer.

All precautions and regulations discussed in the previous sections concerning high voltage equipment apply.

Maintenance preparations. In the risk assessment and work planning stage the “Authorised Person” and the “Competent Person” must be clearly identified and recorded. Ensure the correct risk assessment is used and the correct work permit is issued, i. e. “low/medium voltage permit” (typically for 440 volt) or “high voltage permit” (any circuit exceeding 1 kV).

In the control/starter cabinets for generators/motors there are frequently numerous auxiliary supplies that may not be de-energised when the breaker is isolated, i. e. they remain “live”.

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