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Hazard
Electricity

Hazard Knowledge

Fire and rescue service personnel are at risk of coming into contact with electrical equipment and components at operational incidents, which can result in electrocution. Contact with electricity can cause serious physical injury or prove fatal.

Electrocution can occur in a number of ways:

Direct contact

Direct contact with live electricity is potentially lethal. This can be from direct or alternating current, as well as static discharge from domestic or industrial supplies.

Arcing

Electricity can 'jump' through air, smoke or a column of water. The higher the voltage the more likely, and the further, the electricity may 'jump'. Arcing can also generate intense heat and ignite flammable substances in the vicinity.

Flash down (flashover)

High-voltage electricity has the potential to cause death or serious injury to a person in the vicinity through 'flash down'. This is also referred to as 'flashover' in the electricity industry. For more information refer to the content on Firefighting activities near to high-voltage transmission towers or overhead line equipment.

Carbon tracking Thick smoke with a high carbon content may be generated by fires including rubber tyres or plastics or by wildfires. High-voltage electricity can find a path to earth through this type of thick smoke.

Re-energising circuits and equipment

Substations generally have automatic switches, which are programmed to attempt to re-energise circuits that have been broken. Some re-energising may also be caused by human error. Therefore, it should not be assumed that a circuit is isolated.

Residual current (or stored electrical energy)

Electrical equipment may not be made entirely safe by isolating the supply, as it may hold a residual current.

In high-voltage equipment, the residual current could be sufficient to cause a fatal injury, and may remain a hazard until the equipment is made safe, usually by the electricity supplier.

Electrical feedback

Electrical feedback is a relatively new phenomenon that can occur when electricity supplies in the road or on an industrial estate, are thought to have been made safe and have been disconnected by the electricity supplier. However, domestic and commercial premises producing their own electricity can feed it back into the national grid, thereby re-energising the dead cable.

Refer to the supplementary information for:

National grid

Power stations produce electricity at 25kV. Electricity is sent through the national grid network at 400kV, 275kV and 132kV.

Step-up transformers are used at power stations to produce the very high voltage needed to transmit electricity through the national grid power lines. These high voltages are too dangerous for use in homes and businesses and therefore step-down transformers are used locally to reduce the voltages to a safe level, resulting in the following supplies:

Large industrial consumers - 33kV

Rail network - 25kV to 33kV

Small industrial consumers - 415V to 11kV

Residential and small commercial - 240V

UK electricity distribution - diagram courtesy of London Fire Brigade

Overhead power lines and transmission towers

Overhead lines carry various voltages and are usually uninsulated.

Low-voltage (up to 1000V) lines are suspended by wooden poles and arranged into either:

  • Vertical arrays of up to six lines (with the lowest cable being the neutral)
  • Single lines with live and neutral cores (known as concentric cables)

Some high-voltage (greater than 1000V) lines may also be suspended by wooden poles. These can be identified as they are:

  • Usually in either a horizontal array of two or three lines, or in a six cable array with three lines on each side of the pole
  • Separated from the wooden pole by circular insulators

Transmission towers (often referred to as pylons) have an array of three cables (or sets of cables) to each side of the tower. Each side of the transmission tower represents three phases of electricity; however, each side of the transmission tower may have a different origin and may be operated by a different distribution network operator (DNO).

Figure 2: 132kV transmission towers – photograph courtesy of Peter Martin

Overhead lines are normally uninsulated; if an object gets too close, it is possible that electricity will jump over a distance to reach earth via the object.

It should be assumed that an overhead power line can be fatal. Equipment, appliances and personnel should be kept away from the power lines at all times. Undulating surfaces cause the distance between overhead power lines and the ground to vary; this can result in personnel inadvertently coming too close to lines.

High-pressure jets coming into contact with an overhead line with horizontal conductors may cause them to clash together, resulting in arcing. This could lead to conductor breakage, resulting in live conductors falling to the ground. Water can also cause heated porcelain insulators to shatter, creating needlestick or projectile hazards.

High-pressure jets contacting overhead conductors may also result in earth leakage through the water stream to ground. This may cause the branch to become live, with potentially fatal consequences.

The National Grid has produced Guidance for UK Fire and Rescue Services for dealing with incidents on or near National Grid high voltage overhead lines.

Firefighting activities near to high-voltage transmission towers or overhead line equipment

When firefighting activities are taking place there will be large volumes of smoke, steam, water particles and debris in the air. Water and smoke will always pose a risk of flashover. Consideration should be given to the likelihood of a flashover through the smoke to the tower or ground which may introduce the dangers of touch and transfer potentials.

  • Touch potential – In the event of flashover to the tower or any metal object, such as a lamppost, the current will flow to earth via the metal structure. If a person is standing close by, or touching the structure, some electrical energy may flow through their body.
  • Transfer potential – In the event of a flashover, electrical energy could flow through any conductive object connected to the tower. For example, if a flashover occurred to a tower and a farmer had connected barbed wire to the tower leg, electrical energy could flow to earth through the length of barbed wire, resulting in touch potential hazards. It is very unlikely that a flashover would occur mid-span, as the smoke is more likely to cause a phase-to-phase flashover between one conductor and another of the same circuit, which would not result in any voltage rise at ground level. However, the risk is slightly increased if smoke is affecting the tower and contaminating the insulators.

Fires under or near to high-voltage overhead lines

In the event of a significant fire under or near to the high-voltage overhead line, or where the conductors are subjected to severe heat stresses, there is a risk that part or all of the overhead line may fail and fall to the ground. The conductor may also fall into other spans between the transmission towers.

If overhead lines or conductors were to fall, there may be hazards to the surrounding area including to:

  • Road or rail crossings
  • Navigable waterways
  • Housing or industrial buildings
  • Public footpaths
  • Other power crossings – conductors may fall onto other live power lines which could present further dangers by re-energising the fallen conductors

Health and Safety Executive – electrical definitions

It is important that personnel understand electrical terminology when discussing isolation of electricity supplies with electricity distributors or any attending electrical engineers. Failing to understand this terminology may increase the risks encountered.

The definitions in the table below support the terms used in this guidance. Further information is provided in The Electricity at Work Regulations.

Charged

Means that the item has acquired a charge either because it is live or because it has become charged by other means such as by static or induction charging, or has retained or regained a charge due to capacitance effects even though it may be disconnected from the rest of the system.

Dead

Not electrically 'live' or 'charged'

Designated competent person (also known in some industries as 'authorised person' or 'senior authorised person')

A competent person appointed by the employer, preferably in writing, to undertake certain specific responsibilities and duties, which may include issuing and receiving safety documents such as permits-to-work. The person must be competent by way of training, qualifications and/or experience and knowledge of the system to be worked on.

Note: On a nuclear site, the designated competent person would be referred to as the 'duly authorised person'.

Disconnected

Equipment (or a part of an electrical system) that is not connected to any source of electrical energy

Electrical equipment

Includes anything used, intended to be used or installed for use, to generate, provide, transmit, transform, rectify, convert, conduct, distribute, control, store, measure or use electrical energy

High voltage

Voltages greater than 1000V AC or 1500V DC. Voltages below these values are low voltage.

Isolated

Equipment (or part of an electrical system) that is disconnected and separated by a safe distance (the isolating gap) from all sources of electrical energy in such a way that the disconnection is secure, so that it cannot be re-energised accidentally or inadvertently

Live

Equipment that is at a voltage by being connected to a source of electricity. Live parts that are insulated and exposed so they can be touched either directly or indirectly by a conducting object are hazardous if the voltage exceeds 50V AC or 120V DC in dry conditions.

Live work

Work on or near conductors that are accessible and live or charged. Live work includes live testing, such as using a test instrument to measure voltage on a live power distribution or control system.

Low voltage

Voltages up to 1000V AC or 1500V DC. Voltages above these values are high voltage.

Buildings under construction and demolition

Personnel attending fires in buildings under construction or demolition may be presented with electrical hazards that differ depending on the type of construction or demolition site, such as:

  • Electrical systems that have been partially or completely isolated
  • Temporary electrical supplies
  • Portable generators
  • Uninterruptible power supplies (UPS), which could range from small units to large rooms containing lead acid or lithium-ion batteries

 

Live cabling during construction phase; labels may be destroyed in fire - photograph courtesy of Nick Lacey

 

Unsecured electrical cables during refurbishment - photograph courtesy of Robert Bacon

Dangerous wiring 

This hazard may exist where the wiring has: 

  • Been damaged 
  • Been carried out incorrectly 
  • Deteriorated with age 

If power cables are damaged or inappropriately repaired and they come into contact with unprotected steel framing, scaffolding, water run-off or equipment, personnel may be at risk of electrocution. 

Performing rescues of casualties 

If there is a casualty who is affected by electricity at ground level, it may be necessary to consider performing a rescue. 

However, working within the 5m exclusion zone for high-voltage electricity presents a significant risk of electrocution to personnel.