The National Grid’s Transmission Network Control Centre (TNCC) manages the high voltage transmission system in Great Britain 24/7, 365 days a year. All incidents near overhead electrical assets must be reported by phone on 0800 404090. It is important to give as much information as possible in a clear and concise manner and always follow any warning issued. Details of the information required are provided in the National Grid publication.
This function is provided in Northern Ireland by Northern Ireland Electricity Networks. All incidents near overhead electrical assets should be reported by phone on 03457 643643.
The decision to isolate high-voltage electricity supplies will need to be made in close consultation with the supplier. This will take into consideration the consequences of this action to the community both downstream and upstream of the incident.
Fire and rescue services cannot isolate high-voltage electricity supplies. This can only be achieved by asking the supplier to isolate the supply. However, the request may take a considerable amount of time to achieve.
If there are high-voltage lines, carrying 132 kV, 275 kV and 400 kV (kilovolts), additional safe distances need to be implemented over and above the normal cordoning requirements of an incident.
It should always be assumed that the high-voltage system is live or that there may be a residual current hazard present, until the electricity supplier confirms otherwise, for example, through a permit-to-work certificate.
Personnel will benefit from having access to risk information about equipment and its location such as:
- Substations
- Transformers
- Switchgear
Personnel should not enter any enclosure surrounding electrical assets, or climb any steel tower, structure or pole supporting overhead lines, unless permission has been received from the electricity supplier,
Precautions should be taken when carrying metal ladders or other operational equipment; these should be carried horizontally and as low to the ground as possible to avoid any contact with high-voltage equipment.
Columns or jets of water should not be applied to transmission towers and their components, as solid jets of water coming into contact with any electrical equipment creates a risk of electrocution.
When firefighting in the close proximity of high-voltage assets, branches with spray, fog or mist can be used at ground level, as electricity is less able to conduct through droplets of water.
Risk assessments
Any operational activity under or near to high-voltage equipment, including transmission towers and overhead lines, should be subject to a risk assessment, taking into account:
- The conditions, such as fire or dense smoke
- Wind direction
- Weather conditions, especially lightning
- The equipment being used, such as ground monitors or aerial ladder platforms
Exclusion zones
If there are high-voltage assets, including transmission tower or overhead lines, additional safe distances need to be implemented over and above the normal cordoning requirements of an incident.
Exclusion zones vary depending on the incident:
- Exclusion zones for overhead lines
- Exclusion and safety corridors for major asset failure
- Exclusion zones to minimise touch and transfer potentials
Exclusion zones for overhead lines
If the decision is made to isolate the overhead line, the National Grid TNCC or Northern Ireland Electricity Networks will operate circuit breakers at all ends of the circuit and open isolators (large switches) to disconnect the circuit from any sources of supply. When a circuit is in this condition it is still not safe to allow people within a specified exclusion zone, as it is still possible for the overhead line to be at a high voltage.
It is important to understand that any immediate action taken by the electricity supplier does not totally remove electrical dangers from the overhead line. It is recommended that the following minimum exclusion zones for personnel, equipment and vehicles are maintained.
| Overhead line condition | Minimum exclusion zone |
| Live | 5m |
| Confirmation by the National Grid or Northern Ireland Electricity Networks standby engineer that circuits are isolated, and a national grid safety document is in place to manage safety from the system | Direct access to the equipment as directed by the National Grid or Northern Ireland Electricity Networks standby engineer if required |
Exclusion and safety corridors for major asset failure
Additional precautions should be taken if there is a risk of all or part of the overhead line failing. This may happen as a result of a significant fire under or near to the high-voltage overhead line, or where the conductors are subjected to severe heat stresses. In all cases advice should be obtained from the National Grid or Northern Ireland Electricity Networks standby engineer.
The power lines that span the distance between transmission towers should be thought of as a corridor. If there is a risk of all or part of the overhead line failing, which could lead to power lines falling, a distance of 5m from the outermost conductors or power lines should become an exclusion corridor.
A further 5m from the outermost conductors or power lines should become a safety corridor. Together they form a 10m hazard area that extends either side of the conductors or power lines and for the whole span between affected transmission towers.
For a diagram showing the exclusion and safety corridors, refer to the National Grid publication, Guidance for UK Fire and Rescue Services for dealing with incidents on or near National Grid high voltage overhead lines.
Until otherwise verified by the correct distribution network operator (DNO), transmission operator (TO), National Grid engineer or Northern Ireland Electricity Networks engineer:
- Rescue and firefighting activities in the exclusion and safety corridor should be strictly controlled
- Personnel, equipment and vehicles should not enter the exclusion corridor
- People should be evacuated from the exclusion corridor; a risk assessment may also determine that people should be evacuated from the safety corridor as well
However, if a transmission tower is at risk of failure or collapse, the height of the tower should be added to the exclusion and safety corridor distances.
Exclusion zones to minimise touch and transfer potentials
Exclusion zones should be established and monitored to minimise the hazard of touch and transfer potentials, based on the situation and whether the circuit is live or confirmed as isolated.
| Situation | Live circuits | If the National Grid or Northern Ireland Electricity Networks standby engineer has confirmed that the circuits are isolated |
| Areas of dense smoke | 10m | No restriction |
| Towers or metal structures that carry high voltage electricity | 10m | No restriction |
| Tall conductive objects in span, such as lampposts | 10m | No restriction |
| Towers and metal structures during a lightning storm | 10m | 10m |
| Conductive objects directly connected to towers, such as fencing | 10m | No restriction |
Rescuing a casualtyÂ
If it is necessary to rescue a casualty who is within 5m of high-voltage electricity, the following steps should be taken:
- Request permission from the electricity supplier to rescue a casualty, providing information such as:
- The condition of the casualty
- The distance and position of the casualty in relation to the point of contact of the high-voltage electricity
- Whether the casualty is at, above or below ground level
- Weather conditions
- Transmission tower markings and signs
- Record permission when received
- Carry out a risk assessment
- Proceed with caution