SG3:14 Earthing of Scaffolding Structures

This NASC Guidance Note addresses the issue of earthing temporary scaffolding and the protection of scaffolding from lightning.
In almost all circumstances scaffolding structures will have low electrical resistance.
Although the scaffolding structure is usually in contact with the ground and may have base plates and sole boards, it should never be assumed that the structure is effectively earthed.
The Construction (Design and Management) Regulations (CDM) outlines that clients, CDM co-ordinators and designers should consider hazards at the planning stage. This should include any requirement for earthing.
For instance, there may be a requirement for earthing to be run under the base plates and therefore it is important that the risk of lightning be assessed at scaffold design drawing and pre-tender stage, prior to the erection of scaffolding.
Electrical hazards in the form of lightning, overhead electrical lines and integral lighting and alarm systems can affect all types of structures and if scaffolding is not properly earthed people can be killed or seriously injured and buildings damaged.
The client or user of the scaffolding structure is responsible for ensuring that it is safe for use. The client should ensure that expert advice is sought from a competent person (i.e. a qualified electrician) and suitable protection is installed.

EARTHING OF TEMPORARY SCAFFOLD

Where the scaffolding carries lighting, alarm or similar small power circuits, it is recommended that the structure should be electrically bonded to the protective conductor(s) of the circuit(s) that it carries.
Bonding is not required if the operating voltage of a circuit is below 50V a.c.
For higher power circuits where bonding is required, such as goods and passenger hoists, this should be carried out in accordance with the latest IEE regulations by a competent person in line with the risk assessment, to ensure there is no risk of scaffold operatives being electrocuted while the scaffold is dismantled.
Except where it is necessary for lightning protection purposes, scaffolding external to a structure should not be connected to the means of earthing within the structure that is afforded by the supply authorities. If Earthing Spikes are required, the area must be CAT Scanned by a competent person to detect underground services etc, using equipment that has been calibrated appropriately. This will also help to ensure that any permit conditions are complied with when in place.
It should be noted that individual scaffolding structures if not connected, must be earthed separately.

LIGHTNING PROTECTION

If scaffolding is next to an existing structure which has an external lightning protection system it should be bonded to the earth termination and the air termination network of the lightning protection system by a competent person, arranged by the client.
Where scaffolding is erected next to a building or structure which is readily accessible to the general public and used as a working platform or erected over or as part of the common highway, the client should ensure that it is efficiently bonded to earth.
For example, a simple method of bonding such structures consists of running a strip of metal, other than aluminium, 20 mm x 2.5 mm in size, underneath and in contact with the base plates and ground carrying the vertical members of the scaffolding and earthing at intervals not exceeding 20 m. With public seating
accommodation, only the peripheral members of the structure need to be bonded to earth. Other steel structures, such as those used for pedestrian bridges over main trunk roads, are frequently sited in isolated situations where they may be prone to lightning strikes and should therefore be bonded to earth, particularly at the approach points to the structure.

OVERHEAD ELECTRIC POWER LINES

Other temporary tall metallic structures may require separate earthing electrodes to afford adequate lightning protection. This will depend on the construction of the temporary foundations and footings. BS EN62305-2 should be consulted for further guidance and expert opinion should be obtained if necessary.
When working near overhead power lines it is essential that the voltage being carried is known to determine the likelihood of arcing and the size of any exclusion zone required. Advice should be sort from the power line owners.
Work under overhead electric power lines should not be carried out without consultation with the owners of the lines. Wherever possible the lines should be diverted or made dead before work begins.
Further details are contained in NASC Guidance Note SG5 Overhead Power Lines (latest edition), and HSE Guidance Series GS6 4th Edition 2013.

SCAFFOLDING STRUCTURES AND SENSITIVE BUILDINGS

Scaffolding contractors are often asked to erect scaffolding to sensitive buildings or sites such as historic buildings and monuments and in Atomic or Petrochemical plants. These places often have their own rules for Earthing or Bonding and advice must be sort from the owners of the building or the Principle Contractor to
ascertain local policy, prior to the erection of scaffolding.
Additionally, special earthing requirements will be required when erecting scaffolding adjacent to power supplies such as National Grid stations and National Rail installations, and advice and instruction must be sought from the client, with a risk assessment made by a Senior Authorized Person prior to works commencing.

ERECTION AND USE DURING LIGHTNING STORMS

Scaffolders erecting or dismantling scaffolding and users who observe thunder storms in the distance are strongly advised to stop work immediately and leave the scaffolding until it is clear that the storm is not approaching or is moving away and there is no further risk of a strike.