Electrical equipment that has been properly designed, constructed, installed and maintained does not present a risk of electric shock or burn injury when properly used. The general requirement for the design and construction of electrical equipment is that it is suitable for its intended use and the environment in which it is to be used. Electrical equipment is susceptible to failure from water and dust ingress, damage by impact and general deterioration and should therefore be protected from these effects by installing it in a dry environment free from damage or in suitable weather proofed and protected enclosures.
Electrical installations must conform to relevant British, CENELEC and IEC standards. All equipment must be marked with an appropriate CE marking.
All installations, including those for the control of motor drives, must be capable of being effectively isolated by readily accessible lockable isolating devices. The means of isolation, whether isolating switches, fused isolators, circuit breakers or other devices, must be capable of being locked in the open or off position and clearly labelled to show the equipment or circuit that it controls.
Where emergency stopping devices and or safety interlocking devices are fitted to equipment and machinery these should be designed to be readily accessible, not easily defeated and fail to safety in the event of fault.
The disconnection of faults is extremely important for the following reasons.
- To protect persons from electric shock and
- To protect persons and plant from the risk of fire and or explosion.
Therefore equipment must be designed to withstand all foreseeable excess currents and be protected against faults such as short circuits, earth faults and sustained overloads. The electrical protection used, whether simple fuses or complex tripping systems operating circuit breakers, should be set at the correct level (i.e. the size of the fuses or tripping levels of complex systems) to trip in the minimum time consistent with avoiding nuisance trips. Requirements dealing with tripping times are set out in the current version of BS7671. Requirements for Electrical Installations, but it is recommended that the following disconnection times are adopted.
For 230V / 400V TN earthing systems.
- Production plant and equipment 0.2 seconds for 230V / 400V.
- Offices, weighbridges and welfare facilities 0.4 seconds.
These disconnection times above are only advisory but in all circumstances disconnection times must meet the requirements of those set out in the current version of BS7671 Requirements for Electrical Installations and applies to all low voltage circuits.
Electrically powered hand tools
Where possible all electrically powered hand tools should be either.
- Rechargeable battery type or,
- 110V fed from centre tapped earth supply.
Consideration should be given with regard to the environment where electrically powered hand tools are used, for example inside tanks, silos, mixer drums etc. The risk assessment for the task may identify the use of battery powered hand tools only.
Where 230V hand tools and test equipment have to be used then the equipment should be protected by a suitably rated 30mA RCD.
All 230V / 400V socket outlets should be protected by RCD’s unless they are covered under the exceptions set out in the current version of BS7671 Requirements for Electrical Installations.
Electrical control panels
The design of electrical control panels is an important feature for electrical safety. They must be safe when being commissioned, tested and maintained. To ensure that such equipment is safe when commissioning or fault finding is being carried out the risk of contact with dangerous live parts should be prevented. The internal conducting parts of the electrical panels should be arranged so that when doors are opened, all live conductors above 50 Volts AC 120 Volts DC inside the panel are covered so that accidental or inadvertent contact cannot occur. BSEN 60204 – Electrical Equipment for Machinery sets the minimum standard of protection as IP2X (i.e. finger proof), this should be used as the minimum standard of protection of conductors.
It is also advised to build integral test circuits into motor control electrical panels to allow testing of control circuits, normally live at 110V AC, without the power circuits being energized where possible. These test circuits can also be retrofitted into existing panels to enable safe fault finding.
Mobile Equipment and Machinery
With regard to mobile equipment and machinery where a low voltage generator is used as the source of power the electrical installation should still follow the principles of the current version of BS7671, Requirements for Electrical Installations, and any manufacturer’s standards.
The earthing of mobile equipment and machinery should follow the principles set out in BS 7430 2011 Code of practice for protective earthing of electrical installations. For example.
- For equipment and machinery with an on board low voltage generator the power system neutral / star point should be connected to the frame of the mobile plant which would act as the system earthing point. The mobile plant need not be connected to the general mass of earth. However the frame or chassis of the mobile equipment should not be used as a live or neutral conductor.
- For equipment and machinery fed from a separate low voltage generator the power system neutral / star point should be connected to the general mass of earth via one or more earth electrodes. Where it is not practicable to install to install earth electrodes other measures must be taken to ensure the installation is safe and to comply with Regulation 8 of The Electricity at Work Regulations 1989 and associated guidance – HSR25.
Low voltage systems
The current version of BS7671, Requirements for Electrical Installations, should be the governing standard for the safety of electrical installations within quarries. Wire armoured cables used for fixed installations should generally be XLPE insulated and sized appropriately for the load and fault current. It is advised where possible that the armouring is not used as the sole means of earthing and a separate core, either internal or external, should be used for the circuit protective conductor. Where the armouring is used as the sole means of protection this should comply with the requirements of the current version of BS7671 Requirements for Electrical Installations. Cables should be correctly installed, supported along their length or located where there is no risk of them being damaged. Flexible cables, either armoured or non- armoured, may be used for certain applications such as mobile equipment and machinery, and moving machinery such as screens or feeders. But consideration must still be taken into account with regard to protection against mechanical damage.
Following completion of a new circuit, the installation electrician should provide a completion certificate in a format set out in appendix 6 of the current version of BS7671, Requirements for Electrical Installations, with measurements of earth fault loop impedance or line and protective earth conductor resistance measurements, commonly known as R1 and R2 values, from those values the time to disconnect in the case of fault can be determined.
Earthing of electrical installations.
Proper and effective earthing of the electrical system is necessary for ensuring safety of the electrical installation. There are a number of ways of earthing an electrical system, which are as follows.
- The site may have its own separate earthing point to which the supply system is connected.
- The Electricity Distribution Company may provide an earthing connection to which the supply system will be connected.
- For mobile equipment and machinery as defined in section 5.5 above.
- The use of protective multiple earthing (PME) systems is not advised in Quarries due the difficulty in ensuring the electrical bonding of all metalwork.
Whichever system is in place the earthing must be properly designed to ensure that the system is disconnected safely when a fault develops.
BS 7430 2011, Code of practice for protective earthing of electrical installations, the current version of BS7671, Requirements for Electrical Installations, and associated guidance notes should be used as guidance on earthing systems.
For earthing to be effective, the resistance of all earth paths from any equipment must be sufficiently low to ensure that should a fault occur the power will be disconnected in a short a time as possible and comply with the disconnection times set out in section 5.1 above.
High voltage systems.
High voltage equipment is technically complex and if not correctly managed can be a source of extreme danger. All high voltage systems should be placed under the control of a competent person (employee or contractor) who has the necessary competence to properly design, use and manage the HV system on behalf of the operator. Switchgear in excess of 30 years old should be separately assessed and replaced as and when appropriate.
High voltage switchgear should be installed in locked enclosures with access restricted to competent and authorised persons only.
High voltage installations are not within the scope of BS7671, Requirements for Electrical Installations. These installations therefore should be designed and installed by a person or company competent in high voltage systems and equipment.