Switching off the supply of electricity to allow for safe working is legally mandated, with work on or near to live electrical systems permitted in exceptional circumstances only. Electrical isolation consists of two clear stages: switching off the supply and proving dead. It is important you follow a safe isolation procedure.
Why do we need electrical isolation?
It is vital for safety. The consequences of failing to undertake electrical safe isolation can be dangerous and life-changing. Complacency and incompetence are often the underlying cause. HSE publication HSG85 Electricity at Work, Safe Working Practices is a good source of guidance on the requirements for working on systems made dead and for working on or near to live electrical systems.
Safe isolation procedure
Whilst the applications may differ, the 7 steps for safe isolation is similar for low voltage and for high voltage. The process goes as follows:
- identify suitable point(s) for isolation
- carry out the safe isolation process
- secure the point of isolation (place notices)
- prove dead
- apply safety earths
- place danger notices
- issue electrical permit to work
Identify suitable point(s) for electrical isolation
All works must be planned, the level of planning will depend on the type of work to be carried out and the system that it is to be carried out on. For works on a socket circuit in a domestic environment the planning may be quite informal- checking circuit designations, identifying the circuit breaker to be turned off and getting the children to save their game before they lose power, but on high energy circuits and for high voltage, planning will include reference to record drawings, development of an Isolation and Earthing Diagram and the creation of a safety programme (also known as a switching schedule).
There may be alternative sources of supply, such as interlinks or generator feeds, embedded generation will also need to be taken into account as will UPS supplies and impressed voltages. All of these will need to be taken into account. Neutral connections must be given special consideration- are they to remain connected, or are they to be disconnected as part of the isolation process?
Remember, permission must be given for the power to be isolated, and the work scheduled accordingly- however, refusal to give permission to isolate the supply does not mean that work on or near to live systems is authorised!
Remember, special consideration must be given to plant and equipment that is stationary on arrival, as it may be difficult to confirm that the correct point of isolation has been located. For example, a motor that is part of a process may be non-operational due to a fault, because it is isolated, or because the control system has not energised it at that point in time.
Carry out the safe isolation process
Carrying out the safe isolation process includes cutting off the supply and isolation. In most cases for low voltage, both requirements are completed by the same action, such as turning off an isolator or switching off an MCB, but on high energy circuit breakers and on high voltage systems, cutting off the supply and isolation are two distinct operations, and the system cannot be regarded as being safely disconnected until both have been completed.
For example, the ACB shown here, requires the ‘off’ button to be pressed to achieve the requirements for switching off the supply, and the circuit breaker to be racked out to the ‘isolate’ position to achieve the isolation requirements.
Secure the point of isolation (place notices)
All points of isolation must be secured against inadvertent reclosure. This is usually done using a safety lock with a single, unique, key. The person who has undertaken the isolation retains the key to the safety lock. If more than one person is involved in the work, then a multi-lock hasp is used, and each person applies their own safety lock to the point of isolation. For more complex isolations, other systems of work exist, but the principle of a safety lock for each person working on that system remains.
Circuit breakers, fuses and isolators are all capable of being secured in the open position. Many devices have been developed over the years to achieve this, even when locking off facilities were not incorporated into devices at the time of manufacture. There really are no excuses for not locking off, and the available devices are cheap and plentiful.
Circuit breakers, fuses and isolators are all capable of being secured in the open position and many devices have been developed over the years to achieve this, even when locking off facilities were not incorporated into devices at the time of manufacture. There really are no excuses for not locking off, and the available devices are cheap and plentiful.
This should be second nature to anyone involved in electrical work and should be completed using a suitable Approved Voltage Indicator or test lamp. This should be visually inspected before use and its correct functioning should be verified on a known live supply, such as a proving unit.
A test should be carried out at the point of work to confirm that all conductors are free of voltage or charge, and that conductors are dead in relation to earth, before re-checking that the approved voltage indicator or test lamp is still functional by proving on a known live supply such as a proving unit. It is only at this point that the circuit or equipment should be regarded as being dead.
This is normally not required for low voltage systems, but is part of normal practice at high voltage, with earthing normally being in the form of circuit main earth (CME) applied at the isolation device or protection equipment through various means. Earthing devices, including portable earths must be fault-rated.
There may be a need to earth conductors at low voltage, particularly where impressed voltages may appear on systems that have been made dead- ‘if in doubt, ground it out’.
Place danger notices
This falls under the requirement to take suitable precautions against adjacent live parts, where necessary. There may be no need to place notices, but these serve as a helpful reminder that adjacent cables and equipment have not been made safe, and are intended to provide clarity on how far the created safe work zone extends.
Issue electrical permit to work
For works on high voltage systems and primary low voltage systems the need for a permit to work is clearly understood, such documents should only be issued and cancelled by Authorised Persons. A permit to work should only ever be issued to, and cleared by a Competent Person. Permit to work may also be required when works involves isolation of multiple sources of supply, or where isolation is undertaken on behalf of a third-party.
For competent persons undertaking local isolation of circuits and equipment for their own purposes, the need for a permit to work may not contribute towards safety, and therefore is not required, however, individual clients and sites will have their own rules around this matter.
The need to isolate the electrical supply before commencing work should be well understood, and the consequences of getting it wrong can be fatal. Working on systems that are live, or are of an unknown status is unacceptable, no matter how small the job is, how difficult it is to turn off the power supply or how imperative it is to keep production running.
Best practice for electrical workers and employers
Safe isolation of electrical supplies is something that every company assumes that their maintenance and installation teams are trained to undertake. However, it is often the case that individuals have not received any formal training in such tasks or any training given was so long ago that the basic principles have been forgotten. The Electricity at Work Regulations 1989 requires that those engaged in electrical activities have the knowledge and experience to undertake work safely. Many employers fail with regards to this in that they are often unable to demonstrate that their staff have been deemed competent to undertake safe isolation tasks.
It is often the case that operatives do not fully understand the implications of live working and will attempt many tasks with the power supply connected. Alternatively, they will undertake part of the safe isolation process but omit essential sections such as locking off the supply or fitting a notice, or proving dead using a non-approved device or failing to verify the correct functioning of a voltage indicator both before and immediately after use. At ESUK we have the perfect training course to ensure that your staff are trained in safe isolation procedures and have been observed to ensure that they know how to do it right. We also have Authorising Engineer capabilities to provide an independent assessment of the competency of your staff. Feel free to contact us to find out more.