A switch is a switch…. is not it ?
The subject of switching is often misunderstood, whether for emergency purposes or for mechanical maintenance. While they both seem to have a similar result with fairly straightforward requirements, they have very different implications and can ultimately have serious repercussions when misinterpreted.
Mechanical maintenance shutdown (Chapter 46, Article 464)
As the name suggests here, we are looking to turn off a piece of equipment to perform some form of isolation, which can fall into one of the following categories:
(planned preventive – proactive)
(by damage – reactive)
(through damage, end of useful life or upgrade)
The maintenance carried out may well relate to the non-electrical mechanical parts of a piece of equipment. We switch or isolate the electrical side for the safety of the engineer.
Mechanical switching for this can take many forms, from rotary isolators in an industrial setting to shower or extractor fan isolators in a more domestic setting, Fig. 1 gives representative examples of these devices.
There are a few places where mechanical switching is required in BS 7671, but the one we generally give the most credit to is regulation 132.15.202, which requires every stationary electric motor to have switching that is easily accessible and easy to use.
This regulation does not state that this is an emergency requirement or exactly where switching should be.
Regarding the regulation, this is mainly aimed at industrial and commercial installations to ensure that larger motors can be isolated quickly and safely if necessary; this does not mean in an emergency, see Figure 2 for examples.
Of course, from a domestic scenario, we see motors switched in accordance with this, more by accident than from a regulatory point of view. Bathroom extractors will often have their own multi-pole isolator; cooktop hoods will often be powered by a fusion spur, as will central heating boilers, which will invariably contain an engine of some description.
It can get a bit trickier when household exhaust fans are powered by lighting circuits, but the key here is accessible.
The power consumption unit (UC) must be accessible, which means that we can switch and isolate if necessary, giving us the required multi-pole isolation, all live conductors required by regulation 464.1. Provided, that is to say that we maintain control of the installation throughout any maintenance or repair, see regulation 464.2 for clarity; it can be as simple as a lock kit, a lockable isolator or, for more complex installations, the use of a controlled labeling system.
I should point out that this could also be different in a commercial or industrial environment, where the designer may want to isolate an individual motor for maintenance or where standby and service motors are in place.
So we can see where the confusion can start to set in, where “safe shutdown” stops under regulation 132.15.202, and emergency switching takes over, and to what extent?
Emergency stop (Chapter 46, Section 465)
As the title suggests, we seek to shut down in an emergency, which means that there is an expected or unexpected danger that we need to act on to effectively control the supply. It can be seen immediately that this differs considerably from a simple switchover for mechanical maintenance.
The general principles are the same, with multi-pole insulators disconnecting all live conductors, see Regulation 465.2, but there are some subtle additional requirements and differences.
Switching devices themselves are usually marked specifically for this purpose, and a key requirement is that only one action operates them (See Reg 465.3).
When we say an action, we mean that we are not trying to hold a switch and then turn it, which would probably be two actions; we are looking to trigger the device almost instantly. To instantly disconnect this way we use push to isolate using mushroom style switches, see Figure 3 and Figure 4 for examples of emergency switches.
These push to switch devices are often spring loaded and, once near their locking point, will use the power of the spring to complete the action and lock the switch.
In order to reset the switch, there may be special procedures for this, incorporating mechanical and / or electrical interlocks, to prevent the inadvertent re-energization of an isolated power supply, which may cause a danger (See Regulation 465.4). .
In addition, in addition to these already complex arrangements, it is necessary to ensure that when an emergency switch is actuated, it does not directly or indirectly injure, or endanger, any person associated or likely to ” be affected by the primary insulation by introducing a secondary danger, by the isolation of the primary fault (See Regulation 465.4).
A key area that defines the urgency of maintenance switching is the careful evaluation and design of each system or process. This must be done to avoid the potential ripple effect mentioned previously and clarified in Regulation 465.4.
This will require a risk assessment and an office design process to take into account all possible processes and outcomes; on a production line in an industrial environment, it is a colossal undertaking and critical for the safety of operators or any person coming into contact with an installation or a production line.
Some of the areas that should be considered in a risk assessment and design process, but not limited to:
• Assessment of environmental influences
• Type of process and complexity
• Number of process operations
• Skill level, training and understanding of operators
• Possible injuries due to malfunction or breakdown
• Possible malfunctions or breakdowns
• Level of interaction with the work areas of the equipment
• Historical injuries caused by similar industrial processes
• New innovations to reduce the danger
• Automatic isolation of previous or subsequent operations for safety thanks to emergency isolation of the operation
We can see that the requirements for these two switching methods are different and very specific.
Both have security implications, but emergency switching has unique requirements that almost require in-depth attention and risk assessments before you install them.
Advice on understanding risks, what risk assessments are and how to perform them is given in the NAPIT publication, Code of practice for risk assessment and management. Available as an eBook from www.napitdirect.co.uk