FEATURE SWITCHES & RELAYS
SAFETY’S SALVATION – the modern switch-disconnector
As demand for electricity becomes ever greater, so does the need for specialist switches to isolate circuits, machines or even entire buildings. Switchtec’s Technical Director Jeremy Lester explains the various features of the switch-disconnector
manufacturers provide red and yellow ones for emergency use. Because we cannot see electricity, it is difficult to know if a circuit is live when there is no equipment connected to it. Even then, if that piece of equipment does not power up, it is no guarantee that the circuit is dead. Conforming to IEC/EN60947-3, Telergon switch- disconnectors include a visible on-off indicator, confirming circuit status within an instant. Depending on the magnitude of currents
involved, the terminals on a switch- disconnector are either screw clamps or lugs to accommodate bolt-on cables. It is advisable to choose a switch-disconnector that features generous cable entries to accept all possible sizes of cabling, as it can be dangerous to attempt to terminate too many conductors where they cannot be accommodated. For applications where space is restricted, it is a
T
he need to electrically isolate machinery and buildings is an important safety aspect of the modern workplace. The switch- disconnector is the last – and hence very important - part of a control system that must guarantee isolation, particularly under fault conditions. Switch disconnectors offer a more reliable method of isolation than circuit- breakers, and are designed to switch on and off under load conditions. A circuit-breaker is only designed to open the circuit under over- currents; and of course switch-disconnectors are much less expensive.
ARCING The difference between an ordinary ‘knife’ switch and an independent switch- disconnector is that with the former, the operator can (inadvertently) control the speed at which the contacts open and close – which could lead to arcing. An independent switch- disconnector is ‘independent’ of this effect because when its handle is operated, a spring is charged. When this spring reaches a certain tension it rapidly opens (or closes) the contacts in a fraction of a second. So the operator has absolutely no influence over switching speed, thus arcing of contacts is minimised. Indeed, for a switch-disconnector to meet
IEC/EN60947-3 international standard, it must be capable of closing under short-circuit conditions. To further minimise arching, self-
26 SEPTEMBER 2015 | ELECTRONICS
wiping contacts reduce the transfer of metal from one contact to another during switching under heavy load.
ELECTRICAL ISOLATOR It is important to purchase a switch- disconnector that is capable – without limitation – to fulfil its requirement as an electrical isolator. This may sound obvious, but there are units on the market that are simply not up to the job. For example, the rod that connects the handle to the switch itself is sometimes supplied as plastic units that can be clipped together to form the desired length. This can lead to flexing in the rod, resulting in poor action and worse, even fracture. Telergon switch- disconnectors are supplied with a one- piece steel shaft that can be cut to length using just a hacksaw. As a result, the on-off action between handle and switch has a firm, positive feel. The handle itself should provide a facility to accommodate a padlock, allowing the switch to be secured in the off position. This ensures that only authorised personnel are able to reconnect the power. Colour coding of handles is – in the case of Telergon units – navy blue for general switching duties, and most
Figure 1:
A housed Telergon 3-pole and neutral switch disconnector
good idea to opt for a compact switch- disconnector with built in fuse protection. Offered in the 32 to 800A range, Telergon’s
“For
applications where space is
restricted, it is a good idea to opt for a compact switch-disconnector with built in fuse protection ...”
M11/M21 Series features short circuit protection with load break switching on both sides of the fuse for maximum isolation. During fuse replacement and general maintenance situations, the fuse cover cannot be removed when the switch is in the ‘on’ position, while isolated links on both sides of the fuse ensure the operator is not exposed to live terminals. The early make/late break neutral pole switch-disconnector means that when it is switched on, the neutral makes contact before the three live poles, and when switching off, breaks contact afterwards. This ensures that equipment ‘down the line’ will operate properly as the supply remains as clean as possible,
increasing equipment reliability and complying with IEC/EN60947-1.
SOURCE TO SOURCE The overlapped bypass switch is the latest development in switch technology. It is used to switch from one source of power to another, but ensures that there is no interruption in supply during the switching process.
Switchtec
www.switchtec.co.uk 01785 818 600
/ ELECTRONICS
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