Various types of devices are available for carrying out the switching and protection tasks listed under a title that are specially designed to fulfil the respective requirements. The various parts of IEC 60947 (Low voltage switchgear and controlgear) specify the design, performance and test features of the devices.
Differences between disconnectors, load switches, switch disconnectors and circuit breakers (on photo: Low-voltage disconnect switch max. 1000A – SIEMENS via directindustry.com)
The most important features of the main device types are presented below:
Disconnectors (isolating switches)
Load switches
Switch disconnectors
Circuit breakers
1| Disconnectors (Isolating Switches)
The disconnector is a mechanical device that fulfills in the open position the requirements specified for the isolation function (IEC 60947-1).
!!! The purpose of the isolating function is to cut off the supply from all or a discrete section of the installation by separating the installation or section from every source of electrical energy for reasons of safety.
The key factor here is the opening distance. Isolation must be guaranteed from pole to pole and from input to output, whether this is by means of a visible isolation gap or by suitable design features within the device (a mechanical interlocking mechanism).
A device fulfills the isolating function stipulated under IEC 60947-1 when in the “Open” position the isolation at a defined withstand voltage is assured between the open contacts of the main circuit of the switchgear.
It must also be equipped with an indicator device in relation to the position of the movable contacts. This position indicator must be linked in a secure, reliable way to the actuator, whereby the position indicator can also serve as an actuator, provided that it can only display the position “Open” in the “OFF” position when all moving contacts are in the “Open” position. This is to be verified by testing.
According to IEC 60947-3, an isolator must only be able to make and break a circuit, if either a current of negligible size is switched on or off, or if during switching no noticeable voltage difference between the terminals of each pole occurs.
"Under normal conditions, it can conduct operational currents as well as under abnormal conditions larger currents (e.g. short-circuit currents) for a certain period.
The isolator function can be realized with a variety of devices such as for example in disconnectors, fuse-disconnectors, switch-disconnectors, fuse-switch disconnectors and circuit breakers with isolating functions.
2| Load Switches
Load switches (or only “switches”) are mechanical switching devices capable of making, carrying and breaking currents under normal circuit conditions which may include specified operating overload conditions and also carrying for a specified time current under specified abnormal circuit conditions such as those of short-circuit.
!!! A load switch may have a short-circuit making capacity, however it does not have a short-circuit breaking capacity (IEC 60947-1 and -3).
Short-circuit currents can be conducted (high short-circuit withstand capacity), but not be switched off.
For load switches the range of designs is similarly wide as for isolator switches, for example, “normal” (load) switches, fuse-switches, circuit breakers.
Fuse-switches are not legally permitted in all countries.
3| Switch Disconnectors
Switch disconnectors; Left – ABB’s motorized switch disconnector 160-2500A; Right – Schneider Electric’s low-voltage disconnect switch with free tripping 80-3200A (photo credit: directindustry.com)
Switch disconnectors combine the properties of (load) switches and disconnectors. In this case, too, there are a variety of designs such as “normal” switch disconnectors, fuse- switch-disconnectors and circuit breakers.
Fuse-switch-disconnectors are not legally permitted in all countries.
4| Circuit Breakers
Circuit breakers are mechanical switching devices, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit (IEC 60947-2).
They thus also fulfil the requirements of (load) switches. Circuit breakers are often designed so that they can fulfil the requirements for disconnectors
