Electrical Switchgear
National and international standards define the method for making electrical distribution circuits as well as the purpose and functions of switchgear. There are three main functions, namely:
- Circuit protection takes three main fault types into account:
- Overloads,
- Short-circuits (Both of which adversely affect the lifetime of cables and loads)
- Insulation faults, harmful for persons and equipment.
- Isolation is used to separate a circuit or a device from the remainder of the installation to allow 100% safe operation,
- Control allows the user to take voluntary action on installation operation:
- When such action takes place in normal operating conditions (on load, with no overcurrent) in order to energise or de-energise all or part of the installation, control is said to be “functional”,
- When such action is essential (whatever the load condition of the installation) in order to immediately de-energise all or part of the installation, control is said to be of the “emergency stop” kind.
Three main functions of electrical circuits:
- Protection,
- Isolation,
- Control,
The switch is used for:
- Control,
- Often isolation.
For protection it must be combined with:
- Circuit-breaker,
- Fuses,
- Residual current device (if necessary).
A variety of switchgear perform all or some of these three main functions. The following table situates the role of the main switchgear:
| Functions perfomed Protections | Isolation | Control | |||
| Switchgear | Overloads | Short-circuit | Insulation-fault | ||
| Fuse | Yes (3) | Yes | No | No | No |
| Disconnector | No | No | No | Yes | No |
| Switch | No | No | Yes? (1)(2) | Yes? | Yes |
| Disconnector – fuse | Yes | Yes | No | Yes | No |
| Switch – fuse | Yes | Yes | Yes? (1)(2) | Yes? | Yes |
| Contactor | No | No | No | No | Yes |
| Circuit-breaker | Yes | Yes | Yes? (2) | Yes? | Yes |
Yes – Basic function of the switchgear considered (always performed)
Yes? – Additional function possible (not always performed)
(1) Possible with a switch with automatic opening.
(2) Possible with addition of a “Residual Current Device (RCD)“.
(3) gG fuse only
Resource // Schneider Electric – Low voltage switches
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