Switch-Disconnector, Disconnector and Fuse Switch: Differences and Selection
switch-disconnector selection: A disconnector only provides safe isolation with no load current; a switch-disconnector makes and breaks the rated current on load; a fuse switch adds short-circuit protection through the fuse. Selection follows utilisation category, rated current and short-circuit withstand.
Contents
- Disconnector, switch-disconnector and circuit breaker: where definitions differ
- Utilisation categories: AC-21A, AC-22A and AC-23A
- Fuse switch or switch fuse: which architecture fits?
- Ratings: Ie, Icm, Icw and switching endurance
- Mounting, lockability and maintenance safety
- Key takeaways
- Frequently asked questions
- Related articles
Choosing the device at the incoming section or on an outgoing way is usually reduced to one question: disconnector, switch-disconnector or circuit breaker? They may look similar in the panel, but IEC 60947-3 defines them for different duties. One guarantees isolation, one switches under load, and one combines with a fuse to clear short circuits. A wrong choice compromises maintenance safety and protection coordination at the same time.
Disconnector, switch-disconnector and circuit breaker: where definitions differ
A disconnector guarantees a defined isolating distance in the open position; its duty is not to interrupt power but to prove that the supply is safely separated. A switch-disconnector adds the ability to make and break the rated current under load. A circuit breaker is designed to interrupt fault currents and is characterised by Icu and Ics. In a switchboard the three functions usually coexist: the breaker protects, the switch performs operational manoeuvres, and the isolating function makes maintenance possible.
Utilisation categories: AC-21A, AC-22A and AC-23A
The utilisation category describes how many operations a device can perform against a given load character. AC-21A covers mainly resistive loads, AC-22A mixed resistive and inductive loads, and AC-23A motor loads or highly inductive loads. Using an AC-21A device on a motor feeder means contacts wear far faster than expected under starting current and arc energy. In the catalogue the category is always read together with the rated operational current Ie and voltage Ue.
Fuse switch or switch fuse: which architecture fits?
In a fuse switch-disconnector the switching element and the fuse are in series and the operation is independent of the fuse carrier; in a switch fuse the fuse carrier itself moves. The first gives a more predictable isolation during maintenance, the second is more compact and saves space in sub-distribution. In both cases the fuse size, the gG or aM characteristic and the derating caused by temperature rise inside the base are part of the selection.
Ratings: Ie, Icm, Icw and switching endurance
| Device | Breaking on load | Short-circuit breaking | Typical use |
|---|---|---|---|
| Disconnector | No | No | Safe isolation before maintenance |
| Switch-disconnector | Yes (up to Ie) | No | Incoming and outgoing switching, transfer |
| Fuse switch-disconnector | Yes | Yes (through the fuse) | Sub-distribution, transformer and motor feeders |
| Circuit breaker (MCCB / ACB) | Yes | Yes (Icu / Ics) | Main distribution and selective protection |
A switch-disconnector does not interrupt short-circuit current, so catalogues state the short-time withstand current Icw and the rated short-circuit making capacity Icm instead of a breaking capacity. These values are compared with the current and energy that the upstream protective device will let through. Mechanical and electrical endurance figures matter as well: in a transfer application operated several times a day, endurance can be as decisive as the rated current.
Mounting, lockability and maintenance safety
The main isolating device is arranged for door operation with a clearly readable position indicator. A lockable mechanism is the physical basis of the lockout-tagout procedure, so the number of lock holes and the padlock diameter are checked at the start of the project. Tightening torques at busbar connections, lug selection matching the conductor cross-section and the required clearances are evaluated together with the form of separation.
Key takeaways: switch-disconnector selection
- Category choice — AC-23A is defined for motor loads, AC-22A for mixed loads and AC-21A for mainly resistive loads.
- Short-circuit match — A switch-disconnector does not clear faults; Icw and Icm are coordinated with the upstream device.
- Safe isolation — Position indication, isolating distance and a lockable mechanism are required together.
- Fuse compatibility — NH size, gG or aM characteristic and base temperature derating are checked together.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What is the basic difference between a switch-disconnector and a circuit breaker?
A switch-disconnector makes and breaks normal operating current on load but does not interrupt short-circuit current. A circuit breaker is designed to clear fault currents and is defined by Icu and Ics. In a switchboard the two are usually used together.
Why is category AC-23A required on motor feeders?
Motor starting current is a multiple of the rated value, and at interruption the inductive energy appears as an arc across the contacts. AC-23A proves that the device can perform the expected number of operations safely under those conditions.
How is the fuse rating chosen in a fuse switch-disconnector?
The continuous load current, cable current-carrying capacity and the required short-circuit protection level are evaluated together. Distribution circuits usually use gG, while motor feeders prefer the aM characteristic that withstands starting current.
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Let us define the main isolating device of your switchboard together, based on load type, short-circuit figures and your maintenance scenario; the Devpan engineering team supports you through selection and verification. Related Devpan solution: medium voltage switchgear.
