Understanding Switch-disconnectors: Types and Wiring Diagrams

Short Answer

A changeover switch (or multi-position switch) is a manual switching device where contacts are arranged in a stack of identical sections or "packages" on a common shaft, allowing a single handle to switch multiple circuits simultaneously. A cam switch performs the same function, but each contact group is operated by a profiled cam on the shaft. Below, we examine the structural differences, selection criteria, and typical connection diagrams.

Both changeover and cam switches serve the same purpose: manual switching of multiple circuits using a single handle according to a pre-defined sequence. The difference lies in the contact system construction: in a changeover switch, contacts are assembled in a stack of identical "packages" on a common shaft; in a cam switch, each contact group is opened and closed by a profiled cam on the shaft. The cam switch design offers greater flexibility, allowing for any switching diagram, up to 8–12 or more poles, precise position fixing, and compact mounting on a cabinet door using a 48x48 mm cutout. Changeover switch series are simpler and more cost-effective, typically used for "on/off" functions and reversing small distribution boards.

Selection is based on four parameters: the rated continuous current $I_u$ (available from 10 A to 125 A, with common values of 10, 16, 25, 40, and 63 A), the number of switched poles (most commonly 1–4, or 6, 8, 12 for complex circuits), the number of fixed handle positions (typically 2 or 3 for most devices, up to 8–12 for multi-step models), and the degree of protection for the front face (ranging from IP20 for installation inside a distribution board to IP65 for devices in enclosures on an open panel). The following sections explain how these parameters relate to specific applications.

Changeover vs Cam Switch: What is the difference?

Both devices are multi-position rotary manual switches. Historically, changeover switches were the first to emerge: the contact system is built from identical insulating "packages," each containing a pair of fixed contacts and a moving contact plate on a common shaft. The mechanism allows for rapid switching regardless of the handle rotation speed, but the switching sequence is strictly limited by the package construction.

Cam switches (such as the LW32 series and similar) operate differently: cam discs are mounted on a shaft, where the profile of each disc determines which contact groups are closed at specific handle positions. By changing the set of cams, a manufacturer can use a single platform to produce a "0-1" switch, a reversing "1-0-2" switch, a star-delta starter, or a four-position ammeter switch—essentially any switching diagram. This versatility is why cam switches dominate in control circuits.

Criterion	Changeover/Transfer switch	Cam switch
Contact system	A stack of identical contact sets on a common shaft	Individual contact groups operated by cams
Switching diagram	Limited by construction: ON/OFF, reversing, or simple circuits	Virtually arbitrary, determined by the cam profiles
Number of poles	Typically 1–4	1–4 is common; multi-step series offer 6–12 or more
Number of positions	2–3	2–8 or more (models with 10–12 positions are available)
Mounting	Usually mounted on a base or DIN rail inside an enclosure, or in a housing	Panel mounted (typical 48x48 mm cutout) or mounted on a cabinet door
Typical application	Main switch, reversing small loads	Control circuits, measurement, stepped and reversing schemes

Practical conclusion: if you only need simple ON/OFF switching inside an enclosure, a changeover switch is more cost-effective. If the device is to be mounted on a cabinet door, used for control circuits, or requires a non-standard switching diagram, choose a cam switch.

Selection by Current and Voltage

The primary electrical parameter is the rated continuous current, Iu. This category includes devices ranging from 10 A to 125 A; the most common ratings are:

• Current margin. For continuous load circuits, select a device such that the operating current does not exceed 80–90% of $I_u$; this ensures the contact system lasts longer, especially during frequent switching operations.

Number of poles and positions

How to relate the circuit to poles and positions

• On-Off load switch: 2 positions; poles correspond to the number of conductors being switched (1P for control circuits, 2P for phase + neutral, 3P/4P for three-phase inputs).
• Changeover switch (1-0-2): 3 positions (including a central OFF position); poles correspond to the number of conductors switched for each direction.
• Motor reversing switch: 3 positions, 3 poles, with two phases cross-connected in the second position.
• Three-phase ammeter selector switch: 4 positions (L1-L2-L3-0), 4 poles; the device switches the current transformer circuits without breaking them (contacts operate with overlap).
• Voltmeter selector switch: up to 7 positions — three phase-to-phase voltages, three phase-to-neutral voltages, and "0"; this is a distinct category of apparatus with its own specific diagram.

Note the distinction between "with zero position" and "with spring return to zero position". The former indicates a fixed "all off" position; the latter features a spring-loaded handle that returns to centre automatically (required for push-button control schemes; most standard cam switches have fixed positions).

Typical Application Diagrams

Main-Standby (I-0-II)

A classic application for distribution boards with two power sources: the main supply is position I, the standby supply is position II, and the central position 0 ensures isolation between the sources. For a single-phase board, a 2-pole "1-0-2" switch (switching phase and neutral) is sufficient; for three-phase, a 4-pole switch is required. The current rating is selected based on the incoming MCB: for a 25 A board, use a 25–40 A switch. Important: a standard cam switch is a manual ATS (Automatic Transfer Switch) device; it does not perform automatic switching and will only work with a generating set if the generator is started manually.

Motor Reversing

A reversing switch swaps two phases when the handle is moved from position 1 to position 2 via the zero position. Direct reversing through the power contacts is permissible for small motors (refer to the specific device's datasheet for power limits, as they depend on the series and current rating); for more powerful drives, the switch is placed in the control circuit of two contactors, in which case a 10 A rated device is sufficient. Below is an example of a complete reversing switch in an enclosure:

Ammeter and Voltmeter Selectors

To avoid installing three separate ammeters, a single instrument is connected to the secondary windings of the current transformers (CTs) via an ammeter selector: the handle allows the user to select phase L1, L2, L3, or to disconnect the instrument. A critical requirement is that the CT secondary circuit must never be opened under load; therefore, ammeter selectors are designed with make-before-break contacts (short-circuiting the CT winding during the switching transition). Conversely, a voltmeter selector operates with a break-before-make mechanism to select the measured pair: L1-L2, L2-L3, L3-L1, or phase-to-neutral voltages. A standard changeover switch cannot be used for ammeter applications specifically because it lacks the necessary make-before-break contact overlap.

Step Selectors

Step selectors are the most common type in this category. Typical applications include fan speed control, heating element stages, lighting group selection, and transformer tap changing. The number of positions equals the number of steps plus zero; the number of poles corresponds to the number of circuits being switched at each step. For star-delta motor starters, specialised diagrams are available to ensure the correct switching sequence.

Comparison: Three Cam Switches for Different Applications

Using a single product series as an example, we can see how the device configuration changes based on the task: single-direction (1P), two-conductor (2P), and ammeter configuration (4P, 4 positions):

Installation and Execution

The installation options available in this category include:

• Panel mounting — the primary method for cam switches: the body is mounted behind the panel, with the handle and position plate on the front; the standard front panel size is 48x48 mm. The ingress protection for the front face of most such switches is IP44, while versions with seals offer IP65/IP56.
• Internal mounting — open versions (IP00–IP20) for installation on a base or mounting plate; protection is provided by the enclosure of the distribution board itself.
• Enclosed assembly — the switch is housed in its own enclosure for surface mounting near machinery or on a wall; refer to the IP rating of the complete enclosure.

Regarding the actuator, the vast majority of switches feature a short rotary handle; versions with toggle switches, key-operated handles (for unauthorised switching protection), and push-button types are also available. For main switches requiring lockout functionality during maintenance, handles with locks are provided—a padlock secures the device in the OFF position.

Common selection errors

1. Selecting by current rating without considering load characteristics. A 16 A rating for a low-inductance circuit is not equivalent to 16 A for a motor under load. For motor circuits, refer to the permissible power rating specified in the device's technical datasheet.
2. Using a standard switch instead of an ammeter-rated switch. Breaking the secondary circuit of a current transformer (CT) while under load leads to dangerous overvoltage on the winding. In a CT circuit, use only devices designed for short-circuiting contacts.
3. Using a two-pole device for a three-phase main-reserve input. All active conductors must be switched; for a system with a neutral, a four-pole version must be used.
4. Main-reserve switching without a neutral/off position. A "1-2" switching scheme without a central "0" position does not provide a guaranteed break between sources; for manual automatic transfer switch (ATS) applications, select a "1-0-2" diagram.
5. IP20 rating on a workshop enclosure door. The protection degree of the front face must match the environment: for dusty or damp rooms, use IP54/IP65; for outdoor use, use weather-protected enclosures.
6. Ignoring auxiliary contacts. If the circuit requires position signalling (e.g., a "reserve engaged" lamp or a signal to a controller), select a version with integrated auxiliary NO/NC contacts immediately—it is often impossible to retroactively add them to a device once it is mounted.

Standards

Low-voltage switchgear, disconnectors, and load break switches are manufactured according to EN/IEC 60947-3; the utilization categories (AC-21, AC-22, AC-23) defined in this standard determine the current the device can switch for different load types. General requirements for switchgear and controlgear are covered by EN/IEC 60947-1. When designing distribution boards, comply with HD 60364 requirements for switching devices: the ability to disconnect all phase conductors, the accessibility of operating mechanisms, and clear position marking. The handle position indicator is not decorative but a safety requirement: the operator must be able to clearly see which circuit is closed.

Catalogue contents

Standards and regulations

The requirements discussed in this article follow from these standards used in the European Union:

• EN IEC 60947-3 — Low-voltage switchgear and controlgear — Part 3: switches, disconnectors, switch-disconnectors and fuse-combination units
• EN IEC 60947-1 — Low-voltage switchgear and controlgear — Part 1: general rules