IEC 60947-3: Switches, Disconnectors, Switch-disconnectors, and Fuse

IEC 60947-3

IEC 60947-3 switches and disconnectors

The difference between switching and safe isolation

Turning a panel switch does not mean that one device performs load switching, isolation for maintenance and overcurrent interruption. Correct selection starts with understanding which functions the device performs and under what conditions.

This part of the IEC 60947 series covers switches, disconnectors, switch-disconnectors and fuse-combination units. Its scope includes distribution and motor circuits rated up to 1,000 V AC or 1,500 V DC. Those scope limits do not mean that every product is suitable for those voltages.

The IEC catalogue lists IEC 60947-3:2020+AMD1:2025 CSV, combining the 2020 publication with the 2025 amendment. Assess a product using its declared edition, ratings and application conditions together.

Four device groups with different functions

Switch: Makes, carries and breaks current under its declared load conditions. Load-switching capability alone does not establish suitability for isolation.

Disconnector: Provides the isolating function in its open position. Do not assume that it can interrupt load current; suitability for load breaking must be established separately.

Switch-disconnector: Combines switching under specified conditions with the isolating function. It is used where a machine supply needs both functions.

Fuse-combination unit: Combines a switching or disconnecting device with fuses. Protection depends on the fuse type, rating and verified combination with the device. A fuse carrier does not make every compatible-looking fuse suitable.

Why the utilization category matters as much as amperes

AC-20 describes no-load connection and disconnection; AC-21 covers resistive loads; AC-22 covers mixed resistive and inductive loads; and AC-23 covers motor or highly inductive loads. The suffixes A and B distinguish frequent and infrequent operation. Identical current figures do not make different categories interchangeable.

For a concrete example, Socomec COMO 21153403 data gives Iu as 32 A, an AC-21 rating of 32 A at 400 V and an AC-23 rating of 22 A at 400 V. A motor load drawing 25 A therefore cannot be approved merely because the product is described as a “32 A device”. These figures are not universal for all 32 A products.

Frequent motor starting and stopping requires a separate assessment. The motor-control duty of contactors differs from the duty of a switch-disconnector used for maintenance. An AC-23 marking does not make a device a suitable contactor for every motor-starting cycle.

Withstanding a short circuit is different from clearing it

Icw is the rated short-time withstand current and must be read with its stated duration. A conditional short-circuit current applies with a specified protective device; manufacturer documentation may use the symbol Iq. Neither rating establishes that the device automatically interrupts a short circuit on its own.

The protective fuse or circuit-breaker's type, rating and protective characteristics must match the manufacturer's coordination data. Values in kA declared under different conditions cannot be substituted for one another. Other ratings, such as SCCR, must also be interpreted within their own standard's conditions.

Separate AC, DC and photovoltaic duties

An AC rating cannot simply be transferred to a DC circuit. For DC, check the declared operating voltage, load characteristics, pole arrangement and current direction. Follow the device diagram for any series-connected poles or polarity requirements; do not guess the required number of poles.

For photovoltaic applications, DC-PV0 concerns disconnection without current flowing; DC-PV1 concerns applications without reverse current or significant overcurrent; and DC-PV2 covers applications where significant overcurrent and current flow in both directions may occur. A PV rating alone does not establish suitability for a battery circuit's fault conditions. Select DC fuses for the actual conditions of the same circuit.

Installing the device in a panel or machine

Read the rated operational current Ie for the relevant voltage and category. Installation must account for ambient temperature, enclosure conditions, conductor size, terminal torque and permitted connections. Device conformity does not automatically complete all verification of the IEC 61439 assembly.

For a fused solution, consider the role of the fuse under IEC 60269 together with that of the switching device. IEC 60204-1 is also relevant to isolation of a machine supply. Maintenance planning must address isolation, prevention of re-energisation and verification that the equipment is dead together.

In a system containing a variable frequency drive, a stopped motor does not establish that electrical energy has been removed. Assess stored energy, auxiliary supplies and possible backfeeds. Competent personnel must follow the equipment and system instructions for waiting periods and verification.

Editor's note: Setting a switch-disconnector to 0 does not establish that the panel's incoming terminals or sections fed from another source are also dead. Do not confuse handle position with proving the absence of voltage at the point of work. Identify the isolation boundary and every energy source explicitly.

Related switchgear and controlgear standards

IEC 60947-1 provides general rules; IEC 60947-2 addresses circuit-breakers; and IEC 60947-4-1 covers electromechanical contactors and motor starters. IEC 60947-5-1 concerns control circuit devices and switching elements. IEC 60947-6-1 is the relevant part for transfer switching equipment used between power sources.