Utilisation categories define how demanding a switching task is. They are specified in IEC 60947 and classify loads based on their electrical behaviour during switching.

They consider the type of load, how it operates, and the stress placed on contacts. Instead of focusing only on current ratings, they account for inrush currents, transients, and arcing.

Why are utilisation categories important?

Loads behave differently when switched. A heater is easy to switch, while a motor can draw several times its rated current during startup.

If switchgear is incorrectly rated:

• • Contacts can overheat
• • Arcing increases
• • Equipment fails prematurely

Utilisation categories ensure the device is suitable for real operating conditions.

How do different loads behave?

Resistive loads are simple. Current is stable, and switching stress is low.

Inductive loads, such as motors and transformers, draw high inrush current and produce voltage spikes. Arcing is more severe.

Capacitive loads can create very high inrush currents in a short time, placing heavy stress on contacts.

What does AC-1 mean?

AC-1 applies to resistive or slightly inductive loads, such as heaters.

Switching stress is low. Devices rated AC-1 are not suitable for inductive loads.

Why is AC-3 commonly used?

AC-3 applies to squirrel cage motors used in pumps, fans, and compressors.

It accounts for high inrush current during startup. Devices rated AC-3 can safely switch motors under normal operating conditions.

When is AC-4 required?

AC-4 is used for severe motor duty, including frequent starting, stopping, plugging, and inching.

These conditions create high current and heavy arcing. AC-4 devices are built for this level of stress.

How are lighting and transformers classified?

AC-5 covers lighting loads, which can have high inrush current when switched.

AC-6 applies to transformers and capacitor banks. These loads produce strong transients and require suitable switching devices.

What are AC-20 to AC-23 categories?

These categories apply to switches and isolators.

AC-20 is for no-load switching. AC-21 covers resistive loads. AC-22 applies to mixed loads. AC-23 is for highly inductive loads such as motors.

AC-23 is commonly used for load-breaking in motor circuits.

Why is DC switching more difficult?

DC does not pass through zero current, so arcs last longer.

This increases stress on contacts. Categories such as DC-1, DC-3, and DC-23 define suitable applications.

DC switching is important in solar, battery, and EV systems.

What is the key takeaway?

Utilisation categories define how hard the switching task is.

Underrated devices fail. Overrated devices are safe.

Correct selection ensures reliable and safe operation under IEC 60947.

How should they be applied?

Always consider utilisation category along with current and voltage ratings.

They ensure the switchgear matches the load and operating conditions, leading to better performance and longer equipment life.