Disconnector vs switch-disconnector vs circuit breaker — utilization categories (IEC 60947-3)

The [MCCB vs MCB guide](/guides/protection/vermogensschakelaar-mccb-vs-automaat) covers the difference in breaking capacity between device families. This article covers a different, easily confused distinction: three types of low-voltage switching device that can look almost identical in a panel but have fundamentally different ratings, defined mainly in IEC 60947-3 ("Switches, disconnectors, switch-disconnectors and fuse-combination units").

Disconnector (isolator) — no-load isolation only

A disconnector is designed for exactly one job: to provide a reliable, verifiable isolating distance so that downstream equipment can be safely worked on. Its utilization category is AC-20 (or DC-20 for DC) — connecting and disconnecting under negligible current only, essentially the small capacitive charging/leakage current of an already-unloaded circuit. A disconnector is not rated to make or break normal load current, let alone fault current. Its value lies in the guaranteed open contact gap and reliable open-position indication that satisfy the isolating-function requirements of IEC 60947-3 — the property that makes it suitable as the point of isolation for lock-out/tag-out.

Switch-disconnector — rated load switching, not fault switching

A switch-disconnector combines the isolating-distance property of a disconnector with a rating to make and break actual load current under defined conditions. IEC 60947-3 groups these ratings into utilization categories:

AC-21 — resistive loads, including moderate overloads (e.g. heating circuits, resistive lighting).
AC-22 — mixed resistive and inductive loads, including moderate overloads (e.g. general distribution feeders with a mix of loads).
AC-23 — motor loads or other highly inductive loads, where the making and breaking currents required are substantially higher multiples of the rated current than for AC-21/22, because of motor inrush and the inductive energy that must be interrupted.

Each category is tested at a specified multiple of rated current and a specified power factor (lower power factor and higher current multiples for AC-23 than for AC-21), which is why a switch-disconnector sized for a resistive load is not automatically suitable for switching a motor of the same nominal current — the exact making/breaking current multiples and test power factors per category and current range are tabulated in IEC 60947-3 itself. Crucially, a switch-disconnector is still not rated to interrupt short-circuit current — it depends on an upstream or coordinated fuse (a "switch-fuse" combination) or breaker to clear a fault.

Circuit breaker — rated fault-current breaking

A circuit breaker (IEC 60947-2) is rated, in addition to normal load switching, to make and break short-circuit current up to its rated breaking capacity (Icu/Ics — see the MCCB vs MCB guide). A plain circuit breaker is not automatically suitable for isolation: only a breaker that is additionally tested and marked as meeting the isolating requirements of IEC 60947-3 (typically shown with a padlock symbol combined with an arrow, sometimes referenced as "suitable for isolation") provides the same guaranteed isolating distance as a dedicated disconnector.

Choosing the right device for the job

Requirement	Device
Guaranteed, lockable isolating distance for LOTO, no load switching expected	Disconnector (AC-20)
Regular switching of a resistive or mixed load under normal operation	Switch-disconnector (AC-21/AC-22)
Regular switching of a motor or other highly inductive load	Switch-disconnector rated AC-23, or a contactor
Must also clear a short-circuit fault	Circuit breaker (IEC 60947-2), optionally marked suitable for isolation

Common mistakes

Using a plain circuit breaker as the sole point of isolation for lock-out/tag-out without checking whether it is actually marked as suitable for isolation per IEC 60947-3 — not every breaker provides the guaranteed isolating gap.
Assuming a switch-disconnector can interrupt fault current — it is rated for load switching within its utilization category, not for short-circuit breaking; fault clearing still depends on an associated fuse or breaker.
Applying an AC-21-rated switch-disconnector to a motor circuit — the making/breaking duty of a motor load exceeds what an AC-21-rated device is tested for, leading to accelerated contact wear or contact welding.
Over-specifying an AC-23 device where AC-21 would suffice — not a safety error, but an unnecessary cost; category selection should match the actual load type, not default to the highest category.