Smoke Alarm & Detection Systems for Class 2 Buildings

Specification 20 of the National Construction Code (NCC) describes the installation and operation of automatic smoke detection and alarm systems in Class 2 to Class 9 buildings.

According to convention and the National Construction Code there are TWO types of required automatic smoke detection and alarm systems plus a third option that is a combination of these two systems.

In this article, we will explore how these systems can be used in each building classification. To understand the requirements, apply, we must understand the “type of system” described in Specification 20;

Reading the text here is a little complex, so let's try and summarise it in table form;

In simple terms, wherever a smoke alarm system (S20C4) could be installed, a smoke detection system (S20C4) or a combined smoke alarm and smoke detection system (S20C5) may be used1, BUT a smoke alarm system or a combined smoke alarm and smoke detection system may never be used where a smoke detection system is required.

Another simpler way to put it is this: a smoke alarm system can be used in the following circumstances;

Other than the titles of these systems, “smoke alarm system” or “smoke detection system”, what are the key differences, and why would I choose one over the other?

A smoke alarm system, as defined under Clause S20C3 of the National Construction Code (NCC), refers to mains-powered, wired and interconnected self-contained smoke alarms that are typically installed within each sole-occupancy unit (SOU) of a residential building.

Australian Standard AS 3786 is the Standard that covers Smoke Alarms that include an integrated smoke sensor, an audible alarm, and test facility. They are designed primarily for occupant warning and in the case of a Class 2 building, must be hard-wired and interconnected so that activation of one alarm triggers all other smoke alarms within the same SOU.

In contrast, a smoke detection system, compliant that satisfies Clause S20C4 and complies with Australian Standard AS 1670.1, includes a Fire detection Control & Indicating Equipment (FDCIE), and is typically used in more complex or environments. These systems include various interconnected components like smoke detectors, heat detectors, a control panel, and occupant warning system.

A combination smoke alarm and smoke detection system employs elements of both systems, typically with smoke wired and interconnected smoke alarms in each SOU, with a smoke detection and alarm system used in the common areas of the building with an integrated occupant warning system.

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  A Class 9a building where 6 or less bed patients are accommodated may use a smoke alarm system or a smoke detection system, but not a combined system.

YES, a Smoke Detection System may be used for a Class 2 building in lieu of a Smoke Alarm System or combined system, but there are additional requirements described in both Australian Standard AS 1670.1 and the Specification 20 as generally follows;

• In a Class 2 provided with a smoke detection system, the following applies: Smoke detectors must be installed— within each sole-occupancy unit, in accordance with the requirements for alarms in S20C3(2)(a) and (2)(b) as follows;
  • Smoke detectors must be installed within each sole-occupancy unit, and located on or near the ceiling in any storey—
    • containing bedrooms—
    • between each part of the sole-occupancy unit containing bedrooms and the remainder of the sole occupancy unit; and where bedrooms are served by a hallway, in that hallway; and
    • not containing any bedrooms, in egress paths.
  • Where there is more than one alarm installed within a sole-occupancy unit, alarms must be interconnected within that sole-occupancy unit.
• in a Class 2 building provided with a smoke alarm system in accordance with S20C3(2)(c) the building occupant warning system requirements must comply as follows—
  • the sound pressure level need not be measured within a sole-occupancy unit if a level of not less than 85 dB(A) is provided at the door providing access to the sole-occupancy unit; and
  • the inbuilt sounders of the smoke alarms may be used to wholly or partially meet the requirements; and
  • in a Class 2 building provided with a smoke detection system in accordance with S20C4(2), the sound pressure level from a building occupant warning system need not be measured within a sole-occupancy unit if a level of not less than 100 dB(A) is provided at the door providing access to the sole-occupancy unit.
  • installed in public corridors and other internal public spaces, located in accordance with the requirements for smoke detectors in AS 1670.1; and
  • connected to activate a building occupant warning system in accordance with S20C7.
• and subject to (b), in public corridors and other internal public spaces.

This article looks at smoke alarm and detection systems in Class 2 buildings, focusing on a real example that placed the occupants of the building at risk. The building in question had a combination setup—interconnected smoke alarms (AS 3786) in each SOU and a smoke detection system (AS 1670.1) in the common areas and car park. Everything looked compliant on the surface, but concerns were raised following a cladding audit.

The Municipal Building Surveyor recommended extra smoke alarms near windows and doors to help detect fires that may start outside. This led to questions about the best way to upgrade the existing (and outdated) systems while staying compliant with the NCC.

Specification 20 of the NCC outlines three valid options for Class 2 buildings: a smoke alarm system, a smoke detection system, or a combination of both. This article unpacks the key differences between them, when each can be used, and highlights that while a detection system can be used in Class 2 buildings, it brings additional obligations around detector placement and sound levels.

• National Construction Code (NCC), Volume 1 - Class 2 to Class 9 Buildings
• Australian Standard AS 3786
• Australian Standard AS 1670.1