Fire Engineering Glossary

Access credential

The token, code or biometric a person presents to an access control system; lifecycle management (issue, change, revoke) matters as much as the hardware.

As-fitted drawings

Drawings updated to show what was actually installed — positions, addresses, routes and interfaces — rather than the original design intent.

Aspirating smoke detection (ASD)

A system that continuously draws air through sampling pipework to a central high-sensitivity detector, used for early warning or difficult spaces.

Airflow faults against the commissioning baseline usually mean broken pipe (high flow) or blocked holes/filter (low flow).

Automatic opening vent (AOV)

A vent opened automatically on smoke detection to clear smoke from escape routes such as stair cores and corridors; tested from the mapped trigger through to full travel.

Class B wiring

Radial (spur) wiring; devices beyond a break are lost, so a break shows as multiple missing devices downstream of one point.

Double knock

Cause-and-effect logic requiring a second confirming trigger (same or second device) before a critical output such as plant shutdown or release operates.

Loop

The two-wire circuit connecting addressable devices to the panel, normally wired as a ring so devices remain in communication from either direction if the ring is broken at one point.

Power supply unit (PSU)

The monitored supply providing system power and battery charging; PSU faults commonly present as charger, battery or mains fault indications.

Quiescent current

The current a system draws in normal standby, from panel and all devices together; the first input to any battery calculation.

Loop devices often dwarf the panel's own figure — never use the panel datasheet alone.