VADs are covered by EN 54-23 (the use of visual alarm devices in fire detection and alarm systems in non-domestic premises), which ensures that the device emits sufficient light to attract the attention of anyone in a specific area.
The need for a VAD will depend on factors such as where ambient sound pressure levels are high, if staff or visitors need ear protection, or where people with impaired hearing may be alone.
VIDs, covered by the standard EN 54-3, are used as supplementary indication devices where additional notification is required to supplement alarm sounders. They cannot be used as the only way to alert people to a potential hazard, and can be used where VADs are not a requirement.