Grade A fire alarm systems are typically used in commercial properties and include a full suite of components such as smoke detectors, heat detectors, manual call points, a central control panel, and sounders. They offer the highest level of protection and are designed for larger or more complex buildings. Grade D systems, on the other hand, are commonly used in domestic properties and consist of interlinked smoke and heat alarms, usually powered by mains electricity with a battery backup. Grade D systems are less complex and are suitable for smaller residential buildings.