Raising the alarm

In a sentence, the distinct advantage of such remote switching is its ability to permit switching 250 Vac directly from the loop without any additional interfacing, thus providing considerable installation savings in both costs and labour when equipment of the following kinds of duties needs to be controlled from the fire alarm system:

• magnetic door retainers (automatically release doors in the event of fire);
  
• ventilation control (dampers, extractors, etc);
  
• plant shutdown;
  
• lift control (returns lift to ground floor in the event of fire).

Loop-powered switching
With its capability for 1 kW switching and low current consumption, this cost-effective interfacing device provides system designers with functional versatility in applications where auxiliary mains equipment needs to be under the control of the fire alarm panel. It also offers a method of remotely controlling such equipment from the loop with a single input/output at one address.

The module has been designed to provide a single mains-rated relay output (with flying leads) using simple dual in-line (dil) switches for reliable setting of a single loop address to which a maximum of 127 devices can be attached, exceeding the practical limit.

A general-purpose auxiliary input is also provided for local fire and fault monitoring. This is fully monitored for open and short-circuit which, if required, can be enabled or disabled with bit 8 on the dil switch.

Interference-free comms
This communications platform, dubbed the CHQ-SIO System Interface I/O Module by supplier Hochiki, is widely recognised by leading alarm system panel manufacturers for providing high immunity against false alarms. It is especially suited to applications where there is a need for high-sensitivity signal processing to counter the hazards of interference from mobile phones or commercial and industrial machine noise.

Due to recent technological advances, the most frequently selected fire detectors used for intelligent building controls are now analogue/addressable sensors. Their addressability enables precise indication of the sensor's location and transmission of the condition of the sensing element to alert the control panel to the presence of heat or smoke. Thus, by allowing seamless connectivity with the mains-operation of auxiliary, emergency, heavy-duty equipment, as outlined above, Hochiki's I/O interface module, supported by interference-free alarms data communications, offers OEM designers the potential for an integrated system solution with the benefits of reduced installation costs.

Comparison
To evaluate the benefits of the mains switching interface module in action, figure 1 (over) indicates the additional equipment that is typically required to switch mains for all the equipment shown. In comparison with the low labour and cost expenditure associated with the interface module, additional considerations for an alternative system configuration include: the cost of assembling this equipment into a housing and its wiring; running a mains supply; and the cost of mounting this additional equipment.

From this example, the cost benefits of the interface module over a relay that can only switch 24 V can soon be realised. It should be stated that there exists a requirement for both types of switching capability. It is in this context that Hochiki now gives designers the choice of the correct product for the particular application, removing the additional costs when mains switching is required.

The module has been designed for easy installation and contains one connector block for termination of the loop and input wiring, and flying leads for connection to the mains-rated relay.

The hazards of industrial noise can cause unwanted alarms on some systems due to corruption of the signals between the devices and the control panel. Protocols now exist where the communication between the devices and the panel are digital, and at the end of each transmission a checksum is transmitted.

This checksum enables the data being transmitted between the control panel and the devices to be verified to ensure that no corruption has taken place. If corruption has occurred, the system perceives the data as unreliable and the information is discarded. Retransmission of the data is automatically requested by the system. Hochiki has adopted this style of communication method in its ESP protocol.