PDF of this tutorial- fire, life, and safety (FLS) or fire alarm (FA)
- security and access control (SAC)
- energy management systems (EMS)
- heating, ventilation, and air conditioning (HVAC)
These BMS categories are typically cabled separately by the mechanical and electrical specifications. The voice and data cabling is rarely addressed during construction and is usually not part of the construction budget. Planning and installation are normally accomplished when the floor space is being prepared for occupancy. This means multiple cabling systems and cable delivery methods are installed during various stages of the construction.
With proper planning, the only limiting factor for complete systems integration of the voice, data, video, and BMS may be the FA system. In the United States, Article 760-54 (b) of the 1996 National Electrical Code (NEC) allows conductors of power-limited FA systems and signaling/communications circuits (Article 725/800) to share the same cable, enclosure, or raceway. In addition, Article 760-61 (d) of the NEC allows the use of the same type of cable for FAs that is typically used for the signaling/communications (voice and data) circuits. Some local codes however, especially codes in other countries, may invoke limitations or require special approvals for integrating the FA system. Yet, even if the FA cabling is installed separately, there are still substantial cost reductions and benefits that can be derived from integrating the remaining BMS.
In addition to the code requirements, there is also a need to evaluate the electrical characteristics of the systems. The voice and data systems primarily consist of analog and digital signals and have established guidelines for signal strength over distance. The BMS devices operate on current draw, circuit resistance (contact closure), or consist of analog or digital signals. Basically, each BMS terminal or device will operate over a particular cable type as long as it is located within a specified range from the equipment.
BMS devices are utilized to monitor or control a specific function. This can be equated to an output from the equipment or an input from a device. As an example, there may be a temperature sensor that gathers information and sends a signal to the equipment panel (input) and, as a consequence, the equipment sends a signal to a device that closes a damper or vent (output). Devices are primarily power-limited or communicate using low-speed protocols. The signal distance supported by the devices is usually limited by the current draw and line voltage delivered by the power supply. Typically, 24–American wire gauge (AWG) unshielded twisted-pair (UTP) cable has the capacity to handle 1 Ampere (Amp) of current draw per conductor, with a maximum of 3.3 Amps per four-pair cable.
What does this mean? The current or signal from the equipment leaves at the specified voltage level. The device requires a certain voltage level to operate. As the signal travels through the cable, the voltage drops due to resistance. Cable pair resistance is measured by shorting one end of the cable and taking a resistance reading between the conductors at the other end. A typical 24–AWG UTP cable pair has 57.2 Ohms of resistance per one-thousand feet or .0572 Ohms per foot. Circuit resistance can be measured by dividing the voltage drop by the current draw.
If a 24 Volt (V) device requires .05 Amps of current to operate and the allowable voltage drop is ±10 percent, or 2.4V, the maximum circuit distance using 24–AWG UTP cable is 839 feet (256 meters). This can be easily calculated for any cable and circuit using the following two-step formula:
- voltage drop (2.4 V)/current draw (.05 Amps) = circuit resistance (48 Ohms)
- circuit resistance (48 Ohms)/1 foot cable resistance (.0572 Ohms) = maximum distance (839 feet/256 meters)
Some equipment vendors state that a lower-gauge cable, such as 18 AWG, is required for proper system operation. This is typically found to be unnecessary once the electrical characteristics of the system are analyzed.
