Solutions to Common Issues in Fire Alarm Systems: Line Voltage Drop
During the debugging of bus-type fire alarm control systems, it is common to encounter situations where the fire alarm controller has issued control commands, and the control modules have activated, but some external control devices such as smoke exhaust valves and air supply vents fail to operate. When monitoring the voltage at the control module’s DC24V input terminal on-site using a multimeter, we find that before the fire alarm controller issues the control command, the voltage remains unchanged, but once the control command is issued, the voltage drops by several volts.
Both fire alarm circuits and fire linkage control circuits face issues with voltage drop. Typically, this problem may not be apparent in smaller systems, but in larger buildings with longer wiring, it becomes more pronounced. This issue is often overlooked by construction personnel until it becomes evident, at which point they seek various remedial measures, which not only takes time and effort but is also difficult to resolve completely.
1. Analysis of Causes of Line Voltage Drop
(1) Resistance of Conductors
The wires themselves have resistance. The internal resistance of a wire is inherent to the wire, proportional to the length of the wire and inversely proportional to its cross-sectional area, and is related to the quality of the wire. Some manufacturers produce low-quality wires, which inadvertently increases resistance.
(2) Contact Resistance
Contact resistance refers to the resistance at the junctions of wires, terminals, and connections between wires. When connecting devices, if the terminal connections are not tightly crimped, the contact resistance increases. If wire ends are not soldered and are exposed to air for extended periods, oxidation can occur, leading to increased contact resistance. Additionally, connecting various devices such as smoke detectors, temperature detectors, input modules, control modules, and bus isolators increases contact resistance as the number of connected devices increases.
We refer to the internal resistance of wires and contact resistance collectively as line internal resistance. It is precisely due to the existence of line internal resistance that the voltage drop across the load in the circuit occurs. According to Ohm’s Law, the voltage drop is proportional to the ratio of line internal resistance to load resistance; thus, to reduce line voltage drop, we must find ways to decrease the ratio of line internal resistance to load resistance.
(3) Operating Current
Issues with line voltage drop are significantly impacted in the power bus, primarily due to the high operating current of electromagnetic devices.
Fire curtain doors, fans, and water pumps are controlled via intermediate relays. The resistance of selected relays is generally over 500 ohms, and their operating current is much larger than that of the addressing units in the loop. However, this is not the primary cause of voltage drop issues. Electromagnetic devices such as smoke exhaust valves, air supply vents, and gas fire suppression system activation cylinders are the real “power consumers.” The resistance of electromagnetic valves is generally 36 ohms, with an operating current of about 0.65 amperes. Such a large operating current is sufficient to create a significant voltage drop due to line internal resistance.
Here are a few engineering examples illustrating the adverse effects of line voltage drop in projects.
In the Shijiazhuang Financial Building project, each floor has 2 smoke exhaust vents, 2 air supply vents, 2 sound and light alarms, and 1 strong power switch. After confirming a fire, it is necessary to open the vents on the current floor and the floor above and below, which requires linking at least 12 vents. An electromagnetic valve has a resistance of 36 ohms, and with 12 vents connected in parallel, the resistance is 3 ohms. This building is 30 stories tall, with a height of 3 meters per floor, giving a total vertical height of 90 meters. According to the 2.5mm² copper wire used in this fire protection project, using R=ρL/s, the resistance of the 90-meter length of 2.5mm² copper wire is 0.714 ohms. Based on Ohm’s Law, U_device=24/(3+0.714)*3=19.3V, while the device’s starting voltage is 20V, which means the device cannot start without considering the power consumption of the strong switch and sound and light alarm.
The dry pipe system for fire hydrants has a large electromagnetic valve controlling the direction of water flow. If different floors’ fire hydrants alarm simultaneously while in automatic mode, the electromagnetic valves on different floors will activate at the same time. Due to the high current, not even one electromagnetic valve can start, which is the same situation as above, caused by line voltage drop.
2. Solutions to Line Voltage Drop
Excessive line voltage drop has adverse effects on the system, preventing it from operating normally. This problem is quite common, and based on practical debugging, I propose the following solutions.
(1) Time-division Control
Time-division control can reduce the number of devices that need to be controlled at the same time. Electromagnetic devices that require high current only need pulse signals and do not require continuous power supply. This way, the number of external control devices connected at the same time decreases, and the load on the power bus increases, thereby reducing the impact of line internal resistance. Even for devices like sound and light alarms that require continuous power, time-division starting is effective since their starting current is large, but the current decreases after starting, alleviating the issue of excessive current at the same time.
There are two methods to implement time-division control.
One is software programming, utilizing the delay output function of the fire alarm system itself;
The other is hardware connection, linking similar external control devices through their interlocking control terminals in series to drive external control devices one by one.
In the previous engineering example, we used this method to reduce line voltage drop by driving one air valve every 5 seconds, linking all air supply vents in series, controlled by a single control module. Thus, the subsequent air supply vent only activates after the previous one closes. The number of devices driven by the alarm system significantly decreased.
If all external control devices can be programmed for time-division control, hardware connections are naturally unnecessary. For example, our company’s devices have a well-developed delay function that can generally meet the needs of a 50,000 square meter project. However, some manufacturers’ controllers have less effective delay functions. Delay control affects the operating speed of the controller, so hardware connections are also a factor we need to consider.
Install multiple DC24V power bus main lines. In the design phase, consideration should be given to laying multiple main lines, which can reduce the number of control modules in the line, thereby reducing contact resistance; and avoid a single power bus winding around, which greatly helps reduce line length.
For example, in a project that used 186 combustible gas detectors, the detectors could not operate normally. The external control power output we provided could reach 10A, fully meeting the load requirements. However, due to excessive line voltage drop, the terminal voltage was insufficient, preventing the combustible gas detectors from functioning. Our proposed solution was to lay a pair of 24V voltage lines for each floor, and after implementing multi-route wiring, the system operated normally.
(2) Increase Wire Diameter
In fire alarm system design, some designers easily overlook the wire diameter of the 24V DC power supply lines and the matching issues with the selected electrical devices—especially the wire diameter of control lines for various electric control dampers. Some designers fail to pay attention to the wire diameter of these lines and do not consider how many devices are present on the lines and their instantaneous operating currents. Often, during a fire, a series of interlinked devices should open or close within a specified time. If the power supply cannot keep up, the relays of these devices cannot operate normally, failing to link with relevant fire control devices and potentially damaging the equipment. This issue is particularly pronounced in basements with many interlinked devices.
Let’s calculate a simple example. In a standard layer, what wire diameter should we choose for the power supply? For instance, if each standard layer has one smoke exhaust vent and one positive pressure vent, and after confirming a fire, it is necessary to open the vents on the current floor and the one above and below, linking at least 6 vents. The nominal operating current of these vents generally ranges from 0.5 to 2 amperes, and during testing, the operating current is mostly around 1 ampere. For safety, we take 1 ampere as the value, and the vent’s starting voltage cannot be lower than 20V. Using R_line=(U-U_valve)/I_start=(24-20)/(6*1)=0.67, we find that the resistance of the DC power supply wires in the vertical shaft should not exceed 0.67.
Assuming the building has 30 floors, with a height of 3 meters per floor, the total vertical height is 90 meters. Based on the copper wire used in our fire protection system, we use R=ρL/s to determine that if we use a 2.5mm² copper wire, the resistance for 90 meters is 0.714; if we use a 4 mm² copper wire, the resistance is 0.464. Therefore, we should use copper wire with a cross-section of 4 mm² or more. This shows that our usual estimates are inaccurate. In basements with more interlinked devices, we should conduct more detailed calculations to determine the wire diameter for the power supply.
(3) Soldering Connections
This can reduce both wire internal resistance and contact resistance.
(4) Place DC24V Power Boxes On-Site
This is a remedial measure for the inherent shortcomings of the project, a last resort. However, two points must be noted: first, the on-site AC220V power supply must be a dedicated fire power supply; second, the power can be turned on or off at the fire control center.
Fire alarm devices, once they leave the manufacturer, have only completed the first step. Even the best quality devices rely heavily on installation quality during subsequent operation. This requires us to consider thoroughly before construction and ensure quality during the construction process. For issues like line voltage drop, if we can consider them at every stage—design, construction, and debugging—they should be avoidable.
(5) Construction According to Requirements
During construction, we should practice civilized construction, ensuring that the installation of lines is intact and reliable. First, during embedding, use locknuts to secure connections between pipes and junction boxes and protect the wire entries; exposed parts such as junction boxes should be sealed; when threading wires, clear debris from the conduits to avoid cutting the wire’s insulation layer or the wire core, inadvertently reducing the cross-section of the wire. Second, minimize interfaces, which not only avoids unnecessary potential faults but also reduces contact resistance. If the line is too long or has intermediate lines, appropriate terminals should be used to ensure good contact.
Sharing knowledge is a form of progress! This article is reprinted from Qili Fire Protection, thanks to 【Qili Fire Protection】 for organizing and sharing.
