In the design, application, and selection process of fire automatic alarm systems, most people pay attention to the appearance, sensitivity, stability, and intelligence level of fire detectors, as well as the performance, interface, and functionality of the main unit… However, during actual engineering construction, there are some seemingly simple yet crucial aspects that require our improvement, which are the line issues to be analyzed and discussed below.
Voltage Drop Issues in Bus-based Fire Alarm Control Systems
During the debugging work of bus-based fire alarm control systems, have you encountered such a problem: the fire alarm controller has issued control commands, and the control module has acted, but some external control devices, such as smoke exhaust valves and air supply ports, cannot operate? We monitored the voltage at the DC24V input terminal of the control module on-site using a multimeter and found that before the fire alarm controller issued control commands, the voltage remained unchanged, but once the control command was issued, the voltage dropped by several volts. What is the reason for this? This is the voltage drop issue we will discuss.
Both the fire automatic alarm circuit and the fire linkage control circuit have voltage drop issues. This is generally not apparent in smaller systems, but in larger buildings with longer lines, this problem becomes more prominent. Often, this is overlooked by construction personnel, and only when the problem is exposed do they seek various remedial measures, which is not only time-consuming and labor-intensive but also difficult to resolve thoroughly.
The voltage drop issue is mainly caused by the following two reasons.
1) Resistance of the wire
The wire itself has resistance. The resistance value is proportional to the length of the line and inversely proportional to the cross-sectional area of the wire. Additionally, some manufacturers produce wires of poor quality, which inadvertently increases the resistance.
2) Contact resistance
Various addressing units such as smoke detectors, temperature sensors, input modules, output modules, and short-circuit isolators are connected in the bus. Over time, the exposed terminals in the air will form an oxide layer, which will increase the contact resistance. The more addressing units are connected, the greater the contact resistance.
We collectively refer to the resistance of the wire and contact resistance as line internal resistance. It is precisely due to the existence of line internal resistance that the voltage drop issue at the load ends in the circuit occurs. According to Ohm’s law, the voltage drop value is proportional to the ratio of line internal resistance to load resistance. Therefore, to reduce voltage drop, we must find ways to decrease the ratio of line internal resistance to load resistance.
Bus-based fire alarm control systems generally have three types of buses: loop bus, power bus, and network bus. The loop bus refers to the wiring between the fire alarm controller and each addressing unit; the power bus refers to the lines providing DC24V to control modules, floor displays, etc.; the network lines refer to the communication bus between the centralized fire alarm host, slave units, and floor displays in the system.
Compared to the power bus, voltage drop issues in the loop bus and network bus are relatively rare. Taking the loop bus as an example: since various alarm device manufacturers pay great attention to this issue, clear requirements have been set for the maximum load of the loop, the length of the loop, and the wire diameter. Therefore, as long as the manufacturer’s wiring requirements are met, it will suffice.
The voltage drop issue has a significant impact generally occurring in the power bus, mainly due to the large operating current of electromagnetic valve linkage devices.
Fire shutter doors, fans, water pumps, etc. are controlled through intermediate relays. The resistance values of the selected relays are generally above 500 ohms, and the operating current is much larger than the working current of the addressing units in the loop bus, but this is not the main reason for the voltage drop issue. The electromagnetic valve linkage devices such as smoke exhaust valves, air supply ports, and gas extinguishing system activation cylinders are the real “power consumers.” The resistance value of electromagnetic valves is typically 36 ohms, with an operating current of about 0.65 amperes. Such a large operating current is sufficient to cause a significant voltage drop due to line internal resistance.
Below are some engineering examples to illustrate the adverse effects of voltage drop in projects.
In the Huainan XXXX project, each floor has 2 smoke exhaust ports, 2 air supply ports, 2 sound and light alarm devices, and 1 strong electricity switch. After fire confirmation, it is necessary to open at least 12 air supply ports on this floor and the floor above and below. The resistance of one electromagnetic valve is 36 ohms; with 12 air supply ports connected in parallel, the resistance is 3 ohms. This building has 30 floors, with a floor height of 3 meters, resulting in a vertical height of 90 meters. According to the copper core wire with a cross-section of 2.5mm2 used in this fire protection project, the resistance for a length of 90 meters is calculated as R=ρL/s, which gives us a resistance of 0.714 ohms. According to Ohm’s law, U_device=24/(3+0.714)*3=19.3V, while the starting voltage of the device is 20V. Without calculating the power consumption of the strong cut-off and sound and light alarms, the device is unable to start.
In the Huainan XXXX project, the dry fire hydrant system has a large electromagnetic valve controlling the water flow direction. If fire alarms occur simultaneously in different floors, the electromagnetic valves on different floors will act simultaneously. Due to the large current, even one electromagnetic valve cannot start, as a result of voltage drop.
Understanding the adverse effects of voltage drop, how can we effectively prevent similar issues? I have summarized the following points from the field experience.
1) Implement time-sequenced control for controlled devices
Time-sequenced control can reduce the number of devices that need to be controlled simultaneously. Electromagnetic valves and other high-current devices only require pulse signals, not continuous power supply. This way, the number of external control devices at the same time is reduced, and the load connected in parallel 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-sequenced starting is also effective, as the starting current of the device is large, and the current decreases after starting, which can alleviate the issue of excessive current at the same time. There are two methods to achieve time-sequenced control: one is software programming, using the delay output function of the fire alarm itself; the other is hardware connection, connecting similar external control devices through their interlock control terminals in series to drive them one by one.
In the previous engineering examples, we can adopt such methods to reduce voltage drop, such as driving one air valve every 5 seconds; connecting all air supply ports in series and using one control module to control them, so that the subsequent air supply port only acts after the previous one closes. The number of devices driven simultaneously by the alarm is greatly reduced.
If all external control devices can be implemented through software programming methods for time-sequenced control, there is naturally no need for hardware connections. For example, the Yi Ai brand equipment produced by our company has a very complete delay function, which can basically meet the needs of 50,000 square meters in practical engineering, such as in Hefei Lvdou Shopping Mall and Tianjin Jinlian Building. However, some manufacturers’ controllers have less complete delay functions. The delay control affects the operating speed of the controller. Therefore, hardware connections are also a factor we need to consider.
2) Set up multiple DC24V power bus main lines
In engineering design, it should be considered to lay multiple main lines, which can reduce the number of control modules in the line, thereby reducing contact resistance; and it can also avoid a single power bus winding back and forth, which greatly helps reduce line length.A considerable number of personnel believe that when devices cannot start or cannot work normally, it is due to insufficient power supply, but this is not the case. Most of the time, it is caused by voltage drop. In such cases, adopting the above methods yields better results. For instance, in the Qingdao Vancouver Garden project, 186 combustible gas detectors were used, resulting in the detectors not working properly. In reality, the external power output current of our company is 10A, which fully meets the load, but it was only due to voltage drop. Only by setting up multiple power lines can the issue be resolved. After multiple wiring was implemented, the system functioned normally.
3) Increase wire diameter
In design, some designers tend to overlook the wire diameter of the DC24V power supply lines and the matching issue with the selected electrical devices—especially noteworthy is the wire diameter of control lines for various electric-controlled air valves. Some designers do not pay attention to the wire diameter of these lines or consider how many devices are on this line and what their instantaneous action currents are. Often, when a fire occurs, a series of interlocking devices should open or close within the corresponding time. If the power supply cannot keep up, the action relays of these devices cannot work normally, which not only fails to interlock with the relevant fire control devices but may also damage the equipment. This issue is particularly prominent in basements with many interlocking devices.
Next, let’s calculate how thick the power supply wire should be in a standard floor. For example, if there is one smoke exhaust port and one positive pressure air port on each standard floor, and after fire confirmation, it is necessary to open at least 6 air supply ports on this floor and the floor above and below. The nominal action current of these air supply ports is mostly between 0.5-2 amperes; during actual measurement, the action current is mostly around 1 ampere. Therefore, for safety, we take 1 ampere as this value, and the valve’s starting voltage cannot be lower than 20V. According to R_line=(U-U_valve)/I_start=(24-20)/(6*1)=0.67, we find that the resistance of the DC power supply wire in the vertical shaft in the standard floor section should not exceed 0.67.
Assuming the building has 30 floors, with a floor height of 3 meters, the vertical height is 90 meters. According to the copper core wire used in our fire protection, we can calculate R=ρL/s. If we use a copper core wire with a cross-section of 2.5mm2, its resistance for 90 meters is 0.714; if we use a copper core wire with a cross-section of 4mm2, its resistance is 0.464. Therefore, we should use copper core wires with a cross-section of 4mm2 or more. Thus, it can be seen that our usual estimations are not accurate. In basements with more interlocking devices, we should determine the wire diameter of the power supply line after more detailed calculations.
4) Soldering wire connections
This can reduce the internal resistance of the wire and contact resistance.
5) Place a DC24V power box on-site
This is to compensate for the inherent shortcomings of the project and is a last resort. However, two points must be noted: first, the on-site AC220V power supply must be a dedicated fire protection power supply; second, the power can be turned on or off within the fire control center.
The fire alarm equipment, once it leaves the manufacturer, has only completed the first step. No matter how good the quality of the equipment, its operational status in the future largely depends on the installation quality. This requires us to consider comprehensively before construction and ensure quality during construction. Issues like voltage drop can be avoided if we consider them at all stages, including design, construction, and debugging.
Additionally, many manufacturers’ modules require power supply; some take power from the host, while others are supplied by separate power supply boxes. I personally believe that the power supply for external devices should be separated from the module’s power supply, as this ensures the normal operation of the modules. Due to the high digitalization level of advanced equipment, their power supply requirements are also high, requiring low noise power supplies to reduce external power interference with the system. Especially for third-generation digital systems, if the module and external devices share the same power supply, the instantaneous low voltage caused by large currents during interlocking has a significant impact on the system and equipment.
6) Construction
During construction, civil construction should also be conducted to ensure the integrity and reliability of line installation. First, during embedding, use locknuts to secure connections between conduits and junction boxes and protect the conduit ends; exposed parts of junction boxes should be sealed properly; when threading wires, clear out debris from the conduit to avoid cutting through the wire’s insulation layer, or even cutting the wire core, which inadvertently reduces the wire’s cross-section. Second, minimize interfaces, as this 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 selected to ensure good contact.
Each of the above methods has its pros and cons. In actual engineering, flexible application of various voltage reduction methods according to different situations is necessary to facilitate construction while achieving certain effects.
Grounding Issues in System Lines
When the system program is completed and the corresponding bus, 24V lines, and linkage lines are connected to the controller, if the system runs unstably and the faulty components’ circuits or addresses are not fixed, this is caused by grounding issues. However, it is often difficult to measure short-circuit phenomena with a multimeter, mainly due to the addition of bus isolators in the circuit and the presence of suspended pipe fittings, which leads us to the issue we will discuss below.
During system debugging, grounding issues are often encountered, and the results are relatively difficult to trace. So, can we minimize such troubles? How can we avoid such occurrences?
First, during construction, the construction unit should properly use a megohmmeter to test the inter-wire resistance of the circuit. The metal embedded pipes should have good contact; only when they are completely grounded can the data measured by the megohmmeter be correct and reliable. During construction, wires should be threaded continuously; they should only be cut during installation, and care should be taken to ensure that the wire ends do not touch walls, screws, or grounding bodies.
Second, after the equipment is installed, retest. The method is: connect a DC 24V (UA) power supply to one end of the circuit, disconnect the other end, and then test the voltage UB at a certain point on the circuit. If UB is significantly less than UA, it indicates that there is grounding in the line. The circuit can be divided into two segments, and one segment can be tested first to determine if it has grounded. This can be repeated to gradually narrow down the range and pinpoint the grounding location, eliminating the fault.
Short Circuit Issues Between Different Buses in the System
When the system program is completed, sometimes additional devices may appear in the same circuit. This situation is generally caused by short circuits between different circuit buses, but it cannot be measured with a multimeter in ohm mode, mainly due to the isolators in the circuit.When the positive pole is short-circuited, it cannot be measured in the control room, and even if it is short-circuited with the ground, it cannot be measured. Only on-site measurements can reveal this.