Understanding cable properties for reliable system design
Every cable is more than just a conductor—it's a complex electrical component with resistance, capacitance, and inductance. These properties directly affect system performance, reliability, and compliance. Understanding them is essential for professional fire and security installations.
Resistance is the most visible and problematic cable property. It directly causes voltage drop, which can cripple devices at the end of long cable runs.
Problem: PIR (Passive Infrared) sensors typically draw 10-20mA at 12V DC (Direct Current). Over a 50-meter run of standard 6-core alarm cable (0.092Ω/m), the total loop resistance is:
At 20mA draw, voltage drop is: 9.2Ω × 0.02A = 0.184V. This seems small, but add multiple devices and cable joints, and you quickly approach the 10% maximum drop (1.2V for 12V systems).
Problem: Maglocks and electric strikes draw substantial current (300mA-1.5A). Using inadequate cable creates massive voltage drop that prevents proper operation.
Example: A 12V maglock drawing 500mA over 30m of 1.0mm² T+E (Twin and Earth):
End voltage = 11.46V (acceptable). But with alarm cable (0.092Ω/m): V_drop = 2.76V, leaving only 9.24V—the lock won't hold!
Challenge: PoE (Power over Ethernet) injects power over the same twisted pairs as data. Cat5e uses 24AWG conductors (0.094Ω/m), which is fine for data but marginal for power.
A PoE (Power over Ethernet)+ camera (30W @ 48V) draws ~625mA. Over 90m (PoE (Power over Ethernet) limit):
At 48V input, the camera receives 37.4V—still within the 36-57V PoE (Power over Ethernet) range. However, in tight cable bundles, heat buildup increases resistance by up to 20%, pushing it toward the minimum threshold.
Capacitance is the ability of a cable to store an electrical charge. In security systems, think of it as a "sponge" that soaks up high-frequency signals. Capacitance forms between conductor pairs and between conductors and shields, creating a frequency-dependent impedance that must be charged and discharged with every signal transition.
At low frequencies, capacitive reactance is high (open circuit behavior). At high frequencies, it drops dramatically, creating a low-impedance path that shunts signals to ground or adjacent conductors. This is why capacitance is the enemy of fast digital communications.
The Speed Limit: RS485 (Recommended Standard 485) is the backbone of PTZ (Pan-Tilt-Zoom) camera control and access control keypad bus lines. Maximum distance is theoretically 1200m, but cable capacitance is the real limiting factor.
High capacitance in a cable "rounds off" the square waves of digital data. RS485 (Recommended Standard 485) relies on crisp voltage transitions between logic levels (typically ±200mV differential). When capacitance is excessive, these sharp edges become sloped ramps.
The Result: Bits become indistinguishable, leading to "Comm Faults" or unresponsive keypads. At the receiver, the voltage may not reach valid logic thresholds in time, causing bit errors and complete communication breakdown.
Cat5e has controlled capacitance (52pF/m) specifically because its twisted pairs manage capacitance and maintain signal integrity. At 100m:
At 115200 baud (common for access control), the bit time is ~8.7μs. With this capacitance, the RC time constant through typical bus termination (120Ω) is:
The signal needs ~5τ (3.1μs) to fully transition—eating up 36% of the bit time! This leaves minimal noise margin, making the link fragile.
The Low-Pass Filter Effect: When sending analog video over UTP (Unshielded Twisted Pair) using baluns, high capacitance acts as a Low-Pass Filter. Video signals contain frequencies from DC (black level) to several MHz (fine detail).
Capacitance between the twisted pairs creates a frequency-dependent voltage divider. High-frequency components (edges, fine detail) are attenuated more than low-frequency components (solid colors, slow transitions).
The Result: Capacitance sucks the high-frequency detail out of the video signal, resulting in grainy, smeared, or "ghosting" images at the monitor. You'll see soft edges, loss of text readability, and a generally "fuzzy" picture.
UK Regulatory Requirement: Fire alarm systems in the UK must use fire-resisting cable as mandated by BS (British Standard) 5839-1:2025. BS (British Standard) 5839 defines two levels: standard fire-resisting cables (30 min survival, PH 30 per BS (British Standard) EN (European Norm) 50200) and enhanced fire-resisting cables (120 min survival per BS (British Standard) EN (European Norm) 50200 and BS (British Standard) 8434-2). These cables have higher capacitance due to their robust construction designed to maintain circuit integrity during fires.
How EOL (End Of Line) Monitoring Works: Fire alarm panels use comparator circuits to continuously monitor the DC (Direct Current) voltage across each zone circuit. The EOL (End Of Line) resistor (typically 4.7kΩ or 10kΩ) sits at the end of the circuit, and the panel measures the voltage to determine if the circuit is normal, open (fault), or shorted (alarm). There is no polling—the panel constantly monitors voltage via analog comparators.
Fire-resisting cables (as specified in BS (British Standard) 5839-1:2025) have significant capacitance due to their silicon or LSZH (Low Smoke Zero Halogen) insulation and construction. A typical standard fire-resisting cable has approximately 80pF/m capacitance. A 100m radial circuit has:
The Capacitance Problem: When a device changes state (detector activates or restores), this capacitance must charge or discharge through the EOL (End Of Line) resistor. The time constant is:
It takes 5τ (~188μs) to fully settle. Fire alarm panels are designed to accommodate these capacitive transients in their monitoring and reset cycles.
Why fire-resisting cables have higher capacitance: The silicon rubber or LSZH (Low Smoke Zero Halogen) insulation and fire-resistant construction creates closer conductor spacing and higher dielectric constant compared to PVC (Polyvinyl Chloride) alarm cable. This is an unavoidable trade-off for fire integrity—the cable must maintain circuit operation at high temperatures (typically 800°C+ for 30-120 minutes depending on standard/enhanced rating), which requires robust insulation that inherently increases capacitance.
System Architecture: Conventional fire alarm systems use radial circuits (zones) with an End-of-Line (EOL (End Of Line)) resistor at the furthest device. Multiple detectors and call points wire in parallel back to the panel. When any device activates, it shorts the circuit, and the panel identifies the zone—but not the specific device.
Contrast with Addressable: Addressable systems use loop-wired networks where each device has a unique address. The panel polls devices digitally via the loop, identifying exactly which detector triggered. The digital communication protocol provides much better noise immunity and allows for sophisticated fault detection.
UK Requirement: BS (British Standard) 5839-1:2025 mandates fire-resisting cables (standard or enhanced, as specified in sections 25.5 and 25.6) for all fire alarm circuits in both conventional and addressable systems. These cables have higher capacitance (typically 80pF/m for standard fire-resisting cables) than standard alarm cable due to their silicon or LSZH (Low Smoke Zero Halogen) insulation and robust construction.
How Conventional Panels Actually Work: Conventional fire alarm panels do NOT poll detection circuits. Instead, they use comparator circuits to continuously monitor the DC (Direct Current) voltage across each zone. The panel measures the voltage and compares it against thresholds to determine the circuit state:
The comparator continuously monitors this voltage—there is no polling cycle.
How Capacitance Affects Conventional Circuits: When a detector activates and shorts the circuit, or when it restores, the circuit capacitance must discharge or recharge. This creates a voltage transient, but fire alarm panels are designed to accommodate these transients in their monitoring and reset cycles.
For a 250m radial circuit using fire-resisting cable (typical ~80pF/m) with a 4.7kΩ EOL (End Of Line):
When a detector activates (shorts the circuit), the capacitance discharges through the short. When the detector restores, the capacitance must recharge through the EOL (End Of Line) resistor:
During this ~0.5ms, the voltage at the panel is transitioning. Fire alarm panels are designed to accommodate these transients in their monitoring and reset cycles.
Note on Quiescent Current: Unlike addressable systems that actively poll devices, conventional panels draw minimal quiescent current—just the steady-state current through the EOL (End Of Line) resistors. There's no AC (Alternating Current) component from polling. However, if the panel performs periodic auto-tests by pulsing circuits, those pulses must charge/discharge the capacitance, adding small transient current spikes to battery load calculations.
Inductance opposes changes in current. At high frequencies (fast signal edges), inductance creates "inductive reactance" that impedes current flow, slowing down signals.
The Balancing Act: Cat5e/6 cables are designed with carefully controlled inductance and capacitance to achieve a specific characteristic impedance of 100Ω. This is critical for gigabit Ethernet and PoE (Power over Ethernet).
If inductance is too high (poor cable quality, excessive untwisting at terminations, or mixing cable types), impedance mismatches occur. This causes signal reflections—energy bounces back down the cable instead of being absorbed by the receiver.
75Ω Standard: RG59 and RG6 coaxial cables are designed for 75Ω impedance, critical for clean video signals. The inductance and capacitance per meter are precisely balanced.
Using incorrect coax (e.g., RG58, which is 50Ω for radio applications) creates impedance mismatches that cause ghosting—you'll see faint duplicate images offset in time due to reflected signals.
Resistance, capacitance, and inductance interact to create frequency-dependent behavior. A cable acts as a low-pass filter, attenuating high-frequency components more than low-frequency ones.
The approximate cutoff frequency of a cable is:
Cat6 has lower capacitance (46pF/m vs. 52pF/m) and similar inductance. This gives it a higher bandwidth—certified to 250MHz vs. 100MHz for Cat5e.
For a 100m run at 100MHz:
This 4dB difference is enough to make or break 10Gbps links.
Cables act as antennas, picking up EMI (Electromagnetic Interference) from nearby power cables, fluorescent lights, motors, and radio transmitters.
UTP (Unshielded Twisted Pair): Relies solely on twisted pairs to cancel noise through differential signaling. Works well for short runs away from interference sources.
FTP (Foiled Twisted Pair)/STP (Shielded Twisted Pair): Adds a foil or braided shield around pairs. Essential in industrial environments with heavy machinery or near AC (Alternating Current) power runs.
RS485 (Recommended Standard 485) uses differential signaling: the receiver measures the voltage difference between A and B lines, not their absolute voltage. This is called common mode rejection.
If noise couples equally into both conductors (common mode noise), it cancels out during differential measurement. However:
Voltage drop isn't just about the end-of-line device. It affects all devices on shared power rails and creates ripple effects throughout systems.
Worst Case (all PIRs (Passive Infrared sensors) active):
Every cable has a maximum current rating based on conductor size and insulation temperature rating. Exceeding this causes heating, insulation degradation, and fire risk.
Regulatory Mandate: BS (British Standard) 5839-1:2025 requires fire alarm circuits to use fire-resisting cables that maintain circuit integrity during a fire. The standard defines two performance levels:
Cables must conform to BS (British Standard) 7629-1 (screened, multicore cables) or BS (British Standard) 7846 (armoured cables), both of which specify low smoke and low corrosive gas emissions when affected by fire.
Examples of compliant cables: FP200 (standard fire-resisting), FP400 (enhanced fire-resisting), and other cables meeting the BS (British Standard) EN (European Norm) 50200 test requirements. Standard alarm cable is NOT compliant for fire systems in the UK.
These cables typically use silicon rubber or LSZH (Low Smoke Zero Halogen) insulation that remains intact at high temperatures, ensuring the fire alarm continues to operate even as the building burns.
Resistance under fire conditions: Fire-resisting cables use larger conductors (commonly 1.5mm²) to ensure that even when insulation chars during fire exposure, the resistance increase doesn't cause circuit failure. This is why fire cables use 1.5mm² conductors despite typical fire circuits drawing <200mA—the cable must handle both normal operation AND worst-case fire conditions.
The Capacitance Trade-off: Fire-resisting cables have higher capacitance (typically 80pF/m for standard types like FP200, vs. 50pF/m for alarm cable) due to their fire-resistant construction:
This is why fire system cable runs must be more carefully managed than intruder alarm zones—you're balancing fire integrity requirements with electrical performance limitations.
Cables are not passive wires—they're complex electrical components with properties that fundamentally affect system performance:
Professional system design requires understanding these properties, calculating their effects, and selecting appropriate cables for each application. The Fire & Security Cable Calculator provides the numbers—this guide explains why they matter.