Conductor Resistance (Effects and Implications)
Conductor resistance is the opposition to the flow of electric current within a wire or cable. It’s a fundamental property that affects how efficiently electricity travels through a conductor.
What Determines Conductor Resistance?
Conductor resistance depends on several key factors:
- Material: Copper and aluminium are common conductors copper has lower resistance than aluminium
- Length: Longer conductors have more resistance
- Cross-sectional area: Thicker wires have less resistance
- Temperature: Resistance increases as temperature rises
- Purity and structure: Impurities or poor manufacturing can increase resistance
Here is the formula for calculating resistance:
Higher conductor resistance increases disconnection times because it reduces fault current, potentially delaying the operation of protective devices like fuses, circuit breakers, and RCDs
This can compromise safety and violate regulatory requirements
Conductor resistance plays a critical role in determining how quickly a protective device disconnects the supply during a fault
• Fault current depends on loop impedance:
• Lower fault current = slower disconnection:
Fault current depends on loop impedance: The total impedance of the fault loop (including conductor resistance) determines the fault current. Higher resistance = lower fault current.
Lower fault current = slower disconnection: Protective devices are calibrated to trip at specific current thresholds. If the fault current is too low due to high resistance, the device may not trip within the required time.
Safety Implications
- Delayed disconnection increases the risk of:
- Electric shock
- Fire hazards
- Damage to equipment
BS 7671 Requirements:
- For TN systems, disconnection must occur within 0.4 seconds for socket outlets up to 63A, and 5 seconds for other circuits
- If conductor resistance causes disconnection times to exceed these limits, the installation is non-compliant
Factors That Increase Conductor Resistance
- Long cable runs: More length = more resistance
- Small conductor cross-section: Thinner wires have higher resistance
- High-temperature environments: Resistance increases with temperature
- Poor connections or corrosion: Adds resistance at joints and terminals
