Understanding Maximum Circuit Lengths for Effective Fault Protection

Understanding Maximum Circuit Lengths for Effective Fault Protection

Along with selecting the right protective device, the correct design of the circuit is a crucial part of ensuring the safety of an electrical installation. The maximum allowable length of a circuit is an essential aspect of designing an electrical installation. In this blog, we break down how to assess maximum circuit lengths to guarantee proper operation of protective devices and maintain compliance with safety standards.

Why Maximum Circuit Length Matters

The primary role of protective devices, such as circuit breakers and fuses, is to disconnect the supply during a fault, protecting both equipment and people from electric shock. However, the effectiveness of these devices is influenced by the total impedance of the circuit. If a circuit is too long, voltage drops and impedance can prevent the protective device from operating within the required disconnection time, potentially compromising safety.

Calculating Maximum Circuit Lengths

When designing an electrical installation, the total earth-fault loop impedance (Zs) is a crucial factor. It’s made up of two components:

Z_s=Z_ext+Z_int

• Z_ext – the external impedance of the electricity supply system, including the distributor’s transformer and supply cables.
  
• Z_int – the internal impedance of the installation, which includes the conductors, active and protective earth within the circuit.
  

Refer to our Ausinet Earth Fault Loop Impedance blog to understand the Earth Fault Loop Impedance Path.

When the sizes of conductors are known, the maximum permissible route length (Lmax) can be calculated using a simple formula.

Maximum Circuit Length (L_max in meters) =
(0.8 x Nominal Phase Voltage × S_ph × S_pe) ÷ (Resistivity × I_a × (S_ph + S_pe))

Where:

• Nominal Phase Voltage (V) = typically 230V for single-phase systems
• Resistivity (Ω·mm²/m) =
  • Copper: 22.5 × 10⁻³
  • Aluminium: 36 × 10⁻³

Note – Pure metals have different resistivity values. (Pure copper – 1.72 x 10^-8, Pure Aluminium – 2.83 x 10^-8). In practical applications, pure metals are rarely used. Additionally, temperature plays a crucial role. Resistivity increases for metals with a positive temperature coefficient of resistance.  The above values are according to Australian standards. (Ref. AS/NZS 3000:2018)

• I_a (Amps) = fault current required to trip the breaker
  • Calculated as: I_a = Rated Current of breaker ×  multiplying factor
    • Fault  current needed to trip the circuit breaker – 
    • Type B = 4 x rated current
    • Type C = 7.5 x rated current
    • Type D = 12.5 x rated current
• S_ph (mm²) = cross-sectional area of the active conductor
• S_pe (mm²) = cross-sectional area of the protective earthing conductor
• L_max (m) = maximum allowable route length

Alternatively, use our Ausinet Maximum Circuit Length calculator. Input all the fields and then simply click on the ‘Calculate’ button for the results.

Interpretation of Results

If actual cable run ≤ L_max, then the circuit is compliant, which means that if an active conductor comes into contact with the body of the appliance, it will cause the circuit breaker to trip.

If actual cable run > L_max, then the risk of delayed fault clearance, redesign is required, such as increasing cable size or reducing the route length.

How to use the calculator –

Calculate the maximum route length based on earth fault loop impedance for a 16mm² copper 2C+e, V90.. The protective device used is a type C MCB of nominal current 80A. Nominal voltage – 230V. (This question is adapted from Electrical Trade Practices)

You have an active conductor size of 16mm²

• From AS/NZS 3000, table 5.1, we can find out that, for a 16mm² active cable, the required earth conductor is of 6mm². 

While entering values in the calculator, make sure that these values are in mm².

For convenience, AS/NZS 3000, Table 5.1, is provided at the end of the blog.

• While entering values, enter the nominal current of the circuit breaker in the calculator and select the type of circuit breaker. The calculator will take care of the multiplying factor.