Understanding Earth Fault-Loop Impedance in Electrical Installations

Understanding Earth Fault-Loop Impedance in Electrical Installations

When it comes to electrical safety, one of the most important protective measures is ensuring that earth fault currents (leakage currents) are safely cleared. This is achieved through the automatic disconnection of the supply, which protects people from dangerous touch voltages in the event of an insulation fault.

The effectiveness of this protection depends on two main factors:

• The characteristics of the circuit protective device (RCD or RCBO).
  
  
• The impedance of the earthing system.
  
  

The significance of earthing is to prevent a potential difference between any exposed conductive parts and the ground. Whenever there is an earthing fault, the earthing system provides a path of low resistance, allowing the current to be high enough to activate the protection device and disconnect the circuit as soon as possible.

Why Disconnection Time Matters

The AS/NZS 60479 series outlines two components that establish a relationship between prospective touch voltage and its duration, ensuring that it typically does not cause harmful physiological effects on individuals exposed to that voltage.

The two components are: 

1. The effect on the human body of electrical currents of various magnitudes and durations flowing through the body
   
   
2. The electrical impedance of the human body as a function of touch voltage.
   
   

The necessary correlation between prospective touch voltage and disconnection time is established for standard conditions, as illustrated in Curve L. This figure outlines the maximum duration a person may maintain contact with an exposed live part of a circuit across various touch voltages under normal circumstances (Curve L). 

Normal conditions are characterised by:

 (i) Dry environments, and 

(ii) Floors that provide considerable resistance. 

In specific scenarios, such as damp environments and instances where there is contact with wet or bare skin, it is essential to further restrict touch voltages, as indicated by Curve Lp. Details regarding damp conditions are addressed in Section 6 of this AS3000 Standard.

When a fault occurs, the protective device must act quickly enough to prevent harm. For a typical 230 V supply:

0.4 seconds is the maximum disconnection time for:

• Socket outlets rated up to 63 A.
  
  
• Hand-held Class I equipment.
  
  
• Portable equipment is designed to be moved during use.
  
  

The maximum disconnection time for other circuits is 5 seconds, provided it can be demonstrated that people are not exposed to unsafe touch voltages.

The key here is that the fault current must be high enough to trip the protective device within these limits

The Earth Fault-Loop Path

In a Multiple Earthed Neutral (MEN) system, the earth fault loop is the complete path that the fault current takes. It includes:

• The active conductor up to the fault point.
  
  
• The protective earthing conductor (PE), including the MEN link.
  
  
• The neutral return path, which runs from the installation back to the transformer’s neutral point.
  
  
• The transformer itself, through its winding and neutral connection.
  
  

This closed loop allows the fault current to circulate, activating the protective device.

Actual Circuit Impedance (Z_circuit) = Z_supply + Z_active + Z_earth 

Where:

Z_earth = impedance of the protective earthing conductor

Z_supply = impedance of the supply network

Z_active = impedance of the active conductor

Let’s take an example to understand the actual impedance. Refer to the image below. There is a fault in the load at point D.

The total earth fault impedance (Z_s)in this circuit will be : 

Z_s = Z_AB + Z_BC + Z_CD + Z_DE + Z_EF + Z_FG + Z_GH + Z_HA

Impedance and Safety

For disconnection to occur safely, the impedance of the earth fault-loop must be low enough to allow sufficient fault current to flow. 

Maximum Permissible Impedance in the circuit

Z_s = U_o ÷ I_a

Where:

Z_s = maximum earth fault-loop impedance (Ohms)

• U_o = nominal phase voltage (typically 230V)
• I_a = fault current required to trip the protective device within the specified time
  
  

Refer to our Ausinet Calculator to calculate the Maximum Permissible Impedance in the circuit.