Thursday, October 27, 2016

What is Earth Fault Loop Impedance?

What is Earth Fault Loop Impedance (EFL)?


IEC defined Earth Fault Loop Impedance as


The impedance of the earth fault current loop starting and ending at the point of earth fault. This impedance is denoted by the symbol (Zs).


Source: City and Guilds


Why knowing its value important?


It is important because EFL impedance affects the time for a protective devices to open during fault condition. From basic circuit analysis, higher resistance yield lower currents and lower resistance will also yield the opposite. 

The circuit breaker curve describe how value of fault current affects the time of its opening during fault condition. Because of this the IEC provide a specific values of earth fault loop impedance to maintain the integrity of our protective devices. 


What affects EFL impedance?

The following will affect the value of earth fault loop impedance:

  • Earth Resistance
  • Cable Resistance
  • Transformer Impedance

Among the three values, only the size of the cable can be readily and practically be changed in case during verification the EFL does not conform with the requirement. 

In case it happened, we need to change the size of the cable since changing the value of earth resistance and transformer impedance is impractical. Select the next higher size of cable until such time that we can attain the allowed EFL value. 


How can we determine the exact value of EFL?


IEC provides specific values of EFL for different voltage levels in electrical systems in order to maintain the time required for the protective devices to automatically open during fault condition.


Source: BS 7671:2008
The table shows the maximum value of EFL needed in order for the protective devices to function normally. For example if we put 32 Ampere Type B circuit breaker to protect our load that means the maximum EFL that we need to have is less than or equal to 1.44 ohms. 


Note: The table shown is just one of the example given by IEC for a specific application. For other design and applications you may read IEC and BS 7671 standards to see other tables and you can get the exact information about your specific requirement. 

IEC also provide the maximum disconnection time for specific voltage levels that maybe used by the customer or local electricity provider. 


Source: BS7671:2008

For example if we are using 230 V single phase TN-S system, the maximum time allowed for the circuit breaker to open during fault condition must be 0.4 s only. More than that there will be serious issues such as equipment damage, electricution and other unwanted results. 



How to obtain EFL value?


There are different methods on how to obtain the value of EFL. One method is calculation which will be discussed in details in other article. But the most practical way is the use of EFL tester.



Source: City and Guilds

Procedure on how to measure EFL by using EFL tester:
  1. Isolate and secure the installation main switch in the off position. 
  2. Ensure, for reasons of safety, that all main protective bonding is connected to the main earthing terminal. If the supplier's cable is faulty, a fault current may be introduced to extraneous parts of the installation. 
  3. Disconnect the earthing conductor from the main earthing terminal. 
  4. Check instrument for safety and correct settings. 
  5. Connect the earth clip to the disconnected earthing conductor, then connect the test instrument line probe to the supply line terminal of the main switch. 
  6. Press the test button and note the result. 
  7. For three-phase installations, repeat this test for all line conductors. For three-phase installations, the highest reading obtained is recorded as the external earth fault loop impedance. 
  8. For a single-phase installation, record the value. 
  9. Reconnect the earthing conductor to the MET. 
  10. The installation supply may be switched back on if required. 
Source:

  • IET
  • IEC
  • City and Guilds

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