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WHAT IS EARTH FAULT LOOP IMPEDANCE ?

 

 

When a person comes into contact with a non-live part of an installation that has become live due to a fault condition, they can receive a severe electric shock, resulting in injury or possibly death. An example of indirect contact is when a fault occurs due to heat or perhaps loose wiring within a toaster touches the metal casing becomes live due to viberation, over usage or simply old age. 

 

 

 

An upstream protective device (ie:- RCD, MCB, MCBRCD, ELCB) needs to clear the original fault immediately, in order to protect the person who comes into indirect contact with the metal casing. With high fault loop impedance, the fault current may be too low for the protection devices to respond in a time quick enough to protect the person from an electric shock.

In basic easy to know terms...
All electricians work to the Australian Standard wiring regulations. The Australian Wiring rules (AS/NZS3000:2000) introduced a clause relating to protection against indirect contact (Clause 2.3). Measuring fault loop impedance is mandatory to ensure compliance with this clause. If your appliance becomes faulty, the resistance in the earth return circuit must be low enough to allow the safety switches (RCD, MCB, ELCB) or fuse to trip off (due to a high power usage) before the voltage can rise above the 50 Volts on the outer or metal shell or parts and ground or surrounding materials.

Why do we need to test for Fault Loop Impedance?
In short, the testing is there to show the electrician that if there is a fault the safety switch will operate within the AS regulated time interval.

How do we test for Fault Loop Impedance?
Fault Loop Impedance testing is done between active conductors and earth. To test the loop impedance, simply plug a Earlth Loop Impedance Tester into a GPO and take a reading. However, with testing existing "live" circuits, you must either have a unique Earth Loop Impediance tester which will not trip out the particular circuits RCD or you must confirm that all appliances on that particular circuit which you are testing are not essential services otherwise you may trip the circuit when testing. This is extremely important and many electricians simply forget to confirm this important matter often at their own expense. Can you imagine tripping out office computers inside the stock exchange when a billion dollar transaction was taking place??? need I say more. Another way is to connect the crocodile clips leads from a reliable Loop Impedance tester to the furthest point in the circuit.

We use a Kyoritsu 4120A model for testing and when it states it has a "D-LOK" helps the electrician by not tripping live circuits as stated in the manual - this is not 100% true. We have found this out the hard way however, we were fortunate enough to discover this on residential homes RCD circuits. So, beware of manufactures marketing spins...if you notice they will insert clever BS wording. For example Kyoritsu states "D-LOK circuit - Model 4120A with unique D-LOK circuit avoids the need to by pass most RCD's" The clever word in this statement is "most" but doesn't say what brand it doesn't cover (go figure?) I would like to see you wiggle out of that one when your client sends you a bill for all the information or stock they lost when their circuits went down when you were relying on this D-LOK...

 
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How do we interpret the results?
Take the Earth Loop Impedance reading from the Loop Impedance tester and compare it to the Fault Loop Impedance table in the Australian Wiring Rules. Your readings must be within the AS/NZS3000:2000 tables within the regulations. AS/NZS 3000:2000 (Clause 1.7.4.3.3) requires that the ‘characteristics of the protective devices and the earthing system impedance shall be such that ... automatic disconnection of the supply will occur within the specified time. Using the loop impedance values, the tripping time of circuit breakers or fuses in the circuit can be determined. Clause 1.7.4.3.4 of the wiring rules states the required maximum times for circuit disconnection.

Understanding Loop Imedance and Why it is so Important?
The bottom line is - “WILL MY PROTECTION DEVICE FUNCTION DURING A FAULT CONDITION?”

Using a loop impedance meter is useful, however, what we find very strange is that it is not compulsory. Using a loop impedance meter is optional when testing the circuit which has been governed by an existing RCD before hand (go figure?)  according to the wiring rules. As if circumstances won't change over time eh? What we firmly believe is using an Earth Loop Impedance tester for all circuits is essential when modifing an existing circuit wheter it already had an RCD protecting it. It will improve confidence and safety in ensuring that the desired overload protection will operate correctly at its most vunerable time. It is also very useful in fault finding possible loose connections.

This page was last modified on 27 December 2009 and maybe out of date with regards to newer practices at the time of reading this article

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