A U Sparky Pty Ltd.
Master Electricians.

 

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Testing. Fault Loop Impedance.

Fault loop impedance testing must be carried out as part of any electrical work that involves the connecting of a appliance or outlet to a circuit to guarantee the Electrical safety of a circuit.

AS/NZS 3000 Wiring Rules applies throughout Australia and New Zealand and is referenced in whole or in part in legislation in all states of Australia and in New Zealand, as a minimum requirement.

To that extent, the electrical circuit design and electrical installation of these circuits must meet the requirements of the Wiring Rules standard.

One of the basic safety requirements stated in AS/NZS 3000 Wiring Rules is protection from 'indirect contact' with live parts.

Automatic disconnection of the power supply is the most common method of achieving this.

To comply with Clause 1.7.4.3.3 of AS/NZS 3000 Wiring Rules, each circuit in an electrical installation must be designed and protected so that automatic disconnection of the power supply will occur within the specified time when a short circuit of negligible impedance occurs between an active conductor or an exposed conductive part anywhere in an electrical installation.

To fulfil this requirement of AS/NZS 3000 Wiring Rules when a fault current flows, the impedance of the fault current path (referred to in AS/NZS 3000 Wiring Rules as the Fault Loop Impedance Path) must be low enough to allow sufficient current to flow to cause the protective device to operate within the specified time.

Understanding Loop Impedance and Why it is Important

Measuring or calculating loop impedance is essential when determining the prospective short circuit current in a circuit so that the correct overload protection is used.

A fundamental question must be answered “WILL THE PROTECTION DEVICE FUNCTION DURING A FAULT CONDITION?”

To determine the fault current rating for an overload device the maximum current that can be delivered to that point must be considered. Loop impedance at that point and the previous protective device will determine the correct rating.

In a short circuit the overload device must be able to withstand that current before tripping. In other words the protection device must not melt , the contacts must not weld together while the fault is present and the device must be capable of opening under fault conditions.

Prospective short circuit current measured in an installation is used to consider whether the circuit is capable of carrying sufficient current so as to trip the overload device in the specified time.

Loop impedances can be calculated as per appendix B of the Wiring Rules or measured with a suitable test instrument. Using a test instrument may give you a more accurate reading of the circuit because it will include all factors in the line such as bad connections and back e.m.f.s generated by adjacent electrical lines and other loads being present on the system at the time of the test Note that impedance on the active and neutral lines are dynamic due to other sub circuits attached to that line and their changes. The earth impedance without the MEN connection would be constant.

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.

Using a loop impedance meter is useful in determining if the breaker can carry the load during the time it takes to open the circuit and determine the time the overload device could take to open. Using a loop impedance meter is optional  according to the wiring rules. However its use is essential for testing installations and will improve confidence and safety in ensuring that overload protection will operate correctly. It is also very useful in fault finding such as locating loose connections.

Your Electrical Contractor must have the tools to test and record your fault loop impedance for all circuits worked on.
You don't need to know how or why but as the owner you need to be aware the Electrician has completed these tests.

IN BASIC ENGLISH:

If your appliance becomes faulty, The resistance in the earth return circuit must be low enough to allow the Circuit Breaker or fuse to trip (due to a high power usage) before the voltage can rise above the 50 Volt on the outer or metal shell or parts and ground or surrounding materials. This will help so you are protected from being electrocuted if you touch the faulty unit.

(This requirement and the installation of a Safety Switch will provide a high level of personal Safety). Failure to do this test and not having the correct setup will create a very dangerous situation.