How to Test Fault Loop Impedance with a Multimeter

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Let’s face it. Avoiding shocks and incidents is far more essential than making coffee accessible for your staff. With an ever-growing technological advancement in the workplace, it is simple to ignore the simplest of incidents when you least expect them.

That only indicates that you must take time to perform proper fault loop impedance testing that will help test your power points and guarantee safety in the workplace. 

Testing an earth loop impedance involves having your power points and electric installations tested to prevent electric shocks and harm at your home or workplace. That is performed by ensuring the impedance of the electric circuit is producing enough fault current to run a device. 

A fault loop impedance is the most critical aspect to consider when you are designing an electrical circuit. That’s because it impacts the circuit breaker’s size and the cable’s size used on installation.

Every professional and skilled electrician has heard of that. However, nearly half of all electricians don’t completely understand it. Wiring and designing electrical circuits without a detailed and full understanding of this idea could pose a serious safety risk to the end-user. 

This article will explain in detail the idea of fault loop impedance and how to properly test it using a multimeter. 

What is a Fault Loop Impedance, by the way? 

A fault loop impedance is what its name tells—it’s the impedance (resistance) of the loop (circuit) under fault circumstances. A fault condition is normally categorized as a short circuit at the end of the cable run or the terminal’s load. 

The term loop here talks about the whole path the current will flow in the circuit under fault conditions. You will find two opportunities for a short circuit at the end of the cable run.

  1. A phase-to-phase short circuit
  2. A phase-to-earth short circuit 

Take note that the impedance of the phase-to-earth circuit is utilized in such calculations. That’s because the earth wire is normally smaller than active wires. Therefore, it will have more resistance than a phase-to-phase fault. 

The impedance on the circuit identifies the amount of current if there’s a short circuit. The cable path efficiently functions as a resistor, which has been placed across the circuit breaker. Simply put, if the impedance is high, less current will flow. 

If it’s excessively high, there’s a real possibility that the circuit breaker will take too long to trip, if it trips at all. 

Should You Test Fault Loop Impedance? 

As mentioned earlier, testing a fault loop impedance is performed to ensure that a fault happens in an electric circuit. The electrical fault current will be powerful enough to set off the circuit protection. Keep in mind that circuits could overheat and catch on fire if that fault current is left undetected. 

It takes the path of least resistance to the ground with electricity. Various properties in villages, towns, or even out in the countryside have unique earthing arrangements. So too factories, warehouses, offices, and shops, among other establishments.

Such electrical earth wiring arrangements in buildings are often linked to the ground. In electrical terms, that’s also known as the earth return circuit. The earthing rods or earthing cables are there for electrical current to pass through if a short circuit occurs. 

Possible Impacts When Doing Fault Loop Impedance Tests

Earth loop impedance testing for new installations should post some operational concerns throughout the first verification process. The reason behind that is because the installation will not have been put into service. 

Nevertheless, for an installation in service, there might be some serious impacts for the user of the premises. For instance, computer data may be lost, or a home’s life support system may be turned off because of an accidental interruption of supply throughout the test. The unintentional operation of an RCD may cause it. 

Moreover, involuntary disconnection of a distribution board, circuit, group of circuits, or even an entire installation could take place if an RCD runs when an earth fault loop impedance test is performed. Therefore, a series of methods have been created to lessen the possibility of an RCD running during such a test.

One example of that method is calculation. Another way is to measure the fault loop impedance along with a loop test instrument, which supplies a test current low enough not to trip the RCD. 

So, How Do You Test Fault Loop Impedance?

Here are the basic steps you need to do when testing your fault loop impedance with a digital multimeter:

  1. Find the outermost point on the circuit you want to test. For instance, you’d like to test the outermost socket. 
  1. Connect the test leads to the Earth, Neutral, and Line terminals using the proper Earth Fault Loop Tester. 
  1. Calculate and take note of the results of the corresponding test on the Schedule of Test Results
  1. Keep in mind that you need to choose the No Trip function of the tester, especially if the circuit is RCD protected. Doing this will help you prevent RCD’s nuisance tripping. However, if your multimeter does not have that function, it will help if you link out the RCD. 
  1. After you acquire the value of Zs for each circuit, the next step is to confirm those values are within the specified limits designated by NEC. 

Final Thoughts

Testing a fault loop impedance is a way of making sure you’ve made an electrically harmless ground connection having a suitably low residential resistance. Take note that earth loop impedance testing is crucial since the resulting short-circuit current to the earth could be high enough to cause electric shock or produce sufficient heat to begin a fire. That’s especially true when a live conductor is incidentally linked to an earth conductor in a faulty circuit or appliance. 

Typically, the fuse will fire, or some circuit protection device will trip. Still, a scenario may happen where the short-circuit current in a defective installation is inadequate, and the protection device would take too long to start. 

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