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Fluke Tool Guide - Why and How to Measure Earth Loop Impedance

 
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What is electrical impedance?

In a DC circuit, there is no distinction between resistance and impedance. In an AC circuit however, like we use for our mains electricity supply, impedance is a measure of how much opposition a particular circuit presents to the flow of a current when a voltage is applied. It is described as a ratio of voltage to current.

 

Why measure earth fault loop impedance?

 

Measuring earth loop impedance is critical for electrical safety, forming an integral part of the International Electro-Technical Commission (IEC) guidelines.

 

If a live conductor is connected to an earth conductor by accident in a faulty appliance or circuit, it will cause it to short circuit, in which the current can be high enough to cause an electric shock or start a fire.

 

If this were to happen, the fuse would usually blow or another circuit protection device trip, but it’s possible that in a faulty circuit the short-circuit current would be at an insufficient level to active the protection device quickly enough – this delay could be the difference between life and death!

 

It is therefore necessary to know if the impedance of the path that the current would take in such a case is low enough that it lets enough current flow in the event of a fault, allowing a protective device to operate and shut off the circuit within a safe time limit.

 

 

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Verifying protection by automatic supply disconnection

 

Under IEC 60364, earth fault loop testing falls under the category of ‘verifying protection by automatic supply disconnection’. Different systems (such as TT or IT systems) require specific measurements – the guide below is specific to TN systems which require measurement of the fault loop impedance, as well as verification of the particular characteristics of the protective device in the circuit. This could be for example a visual inspection of the nominal current setting of circuit breakers, current ratings and blow characteristics of fuses and assurance that RCDs are functioning correctly.

 

It is important to measure the earth fault loop impedance of each and every circuit, from the point of use back to the incoming supply connection. It is a good idea to also take a measurement of the external loop impedance of the installation at the main distribution panel or supply point. This value in this reading will form a part of the overall impedance from any part of the circuit installation.

 

Once you know the earth loop impedance you are able to calculate the value of the prospective fault current (PFC) at any specific point within the installation. You can also ensure therefore ensure that every protective device in the installation has a sufficient rating to clear the PFC level.

 

 

How to measure earth loop impedance

 

The impedance of an AC circuit is not necessarily the same as the resistance of a DC circuit, especially if it is rated at 100A or higher, so the fault loop impedance is always measured at 50Hz – the nominal mains frequency.

The earth loop impedance should be measured at every outlet of the installation, for example all light fixtures or distribution panels.

An earth loop impedance test will measure the resistance offered to the flow of current in the path that a fault current would take from line to protective earth. This resistance must be sufficiently low to allow enough current to flow past to trip the circuit protective device in place, whether that’s a fuse, a circuit breaker MCB or something else.

 

Features of the Fluke 1650 Series

 

 

 

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The Fluke 1650 Series testers, from our range of Fluke installation testers, are able to measure prospective short circuit current (PSC) and prospective fault current (PFC), and come with a built-in earth loop impedance testing function.

 

Fluke 1650 Series testers can be used to carry out the necessary tests at a distribution board using the three test leads that are supplied with the tester. They can also be used to carry out the test at an appliance outlet using a dedicated lead fitted with a mains plug, also supplied.

 

Fluke 1650 Multifunction Installation testers can measure the earth resistance component of the total loop resistance, or the basic loop impedance between line and protective earth, displaying the results as an easy-to-read measurement in ohms.

 

Other Fluke 1650 Series features and specifications:

 

  • 'Auto null’ function to automatically remove test-lead resistance from the final measurement
  • Automatic overheat protection
  • Loop measurements displayed with 0.01omh resolution
  • Line impedance measurement ability (the source impedance between the line and neutral, or the line-to-line impedance in a three phase system)

 

In addition, the high tech Fluke 1650 Series has been specially designed to avoid common problems caused by RCDs.

 

Conventional earth loop impedance testing techniques often trip RCDs which prevents further measurement. Up until now, the only solutions had been to either ‘bridge’ the RCD or to temporarily replace it with an MCB with an equivalent rating. These are both time consuming and, more importantly, potentially dangerous practices.

 

The Fluke 1650 Series testers are built using an innovative, patented technology that ensures both electromechanical and electronic RCDs do not trip whilst such measurements are being taken. This leads to faster tests and more consistent results.

 

 

Interpreting the results

It is important that you understand the local regulations (and know how to interpret the results of your earth loop impedance tests) to ensure that the safety characteristics of the installation you are testing are within the limits allowed by local law and safety guidelines.

 

An excessive measurement for example should prompt an investigation to find the cause, so that remedial action can be carried out and the installation re-tested.

 

Note that this guide is not intended to replace the recognised standards as given in IEC 60354, or its local equivalent. Always consult the appropriate standards publication before carrying out work.


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