How to test fault loop impedance with a multimeter

How to test fault loop impedance with a multimeter

Each loop should be tested to ensure that the actual loop impedance does not exceed the specified value of the protective device. Since exposure to electrical faults can be very serious, it is essential to check the ground loop impedance of your electrical system and outlets. Your system is valuable and the circuit must be maintained for the longevity and functionality of your business. In most homes, basic protection against electrical shock is achieved by creating a ground loop with a circuit breaker on the internal circuit. If a fault occurs and the touch voltage exceeds the permissible limit, the power supply to the ground circuit is quickly interrupted. 

In accordance with current national safety regulations, you must perform impedance testing of the circuits in your facility to ensure the safety of all visitors and personnel. Check the grounding of all electrical appliances and outlets to see if there is a fault in the circuit. An effective ground circuit will detect faults in the circuit and facilitate the response of your circuit breaker. A Carelabs technician will determine the resistance value of the ground circuit and inform you if the value is incorrect; it must be low enough for the circuit breaker to function properly. Carelabs will inspect and test the electrical installation. By hiring us for this task, you protect your employees and your liability.

The impedance and time values required depend on the type of installation (TN/TT, etc.) and the type of protection, such as miniature circuit breakers (MCB), box fuses or reusable fuses. Fault currents can occur in both the mains neutral circuit and the mains ground circuit, so the loop impedance of both circuits should be checked.

What should be done to check the ground loop impedance?

It is generally accepted that if the measured impedance of the ground fault loop of the circuit does not exceed 80% of the limit specified in BS 7671, the impedance under ground fault conditions is considered to be low enough to meet the limit specified in BS 7671 and the protective device will automatically trip within the specified time.

If the TT wiring system meets the requirements, it will provide sufficient protection against the risk of electric shock. 

Ra x Ia < 50.

Where "Ra" is the sum of the resistance of the earth bar and the protective conductor and "Ia" is the maximum current of the protective system. Ra multiplied by Ia must not exceed 50 V, i.e. the maximum voltage that can be affected in the event of an earth fault will not exceed 50 V.

A short-circuit loop impedance test is performed between the active conductor and ground. To check the loop impedance, our technicians use a ground loop impedance meter connected to the GPO to perform the measurements.

Our trained personnel perform ground loop impedance tests throughout the country.

How is a ground loop impedance test performed?

It is recommended that an external ground loop impedance test (Ze) be performed first. This test is performed at the electrical panel and indicates the loop impedance of an uninstalled circuit. Next, an installed loop impedance test (Zs) should be performed, which includes the circuit tested in the Ze test and the installed resistance. 

The AC impedance of a circuit may not be the same as its DC resistance, especially for circuits in excess of 100A. Therefore, the short-circuit loop impedance is measured at the same frequency as the nominal frequency of the network (50 Hz).

The impedance measurement of the grounding loop impedance of the Small Baron is performed on the supply side of the switchboard and the main grounding disconnector with the main circuit breaker open and all circuits isolated. The grounding device must be isolated (shoveled) from the equipment grounding system during the test. The mini-bar measurement should confirm that the impedance of the grounding circuit is the sum of the resistances. 

External ground circuit test sequence.

Step 1: Use a ground fault tester or select the ground fault test option on a multifunction tester, e.g. Megger 1553.

Step 2: Perform the test on the incoming side of the installation. Connect one test lead to the line terminal, a second test lead to the neutral and a third test lead (normally green) to the incoming protective conductor.  

Step 3: Press the TEST button. The measurement should give a low resistance value.

Do not forget to record this "Ze" value on the electrical installation certificate.

Once the "Ze" value of the installation has been determined, the "Zs" value of each circuit can be easily calculated.

The maximum measured value of the impedance of the ground circuit (Zs) for each circuit must match the value of Ze + R1 + R2, regardless of the requirements of the corresponding protective device. Test results measured at low currents are not recorded in the test results procedure, but Zs values calculated from the results of individual tests are preferred, i.e. 

The formula for determining Zs.

Zs = Ze + (R1+R2) 

Zs - impedance of the ground loop of the circuit under test.

Ze: impedance of the ground loop outside the network.

(R1 + R2) - The sum of the line resistance and the ground resistance of the circuit under test.

Here Ze is derived from the high current test and R1 + R2 is determined in the continuity test of the circuit. The type of test result recorded and the test method used should be indicated in the appropriate comment column of the test result plan.

The impedance of the ground loop Zs is tested on the outermost side of each circuit. in most cases it is necessary to bypass the circuit breaker. The total ground loop impedance is measured by connecting a loop tester to the tap or, in some cases, using an external ground probe. The value of the ground loop impedance is the sum of the resistances. If an external ground probe is used, the ground loop impedance is measured by applying the external probe directly to the ground rod, collector and ground rod connection points. The same measurement can be made by touching the ground probe to the conductive parts of the equipment exposed to circuits and metal parts. 

Ground loop test sequence.

Step 1: Find the farthest point on the circuit to be tested (e.g., the farthest outlet).

Step 2: Using a suitable ground loop tester, connect the test leads to the line, neutral and ground terminal.  

Step 3: Measure the test result and record it in the test result log.

If the circuit is protected by an RCD, you must select the "no trip" function on the Megger 1553 to prevent the RCD from accidentally tripping. If your tester does not have this option, you must connect an RCD.

Once you have determined the Zs values for each circuit, check that these values are within the acceptable limits described in BS 7671.

 Methods for checking loop impedance  

Currently, most contractors use one of five different test techniques when performing loop impedance tests 

Two-wire high current testing  

Two-wire DC saturation test (obsolete) 

3-wire "no trip" test  

2-wire "no trip" test  

4-wire network impedance test  

2-wire high current test 

This is the traditional loop impedance test. It draws up to 20A of test current and is a simple 2-wire connection and is usually the fastest and most accurate test in everyday use. Most standard loop impedance testers offer this type of test. Due to the relatively high test current, readings are generally unaffected by external influences, providing stable and repeatable readings in most cases.

Two-wire "no-trip" DC saturation testing 

A DC test current is injected into the circuit prior to the standard two-wire high current test. The purpose of this DC current test is to saturate the RCD test coil to allow sufficient time for a high current test. However, due to the increasing use of electronic RCDs, the use of this method is very limited.

3-wire leakage-free testing  

This test method overcomes the need to bypass even new electronic protection devices by using a low test current between wire and ground, while achieving a degree of accuracy. It is clear that RCD/RCBO bypassing saves time. By requiring connections to line, neutral and ground, the tester can now confirm the presence of all three elements and indicate whether a polarity reversal has occurred at the test point. There are no problems with the MCB tripping.

2-wire "no trip" test 

This can be used to test most RCDs and RCBOs without having to pull them down. Since no neutral connection is required, left and right operation is maintained, but there is no longer a reverse polarity indication or no neutral warning. Although the physical test time is similar to the 3-wire method, the time saved by not having to bridge the RCDs makes the test more efficient.

4-wire network impedance test 

The test uses a 4-wire Kelvin connection, which negates the internal resistance of the wires and contacts; the test is so accurate. With test currents up to 1,000 A, measurements can be made to an accuracy of 10 MOhm. Therefore, in this test method there is no "no displacement" option. Since the specific application is the measurement of substations/distribution switchboards, this test instrument ensures that the test engineer can obtain accurate readings.

Circuits protected by RCDs require special attention, as current passes through the return phase protection circuit during the ground circuit test. Therefore, when testing RCD-protected circuits, equipment manufacturers are faced with the difficulty of providing test results similar to those obtained when testing circuits without RCD protection, without tripping the RCD circuit breaker during the test. Therefore, all RCDs must be bypassed by short-circuit connections before ground fault loop testing can be performed. Of course, it is very important that these connections are removed after the test.

At Carelabs, we use a ground loop impedance tester that does not trip the RCD circuits we test. Our team performs all tests and checks in accordance with current safety standards. Testing is mandatory for everyone's safety. Get tested today to make sure your workplace is safe; we will help you comply with all regulations.

As the test results depend on the supply voltage, small fluctuations can affect the test results. Therefore, the test must be repeated several times to ensure consistent results. All persons present should avoid the risk of electric shock when touching and performing the test. When purchasing a loop tester, ask the distributor for a test lead to enable Ze and Zs measurements to be made.

Impedance values.

Testing and recording the impedance of the fault ground circuit The fault ground circuit impedance test is carried out throughout the electrical installation to verify compliance with BS 7671 (IET Wiring Regulations) for short-circuit protection and is normally carried out as follows. 

Test current of approximately 23 A when the circuit is protected only by an overcurrent device such as a fuse or circuit breaker; or  

a test current of approximately 15 mA to avoid unnecessary tripping on circuits protected by 30 mA or other RCDs. 

In general, test results are broadly stable at high currents (23 A) in the range of 0.1 Ω to 1.0 Ω with a resolution of 0.01 Ω. For low current tests (15 mA) the resolution is 0.1 Ω, but attempts to reduce this to 0.01 Ω have largely failed to produce similar stable results for readings below 1.0 Ω. 

A recent study by a major UK meter manufacturer, using meters from seven different manufacturers under controlled conditions, showed significant differences in meter readings. Subsequent tests showed that the problem was mainly due to low test currents caused by power quality fluctuations due to voltage magnitude, transients, harmonics, etc. Similar tests with a stable power supply with a pure 50 Hz waveform also yielded results. However, it should be noted that these variations are usually in the range of 1.0 Ω or less and are not important to the proper operation of the RCD. 

Once the test is completed, we will provide you with a new test date (for the next ground loop impedance test), which conforms to national standards. When the time is up, our team will inform you and the test will be repeated. All results are recorded in a detailed report that is delivered to each customer. This report will classify your equipment as pass or fail. This document is kept on file so that it can be consulted at a later date and checked for compliance if necessary. We offer our customers a wide range of inspection and testing services so that you can guarantee your entire installation in a single visit. We can also offer other inspection services after impedance testing. With such a wide range of services, there is no reason to go elsewhere for safety checks.

With a ground loop impedance test you can be sure that you have made an electrically safe ground connection with a sufficiently low residual resistance. The ground loop impedance test is very important because if a live conductor is accidentally connected to a faulty electrical appliance or grounding conductor in a circuit, the resulting short-circuit current to ground could be high enough to cause an electrical shock or generate enough heat to start a fire.... Normally, a fuse or other protective device in the circuit will trip, but the actual short-circuit current in the faulty device may not be high enough and the protective device may take too long to trip. Delays can have catastrophic consequences to life and property.