Measurement of ground loop impedance and determination of prospective fault current (PFC) are critical to safety and are an integral part of the International Electrotechnical Commission (IEC) guidelines. The IEC 60364 standard (and its various national equivalents) specifies requirements for fixed electrical installations in buildings, including verification of protective functions by automatic disconnection of the power supply. The new Fluke 1650 Series Ground Resistance Tester features loop impedance testing capabilities and is also capable of measuring prospective short circuit current (PSC) and fault current (PFC).
Why measure ground loop impedance?
Ground loop impedance testing is essential because if a live conductor is accidentally connected to a grounded conductor in a faulty piece of equipment or circuit, the resulting short-circuit current to ground can easily be large enough to cause an electric shock or generate heat. Enough to start a fire. Normally, a fuse will blow or other circuit protection devices will trigger; however, it may be the case that when the actual short-circuit current in a faulty installation is insufficient to reach a certain level, protective devices will take too long to activate. This delay will pose a huge threat to life and property. Therefore, it is important to know whether the impedance of any fault current path is low enough to allow a large enough current to pass when a fault occurs, and whether any installed protective equipment can start operating within a safe time limit.
Since the circuit loop impedance measurement is performed when the power is normally turned on, precautions need to be taken to avoid possible electric shock and danger to workers near the circuit under test.
Testing the protective function by automatically disconnecting the power supply
In IEC 60364, the category of the fault loop test is "Testing the protective function by automatically disconnecting the power supply". This section covers the effectiveness of protective measures and how applications should be tested depending on the type of system. For example, the IT system requires measurement of the ground electrode resistance of the exposed conductive parts of the facility, however, the IT system uses the calculation or measurement of the first fault current. This application case is focused on TN systems, which require measurement of the fault loop impedance and verification of the characteristics of the associated protective equipment (that is, the nominal current setting of the circuit breaker, the current rating and fusing characteristics of the fuse, and the correct functioning of the RCD Inspection).
The ground loop impedance of each circuit needs to be measured from the point of use to the point of incoming power connection.
The external loop impedance of the installation may also be measured individually at the point of introduction or at the main switchboard, and this value will be part of the overall loop impedance of any part of the installation where the final circuit is installed. Once the ground loop impedance is known, the prospective fault current (PFC) can be calculated at any point in the facility and ensure that all installed protective devices are rated adequately to clear the potential fault current.
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Measuring Ground Loop Impedance
Since the AC impedance of a circuit is different from its DC resistance - especially for ratings over 100 A Circuit - Measure the fault loop impedance using the same frequency as the nominal mains frequency (50 Hz). Earth Loop Resistance Electrical and electronic type RCDs measure the resistance of the fault current path between phase conductors and protective earth in the loop impedance test. This resistance must be low enough to allow enough current to flow through it to trigger a circuit protection device such as a fuse or mini circuit breaker.
Using the three test wires provided with the instrument, the Fluke 1650 series ground resistance tester can be tested at the distribution board, or at the equipment socket using a special test wire with a power plug. The instrument also provides plugs that comply with relevant national standards.
1650 Series Special Functions
Fluke 1650 Ground Resistance Tester has special functions to measure the ground resistance component of the total loop resistance, and also has special functions to measure the phase line and protective earth. Measurement of the basic loop impedance between two meters, and the result can be directly displayed in ohms. Measurements shall be made at all sockets in the installation, including lighting fixtures and all (sub)switchboards.
The 1650 Series Ground Resistance Tester offers several useful features to aid in ground loop testing, including an "auto-zero" feature that eliminates test lead resistance from measurements, and automatic overheating protection. Loop measurements are displayed with a resolution of 0.01 ohms.
The instrument can also measure line impedance, which is the source impedance between line and neutral or line-to-line impedance in a three-phase system.
Traditional methods of measuring loop impedance often cause the RCD to trip, preventing further measurements. Often the only way to get around the above problems is to "bridge" the RCD, or to replace the RCD with an MCB of the same rating during testing - both methods are dangerous and time-consuming. To overcome this, in the 1650 Series Ground Resistance Testers, Fluke uses innovative patented technology to ensure that electromechanical and electronic type RCDs do not trip during loop impedance measurements. This technology can provide more consistent results and test times are significantly shorter than those required by most traditional safety testers.
Calculate Prospective Fault Current
In addition to measuring ground loop impedance, the 1650 Series Ground Resistance Tester automatically calculates Prospective Fault Current (PFC) and displays it on the dual display The lower part shows the measured values. Determination of PFC is important to ensure that the tolerances of fuses and overcurrent circuit breakers are not exceeded.
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The tester can also directly read the prospective short circuit current (PSC).
Interpretation of results and remedial actions
Remember, simply conducting a test and recording the results is not enough. Knowledge of local regulations – and knowledge of how to interpret the results – is also required to ensure that the safety characteristics of the installation comply with prescribed limits. For example, if the ground loop impedance value is too large, the cause should be investigated promptly. Remedial measures should then be taken and the installation retested.
Attention!
This application note does not replace or supersede the recognized standards in IEC 60364 (or national equivalent), but it summarizes fault loop impedance test considerations. Please be sure to refer to the corresponding promulgated standards.