If a fault occurs between a phase conductor and the protective conductor in an electrical installation, the assigned protective device must disconnect quickly and safely. This is exactly where loop impedance is decisive. If it is too high, the fault current may be too low. The fuse or miniature circuit breaker may then not trip quickly enough. This can prolong dangerous touch voltages and impair protection in the event of a fault.
Loop impedance measurement is therefore one of the most important tests during initial verification, periodic inspection and troubleshooting in electrical installations. Especially with long cable runs, extensions of socket circuits, sub-distribution boards, workshops, older installations or changed protective devices, the measurement shows whether the disconnection conditions can still be met. This article explains what the fault loop is, why the measured value Zs is so important and how typical causes of poor values can be identified.
Suitable measuring instruments can be found, among others, in the category installation testers / VDE 0100 installation testing, for example the GSC60 VDE0100 installation tester, the M74 compact VDE 0100 tester or the CA 6117 installation tester.
Table of contents
- What is loop impedance?
- The fault loop: Which path does the fault current take?
- Why loop impedance is important for disconnection conditions
- Zs and Ik: Relationship between loop impedance and short-circuit current
- Cable length, conductor cross-section and contact resistance
- Measuring loop impedance at sockets and distribution boards
- Correct procedure for measurement
- Loop impedance measurement in circuits with RCD / residual current device
- TN, TT and IT systems: Why the earthing system must be considered
- Evaluating measured values: When is a value critical?
- Typical causes of excessive loop impedance
- Special considerations in older installations and later extensions
- Documenting and evaluating measured values traceably
- Suitable measuring instruments for loop impedance and VDE testing
- Practical example: Socket circuit extended in a workshop
- Conclusion: Loop impedance shows whether protection works in the event of a fault
- FAQ: Frequently asked questions about loop impedance measurement
What is loop impedance?
Loop impedance describes the electrical resistance or impedance of the fault loop. This fault loop occurs in the event of a fault when a phase conductor makes a conductive connection to the protective conductor or to an earthed conductive part. The fault current then flows via the phase conductor to the fault location and back to the source via the protective conductor, earthing system, network return path and transformer.
In practice, the measured value Zs is often used. This value describes the impedance of the entire fault loop at the measured point. The lower the loop impedance, the higher the possible fault current in the event of a fault. The higher the loop impedance, the lower the possible fault current becomes.
This is decisive for the protective function. A miniature circuit breaker or fuse only trips quickly if the fault current is high enough. If the cable run is too long, the protective conductor has a poor contact or contact resistances are present, the loop impedance can increase. In that case, the fault current may not be sufficient to trip the protective device within the required time.
Loop impedance measurement is therefore not just a numerical check. It answers a safety-relevant question: Can enough current flow in the event of a fault for the protective measure to be effective?
The fault loop: Which path does the fault current take?
To understand loop impedance, the path of the fault current must be considered. In the event of a fault between phase conductor and protective conductor, the current does not simply flow “somewhere”, but through a defined loop back to the power source. This loop consists of several parts of the installation.
These include the phase conductor up to the fault location, the fault location itself, the protective conductor or PEN conductor, terminals, connections, distribution boards, earthing and equipotential bonding connections as well as the return path to the source. Every part of this loop contributes to the total impedance. Terminal points, plug connections, fuse contacts and cable lengths also influence the value.
It is particularly important to understand that the protective conductor is not just an additional wire. It is an essential part of the protective measure. If its connection is poor, the loop impedance increases or, in the worst case, the fault loop is interrupted. A dangerous touch voltage can then remain present in the event of a fault.
| Part of the fault loop | Influence on the measurement | Typical risk |
|---|---|---|
| Phase conductor | Cable length and cross-section influence resistance | Cable run too long or cross-section too small |
| Protective conductor | Must be low-resistance and reliably connected | Loose terminal, interruption or poor contact |
| Terminals and connections | Contact resistance increases Zs | Corrosion, loose screw connection or old plug connection |
| Distribution board and protective device | Contact points and wiring also have an effect | Incorrect assignment or poor connection |
| Source and earthing system | Determine the return path of the fault current | Evaluation depends on network system and protection concept |
Measurement at the end of a circuit is particularly meaningful because the entire cable run is taken into account there. A measurement only in the distribution board may look good, while a distant socket may already be critical due to the long cable length.
Why loop impedance is important for disconnection conditions
Protective devices must disconnect within a permissible time in the event of a fault. Whether this succeeds depends, among other things, on how high the fault current becomes. The fault current, in turn, is directly related to loop impedance. A low loop impedance enables a high fault current. A high loop impedance limits the fault current.
If the fault current is high enough, the protective device trips quickly. If it is too low, disconnection may be delayed or may not occur at all. The faulty installation then remains live for longer. This can be dangerous for people who touch a conductive enclosure or another accessible conductive part.
This consideration is particularly important for miniature circuit breakers. The circuit breaker must operate within its intended tripping range when a fault current occurs. Whether this fault current is reached at all is shown by the loop impedance measurement together with the displayed or calculated prospective short-circuit current.
In VDE testing, loop impedance measurement is therefore a practical verification that automatic disconnection of the power supply can work. It does not replace proper planning of the installation, but it shows during testing whether the real condition of the installation matches the protective measure.
Zs and Ik: Relationship between loop impedance and short-circuit current
During measurement, two values are often considered: the loop impedance Zs and the possible short-circuit current or fault current Ik derived from it. Both values belong together. If Zs is low, Ik can be high. If Zs is high, Ik is correspondingly lower.
Many installation testers display the prospective short-circuit current directly in addition to the loop impedance. This makes evaluation easier because the user not only sees a resistance value, but also receives an indication of which current could be expected to flow in the event of a fault.
However, it is important that the evaluation cannot be made in a general way. Whether a value is sufficient depends on the protective device, its tripping characteristic, rated current, earthing system, disconnection conditions and application. A measured value that is sufficient for a circuit with a low protective rating may not be sufficient for another circuit with a different protective rating.
| Value | Meaning | Practical statement |
|---|---|---|
| Zs | Loop impedance of the fault loop | The lower the value, the better for high fault currents |
| Ik | Possible short-circuit or fault current | Must match the protective device and the disconnection condition |
| Tripping characteristic | Behavior of the protective device under overcurrent | Determines which fault current is required for fast disconnection |
| Cable length | Influence on the resistance of the circuit | Long cables often increase loop impedance |
The mere indication that “a measured value is present” is therefore not sufficient. The decisive question is whether the measured value matches the specific protective device and protection concept.
Cable length, conductor cross-section and contact resistance
Loop impedance is strongly influenced by cable length and conductor cross-section. The longer a circuit is, the higher the conductor resistance becomes. The smaller the cross-section, the greater the effect of the cable length. This is particularly relevant for subsequently extended socket circuits, large workshops, outdoor areas or distant sub-distribution boards.
Contact resistances can also worsen the measured value. A loose terminal, an aged plug connection, a corroded connection or a poorly connected protective conductor can significantly increase loop impedance. Such faults are particularly tricky because the socket appears to work in everyday use. Only in the event of a fault does it become clear whether the protective path is really sufficiently low-resistance.
During troubleshooting, the cable length should therefore not be the only factor considered. Terminal points, distribution boards, sockets, protective conductor connections and transitions between old and new cable sections must also be checked. In many cases, the problem is not one single major fault, but the sum of several small resistances.
A measurement at the most distant point of the circuit is often particularly important. This is where the cable run is longest and the loop impedance is usually highest. If the disconnection conditions are met there, closer measuring points are often less critical. Nevertheless, individual poor terminal points can also cause problems at intermediate points.
Measuring loop impedance at sockets and distribution boards
Depending on the test task, loop impedance is measured at different points. In socket circuits, the measurement is often carried out directly at the socket. This records the entire cable run up to the socket. In distribution boards, measurements are taken at outgoing circuits, terminals or supply points in order to assess individual circuits or parts of the installation.
A measurement at the distribution board shows the conditions at that point. However, it does not automatically indicate whether the values at the end of a long circuit are still sufficient. For this reason, the relevant end points should be checked during acceptance tests or extensions, especially the most distant or most critical loads.
At sockets, the correct assignment of phase conductor, neutral conductor and protective conductor is also important. A loop measurement can only be evaluated meaningfully if the socket is correctly connected and the protective conductor is properly present. In practice, loop impedance measurement is therefore part of a test sequence that also includes visual inspection, protective conductor testing, insulation measurement and, where applicable, RCD testing.
In distribution boards, it should be clearly documented which outgoing circuit was measured. Especially in workshops, office buildings or machine areas, several circuits may be labelled similarly. Without clear assignment, the measured value is difficult to trace later.
Correct procedure for measurement
The exact operation depends on the installation tester used and on the respective installation. In principle, it must be clear before the measurement which circuit is being tested, which protective device is assigned and which earthing system is present. The measuring instrument must also be suitable for the existing voltage, measurement category and test task.
During the measurement, the test instrument is connected to the intended measuring points. Depending on the measurement type, it measures the loop impedance between phase conductor and protective conductor or evaluates corresponding network and fault loops. Many modern devices display the calculated or determined short-circuit current Ik in addition to Zs.
Before evaluation, the plausibility of the measured value should be checked. Unusually high values, strongly fluctuating measured values or non-reproducible results can indicate poor contacts, incorrect measuring points or installation problems. In such cases, the first displayed value should not simply be accepted.
Work on electrical installations may only be carried out by qualified electricians or under their responsibility. The measurement itself is performed on live installations. Safety rules, personal protective equipment, suitable test leads and the operating instructions of the measuring instrument must therefore be observed.
| Step | Why it is important |
|---|---|
| Identify circuit and protective device | Only then can the measured value be evaluated correctly |
| Clarify earthing system and protection concept | Evaluation depends on the earthing system and protective measure |
| Select a suitable measuring point | End points and long cable runs are often particularly critical |
| Connect the measuring instrument correctly | Incorrect connections lead to wrong or dangerous measurements |
| Check plausibility of Zs and Ik | The measured value must match the installation and protective device |
| Document the result | Traceability for test report and later troubleshooting |
A loop impedance measurement should therefore never be considered in isolation. It is part of a complete safety-related inspection of the electrical installation.
Loop impedance measurement in circuits with RCD / residual current device
In many modern installations, circuits are protected by RCDs or residual current devices. This is very important for personal protection, but it can influence loop impedance measurement. A classic loop measurement with a higher test current may under certain circumstances trip the RCD.
For this reason, many installation testers offer measurement functions for loop impedance measurements without RCD tripping or with reduced test current. These functions are particularly helpful when socket circuits need to be tested without unnecessarily interrupting operation. Nevertheless, the user must know which measurement method is being used and how the measured value is to be evaluated.
It is also important to understand that an RCD does not automatically replace every consideration of loop impedance. Depending on the earthing system, protective measure and test task, both the function of the RCD and the conditions of the installation must be evaluated. In practice, RCD tripping time, RCD tripping current, protective conductor testing and loop impedance often belong together in one test sequence.
If the RCD unexpectedly trips during the measurement, there can be various causes: incorrect measurement method, existing leakage currents, already loaded RCD groups, faulty installation or a sensitive or damaged RCD. The cause should then be checked specifically instead of simply discarding the measured value.
TN, TT and IT systems: Why the earthing system must be considered
The evaluation of loop impedance strongly depends on the earthing system. In TN systems, the fault current flows back to the source via the protective conductor or PEN conductor. Here, loop impedance is closely connected to the possible short-circuit current and disconnection by overcurrent protective devices.
In TT systems, the situation is different because the consumer’s earthing system and the source earthing are separate. The earth resistance and the use of RCDs play a special role here. The mere consideration of a high short-circuit current is not always the central verification in this case.
In IT systems, the first fault assessment is different again because the first fault does not necessarily lead to a high fault current. Insulation monitoring and special protection concepts are used here. A general evaluation of loop impedance without knowing the earthing system would therefore be technically incorrect.
In practice, this means: Before the measurement, it must be clear which network system is present and which protective measure is to be evaluated. The installation tester can provide measured values, but professional interpretation remains the responsibility of the qualified electrician.
Evaluating measured values: When is a value critical?
A loop impedance value is critical if the resulting fault current is not sufficient to reliably trip the assigned protective device within the required time. The limit value depends on the protective device, tripping characteristic, rated current, mains voltage, earthing system and the specific protective measure.
For this reason, there is no simple general statement in practice such as “this value is always good” or “this value is always bad”. A value must always be evaluated in the context of the installation. Many installation testers support this evaluation by displaying the possible short-circuit current, using stored limit values or offering comparison functions. However, responsibility for assessment remains with the competent person.
A good test procedure also considers plausibility in addition to the measured value. If a distant socket shows a significantly better value than a nearby socket, this may indicate a measurement mix-up or different circuits. If measured values fluctuate strongly, a loose connection or contact problem may be present.
Critical values should not only be documented, but also investigated. Possible measures include checking terminal points, checking the protective conductor, adapting the protective device, changing the cable route, using a larger conductor cross-section or implementing another protective measure. Which measure is permissible depends on the installation.
Typical causes of excessive loop impedance
Excessive loop impedance is often caused by long cable runs. If a circuit is extended later, the cable length increases. This can quickly become relevant, especially for socket circuits in workshops, storage areas or outdoor installations.
A conductor cross-section that is too small can also lead to unfavorable values. This applies not only to the phase conductor, but also to the protective conductor. If cross-section, cable length and protective device do not match, the disconnection condition can become critical in the event of a fault.
Poor terminal points are also very common. Loose screws, corroded contacts, damaged sockets, old connections or repeatedly reconnected conductors can cause contact resistance. This increases loop impedance and can also create thermal problems.
Incorrect assignments in the distribution board can also cause problems. If circuits are not clearly labelled, protective conductors have not been correctly assigned or old installation sections are combined with new ones, troubleshooting can become time-consuming.
| Cause | Typical indication | Possible check |
|---|---|---|
| Long cable run | Zs at the cable end significantly higher than in the distribution board | Measurement at several points along the circuit |
| Cross-section too small | High voltage drop and unfavorable loop impedance | Compare cable data with protective device and length |
| Loose terminal | Fluctuating measured values or local heating | Check terminal points and repeat measurement |
| Corrosion or old socket | Poor contact, unreliable measured values | Check socket and connections |
| Incorrect protective device | Ik is not sufficient for fast tripping | Evaluate protective device and disconnection conditions |
| Faulty protective conductor connection | Abnormal protective conductor or loop measurement | Check protective conductor continuity and terminal points |
Loop impedance measurement is therefore not only a required value for the test report. It is also a very helpful diagnostic tool for making weak points in the installation visible.
Special considerations in older installations and later extensions
In older installations, loop impedance measurement is particularly important because installations have often been modified over time. Sockets have been added, cables extended, sub-distribution boards expanded or protective devices replaced. The complete protective measure was not always reassessed in the process.
A typical mistake is assuming that an extension is uncritical because the socket works after connection. Electrical function and protection in the event of a fault are two different things. A socket can supply a device without any problem and still have excessive loop impedance, meaning that disconnection in the event of a fault may not be safe enough.
Old terminal points can also be problematic. Over the years, screw connections can loosen, contacts can corrode or conductors can be mechanically stressed. If a new cable is then connected to an old circuit, existing weak points can become more significant.
For later extensions, it should therefore always be checked whether the existing protective device, cable length, cross-section and protective measure still match. Loop impedance measurement at the new end point is a central verification in this process.
Documenting and evaluating measured values traceably
Documentation of the loop impedance measurement is important because it later makes it traceable which circuit, measuring point and protective device were tested. A single value without assignment is of little help. Especially in larger installations, measured values must be clearly assigned to a distribution board, circuit, room, socket position or machine connection.
In addition to the measured value, the test report should include information on the measuring instrument used, test date, measuring point, protective device, earthing system and evaluation. If the measuring instrument displays the possible short-circuit current Ik, this value should also be documented if it is used for evaluation.
In borderline cases, clean documentation is particularly important. If a measured value is close to the permissible limit, it should be recorded how the evaluation was made and which boundary conditions were present. This makes it possible to identify during later periodic inspections whether the installation has deteriorated.
| Documentation point | Why it is important |
|---|---|
| Measuring point | Shows exactly where the loop impedance was measured |
| Circuit / distribution board | Enables clear assignment within the installation |
| Protective device | Required for evaluating the disconnection condition |
| Zs value | Direct measured value of loop impedance |
| Ik value | Helps evaluate the possible fault current |
| Evaluation | Documents whether the disconnection condition is fulfilled |
| Measuring instrument and calibration status | Important for traceability and quality assurance |
Modern installation testers with memory function can significantly simplify documentation. Nevertheless, the user should ensure that measured values are not only stored, but also meaningfully named and clearly assigned.
Suitable measuring instruments for loop impedance and VDE testing
Suitable VDE installation testers and installation test instruments are required for loop impedance measurement. They support typical tests of electrical installations, for example protective conductor testing, insulation measurement, RCD testing, loop impedance, line impedance, phase rotation testing and documentation.
The GSC60 VDE0100 installation tester is suitable for extensive testing tasks on electrical installations and combines installation test functions with network and energy analysis. For loop impedance measurements, measurement of line and loop impedance with Ik display as well as functions for measurement without RCD tripping are particularly interesting.
The M74 compact VDE 0100 tester is a suitable solution when a handy combination instrument with typical VDE 0100 measuring functions is required. These include, among others, protective conductor testing, insulation measurement, RCD testing, loop resistance and phase rotation measurement.
For comprehensive installation testing in residential, commercial and industrial environments, the CA 6117 installation tester can also be used. It is designed for checking electrical installations and is suitable for users who need a universal test instrument for different installation environments.
The selection of the appropriate device depends on which tests are performed regularly, whether RCD types, memory functions, auto sequences, documentation, network analyses or measurements in more complex installations are required. For pure loop impedance measurement, the measuring function alone is not enough; ease of use, documentation and safe application in the real installation are also decisive.
Practical example: Socket circuit extended in a workshop
In a workshop, an existing socket circuit is extended because additional workstations are being set up. After connection, the new socket works without any problems. Machines and chargers can be switched on, and no fault is initially noticeable during normal operation. Nevertheless, it must be checked whether the protective measure remains effective after the extension.
The qualified electrician measures the loop impedance at the new, most distant socket point. The measured value is higher than at the existing sockets. This is generally expected because the cable run has become longer. The decisive question now is whether the resulting possible fault current is still sufficient for the assigned protective device to trip quickly enough in the event of a fault.
The evaluation shows that the value is critical. The socket works during normal operation, but the disconnection condition is not safely fulfilled at the new end point. The cause lies in the combination of long cable run, existing cross-section and protective device. Terminal points are also checked to rule out contact resistance.
The installation is then adapted. Depending on the permissible solution, this may involve a different cable route, a larger conductor cross-section, a changed protective device or an adapted protective measure. After the modification, measurement is repeated and the result is documented.
The example shows: A socket is not automatically safe just because it supplies voltage. Only loop impedance measurement shows whether enough current can flow in the event of a fault for the protective device to disconnect reliably.
Conclusion: Loop impedance shows whether protection works in the event of a fault
Loop impedance measurement is a central test in electrical installations. It shows whether the fault loop is sufficiently low-resistance and whether enough current can flow in the event of a fault for fuses or miniature circuit breakers to disconnect safely. This test is indispensable, especially for long cable runs, extensions, older installations and workshops.
A high Zs value can indicate long cables, cross-sections that are too small, poor terminal points, faulty protective conductor connections or an unsuitable protective device. The decisive factor is always evaluation in connection with the protective device, earthing system and disconnection conditions. The measured value alone does not replace competent assessment.
For practical testing, VDE installation testers such as the GSC60, the M74 or the CA 6117 installation tester are suitable. It is important that the measuring instrument, measuring method, documentation and professional evaluation match the installation. Only then does loop impedance measurement provide a reliable statement about electrical safety.
FAQ: Frequently asked questions about loop impedance measurement
What is loop impedance?
Loop impedance is the impedance of the fault loop through which fault current flows back to the power source in the event of a fault. It is often referred to as Zs and is decisive for automatic disconnection of the power supply.
Why do you have to measure loop impedance?
The measurement shows whether a sufficiently high fault current can flow in the event of a fault so that the fuse or miniature circuit breaker trips quickly enough. If loop impedance is too high, disconnection may be delayed or insufficient.
What does Zs mean in electrical testing?
Zs refers to the loop impedance at the measured point. The value describes how strongly the fault loop limits the possible fault current.
What is Ik in loop impedance measurement?
Ik is the possible short-circuit or fault current derived from the measured loop impedance. It helps evaluate whether the protective device can trip safely in the event of a fault.
Where is loop impedance measured?
Depending on the test task, loop impedance is measured at sockets, machine connections, distribution boards or circuit end points. The most distant points of a circuit are often particularly important.
Why are long cable runs critical?
Long cable runs increase the resistance of the fault loop. This reduces the possible fault current. If the fault current is too low, the protective device may not disconnect quickly enough.
Can a socket work and still have excessive loop impedance?
Yes. A socket can supply voltage and operate connected devices during normal operation. Nevertheless, the loop impedance may be too high, meaning that disconnection in the event of a fault is not safe enough.
Why does the RCD sometimes trip during loop measurement?
With certain measurement methods, a test current can flow that trips the RCD. Modern installation testers therefore often offer measurement functions for loop impedance measurements without RCD tripping or with reduced test current.
What causes poor loop impedance?
Typical causes include long cables, conductor cross-sections that are too small, poor terminal points, corroded contacts, faulty protective conductor connections, incorrect protective devices or later extensions without reassessment.
Which measuring instruments are suitable for loop impedance measurement?
VDE installation testers and installation test instruments with loop impedance or loop resistance measurement are suitable, for example the GSC60, the M74 or the CA 6117 installation tester.