The insulation resistance of an electrical installation is to be tested. The circuit has been isolated, secured against reconnection and the absence of voltage has been verified. The installation tester is connected and the insulation measurement is started. Instead of a clearly high resistance value, however, the instrument shows only a comparatively low or unstable value.
The obvious assumption is: the cable has an insulation fault.
In modern installations, this conclusion is often reached too quickly. Surge protection devices, mains filters, switched-mode power supplies, LED drivers, frequency converters, electronic control systems or other assemblies may be connected in the circuit. These components create additional electrical paths between active conductors, protective conductor and earth. The insulation tester may therefore no longer be measuring only the insulation of the cable, but also part of the connected electronics.
Surge protection devices are a typical example. An SPD is specifically designed to respond to overvoltages. Depending on the protection circuit, arrester technology and test voltage, a connected SPD can influence the insulation measurement, become conductive during the test or be electrically stressed in a way that is not intended for normal operation.
The test voltage must therefore not be selected simply according to the rule “500 V is usual for 230 V installations” and then blindly applied to a fully wired circuit. Before testing, it must be known which equipment is actually located in the measurement path.
The most important rule is: An insulation measurement always tests the electrical path to which the tester is connected. Surge protection and sensitive electronics must therefore be identified before measurement and considered in accordance with the applicable standard, device manufacturer and installation design. An unusually low reading should only be assessed as an insulation fault in the installation once alternative current paths through connected equipment have been ruled out.
What does an insulation tester actually measure?
An insulation tester applies a DC voltage between two defined points and measures the resulting very small current. The insulation resistance is calculated from voltage and current.
In simplified form:
RISO = Utest / Imeas
In an ideal cable with intact insulation, only a very small current flows and the resulting resistance is correspondingly high. In a real installation, however, the tester does not see “the cable” as an isolated component, but all electrically effective components located between its two test connections.
If, for example, a surge protection device is installed between line and protective conductor, its protection circuit is also part of the measurement path. The same applies to suppression capacitors in an EMC filter, Y capacitors in switched-mode power supplies, insulation monitoring devices or other electronic assemblies connected to the protective conductor.
The displayed insulation value is therefore always the result of the entire connected electrical structure.
This is crucial for troubleshooting. A tester cannot determine whether the measured current flows through damp cable insulation, a varistor, an EMC capacitor or an electronic input circuit. It simply displays the electrical result at the selected measuring points.
Why surge protection can influence the measurement
A surge protection device normally remains largely high-resistance during regular operation. As soon as a sufficiently high overvoltage occurs, its electrical behaviour changes and part of the surge energy is diverted to earth or between the relevant conductors.
It is precisely this function that makes an SPD relevant during insulation testing.
During a DC insulation test, a voltage may be applied that is higher than the normal operating voltage. Whether and how strongly an SPD responds depends on its technology, rated voltage, circuit arrangement and the actual test voltage applied.
A connected SPD can therefore produce several effects. It may reduce the measured insulation resistance, cause an unstable or voltage-dependent reading, or affect the tester’s ability to build up the required test voltage. If the test method is unsuitable for the specific protection device, unnecessary electrical stress may also occur.
It is therefore important not to generalise from experience with one particular SPD type to all surge protection devices.
Many modular surge protection devices have plug-in protection modules. This allows the actual arrester elements to be removed from the base unit for maintenance or testing purposes. Whether this is necessary before an insulation test must be determined from the manufacturer’s instructions and the applicable test procedure.
However, a removed SPD module must be reinserted and checked just as consistently after the test. A perfect insulation measurement followed by forgotten surge protection would not represent a successfully completed test procedure.
Identify sensitive electronics in the test path
Surge protection is only one possible influence. Modern circuits often contain significantly more electronics than is immediately obvious.
A socket outlet may, for example, supply a power supply containing filter capacitors between the mains side and protective conductor. A luminaire may contain an electronic LED driver. A heating system may include a controller, mains filter, pump electronics and communication interfaces. Industrial installations may additionally contain frequency converters, servo drives, PLC power supplies and transmitters.
Before an insulation measurement, it must therefore first be clarified whether the test is intended to cover the fixed electrical installation, a connected item of equipment or the combination of both.
Especially during initial verification, it is advantageous to test the insulation condition of the installation during construction, before sensitive equipment is permanently connected. In existing installations or periodic testing, this clean separation is often more difficult.
The circuit diagram then becomes particularly important. The tester must be able to determine which components are connected to the relevant circuit and what electrical connections those components create between the conductors being tested.
| Connected component | Possible influence on insulation measurement | Clarify before testing |
|---|---|---|
| SPD / surge arrester | Voltage-dependent discharge path, possibly lower reading | Manufacturer requirements for insulation testing |
| EMC filter | Capacitors to PE influence charging current and indication | Circuit arrangement and permissible test voltage |
| Switched-mode power supply | Filter and protection circuits in the measurement path | Whether the device may remain connected during testing |
| Frequency converter / servo drive | Power electronics and mains filters can influence the result | Cleanly separate the test object from the converter if required |
| Electronic control system | Internal protection circuits and capacitances | Manufacturer instructions and specific test circuit |
| Insulation monitoring device | Direct connection between network and earth can influence the result | Observe device-specific test instructions |
Understand the measurement path before testing
The most important preparation for an insulation measurement takes place before the test button is pressed.
The tester must first understand between which points the DC test voltage is actually being applied. If, for example, the measurement is carried out between combined active conductors and PE, electronics connected between L and N are in a different electrical condition than during a test between L and N.
A power-supply input circuit may contain components between L and N that are directly subjected to the test voltage during a conductor-to-conductor test. If L and N are instead connected together for a suitable test against PE, there is no corresponding test-voltage difference between L and N. However, all components connected between the active conductors and PE then become part of the measurement path.
This is precisely why there can be no general instruction stating that electronic devices should simply remain connected or that everything must always be disconnected.
The required test, the specific circuit and the characteristics of the connected equipment are decisive.
The necessary electrical safety measures must of course be applied before working on the installation. The insulation measurement itself must be carried out by an appropriately qualified electrical specialist or by a person authorised within the scope of the specific test assignment.
250 V, 500 V or 1000 V: do not select the test voltage indiscriminately
Insulation testers for electrical installations often provide several DC test voltage levels. Typical instruments offer 250 V, 500 V and 1000 V DC, while more comprehensive installation testers may also provide lower voltages such as 50 or 100 V.
This selection does not mean that the user may freely choose the test voltage simply according to personal caution.
The required test voltage depends on the specific installation, test method and applicable normative requirements. Typical low-voltage circuits are often tested at 500 V DC. Under certain conditions, a reduced test voltage of 250 V DC may be specified or permitted if connected equipment cannot reasonably be disconnected. However, this does not mean that the requirements for assessing insulation resistance can be changed arbitrarily.
One point is particularly important: a measurement at 250 V is not automatically “the same test, just a little gentler”. If the test voltage is reduced, the behaviour of voltage-dependent fault locations can also change. Damaged insulation, a contaminated creepage path or an electronic leakage path may behave differently at 250 V than at 500 V.
The reduced test voltage is therefore a defined test strategy and not a substitute for understanding the measurement path.
| Test voltage | Typical application | Important note |
|---|---|---|
| 50 / 100 V DC | Special sensitive circuits and device-specific tests | Do not use as a substitute for higher test voltages required by standards |
| 250 V DC | Depending on the test procedure, circuits containing special electronics or defined special cases | Clarify permissibility and limit value for the specific test |
| 500 V DC | Common test voltage in low-voltage installations | Consider connected electronics and SPDs beforehand |
| 1000 V DC | Suitably rated circuits or special equipment | Do not apply indiscriminately to typical 230/400 V electronics |
Combine active conductors or test them individually?
The circuit configuration during the measurement also significantly changes which components are stressed and which faults can be detected.
When measuring L against N, the test voltage is applied directly between the two active conductors. Equipment connected between L and N is therefore located directly in the test path.
When measuring combined active conductors against PE, the active conductors are at approximately the same potential from the tester’s point of view. Connected loads between the active conductors are therefore subjected to a different electrical condition.
At the same time, protective circuits or filters between an active conductor and protective earth remain electrically effective.
For surge protection devices, it is also important to know between which conductors the individual protection paths are arranged. In a 3+1 or 4+0 SPD concept, different arrester paths are included in the measurement path than when considering a simple individual L-PE arrester.
The complete circuit of the distribution board is therefore more important than simply knowing that “an SPD is installed”.
Which components may need to be disconnected
For many insulation measurements, the cleanest approach from a measurement perspective is to separate the installation from components that can distort the measurement path.
With plug-in surge protection devices, this can for example be achieved by removing the intended protection modules. Other components are electrically disconnected according to the manufacturer’s information, or the test area is selected so that the sensitive assembly is not subjected to an unsuitable test voltage.
It is important not to disconnect protective conductors, neutral conductors or protective devices indiscriminately. An insulation test must be planned. Before disconnecting anything, it should be clear what electrical condition this creates and which protection or monitoring function is temporarily taken out of service.
After the measurement, the installation must be completely restored to its intended operating condition. For plug-in SPDs, this includes correctly reinstalling the modules and checking the status indicator. Any disconnected controllers, power supplies or measuring instruments must also be reconnected correctly.
For extensive distribution boards in particular, a documented procedure is advisable. Otherwise, there is a risk that the actual measurement is performed correctly but a protection module or auxiliary contact is forgotten afterwards.
Why a low insulation value does not automatically mean a cable fault
Assume that only 0.6 MΩ is measured on a circuit. This value is clearly noticeable. However, it does not yet reveal where the current is flowing.
A genuine insulation fault may be caused by moisture in a junction box, a damaged cable, a crushed conductor or contaminated insulation surfaces.
The same measured value may also result from several electronic components, each with a relatively high leakage resistance, being connected in parallel in the circuit. Parallel resistances reduce the total resistance.
In simplified form:
1 / Rtotal = 1 / R1 + 1 / R2 + 1 / R3 + ...
Ten connected assemblies can therefore together produce a significantly lower value than each individual assembly on its own.
Voltage-dependent protection components can also produce an unusual reading even though the cable insulation itself is intact.
Comparing different installation states is therefore very useful for troubleshooting. If the insulation resistance changes significantly after an SPD module has been removed in accordance with the manufacturer’s instructions, this is important diagnostic information. If the value remains equally low, the affected circuit must be narrowed down further.
Correctly interpret capacitive effects and rising readings
In addition to purely resistive leakage paths, an electrical installation also contains capacitances. Long cables, EMC filters, frequency converters and power supplies can create considerable capacitance to protective conductor or earth.
At the beginning of an insulation measurement, these capacitances must first be charged to the test voltage. The tester therefore briefly supplies a charging current.
The display may consequently show a low resistance initially and then rise significantly.
This behaviour is not the same as a permanently low insulation resistance.
A high-quality insulation tester often displays the actual test voltage reached in addition to the resistance. If the instrument does not reach the selected 500 V at all, this is also valuable diagnostic information. A large capacitance, a conductive protection path or an actual insulation fault may be responsible.
With larger capacitive installations, safe discharge after the measurement must also be considered. The test object may have stored electrical charge even though the mains supply has been switched off for a considerable time.
Practical example: low insulation value caused by an SPD
A new sub-distribution board supplies several socket and lighting circuits. The distribution board also contains a multi-pole Type 2 surge protection device.
During insulation testing between the combined active conductors and PE, the installation tester displays a significantly lower value than expected. The circuit has been newly wired and a visual inspection initially reveals no indication of damaged cables.
Instead of immediately disconnecting individual cable sections from the installation, the actual measurement path is checked first. It is then noticed that the SPD modules are fully inserted in the distribution board during the measurement.
The manufacturer’s documentation for the surge protection device specifies that the protection modules must be disconnected for the insulation test.
After the SPD has been disconnected in accordance with the applicable standard and manufacturer instructions, the measurement is repeated under otherwise identical conditions. The insulation value increases significantly and is now within the expected range.
This does not mean that the diagnosis is simply “SPD defective”. The first reading was not a pure measurement of the cable insulation. It also included the electrical influence of the connected surge protection.
The test is documented, the SPD is then fully reinstalled and its operating condition checked.
The example demonstrates the crucial difference between a poor measured value and a proven insulation fault.
Systematically isolate faults
When an abnormal insulation value is obtained, step-by-step isolation is usually much more informative than repeatedly measuring at increasingly higher test voltages.
| Observation | Possible cause | Sensible next step |
|---|---|---|
| Measured value clearly too low, SPD connected | Arrester is in the measurement path | Check manufacturer instructions and consider the SPD accordingly |
| Value rises continuously during measurement | Capacitive charging | Observe test voltage and time response |
| Tester does not reach the target test voltage | Strong leakage path, capacitance or fault | Divide the circuit step by step |
| Value improves significantly after electronic equipment is disconnected | Electronics influenced the measurement path | Assess installation and equipment separately |
| Value remains low even in an isolated cable section | Actual cable/installation fault more likely | Further isolate branches and cable sections |
| Fault occurs only in humid conditions | Moisture, contamination or damaged insulation | Investigate the affected part of the installation specifically |
The measurement should be carried out as reproducibly as possible. If test voltage, wiring and connected equipment are all changed between two measurements, it becomes difficult to determine afterwards which change was responsible for the new result.
For structured troubleshooting, only one influencing factor should therefore be changed at a time wherever possible.
Restore the installation after testing
The end of the insulation measurement is not the end of the test procedure.
First, it must be ensured that any stored electrical charge has been discharged. Many modern insulation testers provide an automatic discharge function. With capacitive test objects, the displayed residual voltage should be monitored and the safe condition confirmed in accordance with the applicable test procedure.
All equipment disconnected for the test must then be restored to its intended operating condition.
For surge protection devices, this does not simply mean mechanically reinserting the plug-in module. The status indicator, locking mechanism and, where applicable, remote signalling contact should also correspond to the intended system condition.
The same applies to previously disconnected control systems, power supplies, sensor supplies and communication devices.
A final check before re-energising is important. Especially in complex distribution boards, a forgotten component can result in an installation that operates electrically but no longer has its intended surge protection or another required protection function.
Document the test conditions
An insulation resistance reading can only be meaningfully compared later if the conditions under which it was obtained are known.
The test report should therefore not simply state “RISO = 120 MΩ”. The circuit assignment, test voltage used and installation condition during measurement are equally important.
If an SPD was removed, this should be traceable. If certain electronics remained connected, this information is also useful for later interpretation. If a reduced test voltage was used, the associated test strategy should be documented.
This is particularly valuable for periodic testing. A measured value of 80 MΩ at 250 V with electronics connected cannot be directly compared with an earlier value of 300 MΩ at 500 V with all loads disconnected.
Good documentation therefore not only provides proof of testing, but also improves future condition diagnostics of the installation.
Common practical mistakes
Setting 500 V and testing without checking the installation
The test voltage must not be derived solely from the nominal voltage of the circuit. It must first be known which equipment and protection circuits are actually located in the measurement path.
Interpreting every low reading as a cable fault
SPDs, EMC filters and electronic equipment can significantly reduce the total resistance. The reading must therefore be evaluated in the context of the connected installation.
Always leaving the SPD connected because it is designed for overvoltages
Protection against transient overvoltages is not equivalent to arbitrary DC insulation testing. The manufacturer’s requirements for the specific SPD are decisive.
Always removing the SPD without understanding the measurement path
The opposite extreme is also problematic. It must first be clear what is being tested and which protection paths exist in the specific circuit.
Treating 250 V merely as a “gentler 500 V measurement”
The reduced test voltage is a separate, defined test condition. It cannot be used arbitrarily simply to avoid considering connected electronics.
Disconnecting electronics and forgetting to reconnect them after testing
Every installation configuration changed for the test must afterwards be completely and systematically restored.
Looking only at the resistance value
The actual test voltage achieved and the time response of the reading provide additional information about capacitances and possible leakage paths.
Touching capacitive installations immediately after the measurement
Cables, filters and other capacitive components can retain electrical charge after the insulation test. Before further work, a safely discharged condition must be established in accordance with the intended test method.
Suitable test equipment at ICS Schneider
ICS Schneider Messtechnik offers both dedicated insulation testers and complete installation testers for electrical safety testing.
The Chauvin Arnoux CA 6117 is a universal installation tester for different network systems. In addition to insulation testing, it provides continuity, earth, loop impedance and RCD testing, among other functions. This makes the instrument particularly suitable for complete test procedures on electrical installations.
The CA 6116N also covers the essential tests for electrical installations and is suitable for TT, TN and IT systems.
The GSC60 combines an installation tester with a network and energy analyser. Insulation test voltages of 50, 100, 250, 500 and 1000 V DC are available. This selection is particularly useful in installations containing different circuit types, provided that the permissible test voltage has first been determined from the specific test requirements.
For maintenance and targeted insulation diagnostics, the C.A series insulation testers are also available. Depending on the model, different test voltages, memory functions and additional measuring functions are provided.
The C.A 6528, for example, is suitable for insulation and continuity measurements in maintenance and service work. The instrument detects an already present hazardous voltage and prevents insulation testing in this condition.
When selecting an instrument, the maximum measurable insulation resistance should not be the only criterion. More important are the required test voltages, the installation or equipment category, requirements for documentation and memory, and whether RCD, loop, earth or continuity tests must also be carried out.
Insulation testers at ICS Schneider
Installation testers for electrical installation testing at ICS Schneider
Further reading: Preparing for VDE testing – which measurements must be carried out on site?
Further reading: Insulation measurement on a motor with frequency converter
Conclusion
An insulation measurement provides a meaningful assessment of the installation only if the actual measurement path is known.
Surge protection devices and sensitive electronics are not minor details; they can form an important part of this measurement path. A connected SPD can influence the insulation value. EMC filters, switched-mode power supplies and control systems can create additional leakage and capacitance paths. An unusual measured value therefore initially proves only that a corresponding current is flowing between the connected test points.
Only by systematically separating the installation from connected equipment can it be determined whether there is actually a damaged cable, moisture or another insulation fault.
The choice of test voltage is equally important. 250 V, 500 V and 1000 V are not freely interchangeable convenience settings. The test voltage is derived from the test task, applicable standard, installation configuration and manufacturer requirements.
In modern electrical installations, good insulation testing therefore involves more than placing two probes and reading a megaohm value. Circuit diagram, SPD concept, electronic loads, measurement path and test voltage all form part of the professional assessment.
Anyone who considers these points before the measurement avoids both unnecessary stress on sensitive electronics and misdiagnoses in which a completely intact cable section is judged to be defective merely because a protection or filter component remained connected.
FAQ on insulation measurement in installations with surge protection
Does an SPD have to be removed before an insulation test?
This depends on the specific SPD, its circuit arrangement, the test voltage and the manufacturer’s instructions. Many manufacturers specify that surge protection devices or plug-in arrester modules must be disconnected before insulation resistance testing. The requirements of the actual system are decisive.
Why can an SPD cause a poor insulation reading?
A surge protection device contains voltage-dependent discharge paths between conductors or to earth. During an insulation test, these can cause additional measuring current and therefore reduce the displayed insulation resistance.
Does a low value with the SPD connected automatically mean that the SPD is defective?
No. The reading may initially simply include the normal electrical behaviour of the connected protection path. Assessment must be based on the manufacturer data and, where necessary, a separate test.
Can an insulation test damage sensitive electronics?
An unsuitable test voltage or test circuit can unnecessarily stress or damage electronic components. Before testing, it must therefore be clarified which equipment is located in the measurement path and which test conditions are permissible.
Why is 500 V DC frequently used?
500 V DC is a specified test voltage for many insulation tests on typical low-voltage circuits. The voltage to be used in the specific case, however, depends on the relevant test procedure and installation configuration.
When can 250 V DC be used?
A reduced test voltage may be used under certain normative and installation-specific conditions, for example if certain connected equipment cannot reasonably be disconnected. The exact permissibility and corresponding minimum insulation value must be clarified for the specific test.
Is a 250 V measurement automatically safe for all electronics?
No. Even 250 V DC may be unsuitable for certain electronic circuits. The permissible test conditions specified for the equipment are always decisive.
Can a frequency converter remain connected?
This depends on the measurement path and test task. When testing a motor or motor cable, the frequency converter is often electrically separated from the test object so that its power electronics and filters do not form part of the insulation measurement.
Why do EMC filters influence insulation measurements?
EMC filters often contain capacitors between active conductors and protective earth. When a DC test voltage is applied, an initial charging current flows and, depending on the circuit, additional leakage paths may also be present.
Why does the displayed insulation resistance increase during the test?
Capacitances in the test object are initially charged. As the charging current decreases, the resistance calculated by the tester increases. The time response can therefore provide useful diagnostic information.
What does it mean if the tester does not reach the selected test voltage?
Excessive measuring current may prevent the voltage from building up fully. Possible causes include an actual insulation fault, connected electronics, a surge protection device or a large capacitive load.
Can active conductors be combined for insulation testing?
In appropriate test procedures, active conductors can be considered together for measurement against protective conductor or earth. Whether this is permissible and appropriate for the specific test depends on the applicable standard, installation design and test task.
Why is the measurement path more important than the test voltage alone?
The same test voltage can stress completely different components depending on how the tester is connected. Only when the points between which the voltage is applied are known can it be determined which components are affected by the measurement.
Can I isolate a poor insulation reading by disconnecting individual circuits?
Yes. Step-by-step division of the test area is a common diagnostic method. Changes should be carried out systematically so that the influence of individual cable sections or equipment remains traceable.
What must be considered after removing an SPD module?
After testing, the surge protection must be fully and correctly restored. Plug-in modules, status indicators, locking mechanisms and, where applicable, remote signalling contacts should be checked.
Why should the test voltage used be documented?
Insulation resistance can depend on the test voltage and the installation configuration. Without documenting the test voltage, readings from different tests can only be compared to a limited extent.
Should it also be documented whether SPDs and electronics were connected?
This is very useful for a traceable test. An insulation value measured with electronic equipment connected cannot automatically be compared with a value obtained from a completely disconnected installation.
Why is discharge required after an insulation measurement?
Cables, filters and other capacitive components can store electrical charge as a result of the DC test voltage. Before further work, a safe discharged condition must be established or verified.
Which tester is suitable for installations requiring different test voltages?
Installation testers such as the GSC60 provide several insulation test voltages from 50 to 1000 V DC. Alternatively, dedicated insulation testers and complete installation testers such as the CA 6116N and CA 6117 are available. The decisive factor is that the instrument supports the measuring functions and test voltages required for the specific test task.