CEE sockets and three-phase connections are widely used in workshops, industrial plants, construction sites, machine rooms, event technology and mobile systems. They supply machines, compressors, welding equipment, pumps, hoists, construction site distribution boards or mobile loads with 400 V three-phase power. Precisely because these connections often transmit high power, professional testing is particularly important.
At first glance, a CEE socket may seem simple: plug in the connector, switch on the machine, done. In practice, however, numerous faults can occur. A protective conductor may be interrupted, phase conductors may be swapped, the neutral conductor may be missing, contacts may be loose or thermally damaged, an RCD may be incorrectly selected or the loop impedance may be too high. Such faults are not only disruptive, but can become dangerous for people and connected machines.
This article explains what matters when testing CEE sockets and 400 V three-phase connections. The focus is on protective conductor testing, network type, phase conductors, neutral conductor, phase sequence, loop impedance, short-circuit current, RCD testing, typical wiring errors, loose contacts, testing before machine connection and suitable measuring instruments such as COMBI519, COMBI521, EASYTEST and two-pole voltage testers as supplementary test instruments.
Table of contents
- Basics: what makes CEE sockets special?
- Safety: testing only by qualified specialists
- Visual inspection: detecting mechanical damage and thermal traces
- Clarifying network type, protective measure and connection type
- Protective conductor testing: why PE is decisive for CEE connections
- Checking voltages between L1, L2, L3, N and PE
- Phase sequence: why phase order is important before connecting machines
- Neutral conductor: when a missing or incorrect N becomes critical
- Evaluating loop impedance and short-circuit current
- RCD testing on CEE sockets
- Loose contacts, heating and recurring faults
- Testing before machine connection and commissioning
- Documentation and recurring testing
- Typical faults in CEE and three-phase connections
- Practical example: testing a CEE socket in a workshop
- Which measuring instruments / products are suitable?
- Conclusion: do not test CEE sockets only for voltage
- FAQ: frequently asked questions about testing CEE sockets
Basics: what makes CEE sockets special?
CEE sockets are often used for three-phase connections with 400 V. Typical versions are 16 A, 32 A, 63 A or 125 A. In many systems, five-pole CEE sockets with L1, L2, L3, N and PE are used. However, there are also four-pole versions without a neutral conductor, for example for loads that do not require N.
The main difference compared with a standard 230 V socket lies in the higher power, the three-phase supply and the greater dependence on correct conductor assignment. With a three-phase connection, it is not enough for voltage to be present. It must also be checked whether the protective conductor is reliably present, whether the phase conductors are correctly connected, whether the neutral conductor matches the application and whether the phase sequence is correct.
It is particularly critical that many machines depend on the correct phase sequence. A motor running in the wrong direction can drive pumps, fans, compressors or machine axes in the wrong direction. Depending on the application, this can lead to damage, dangerous movements or process problems.
A complete assessment of a CEE socket therefore consists of several test steps. A simple voltage indication is not sufficient. Only the combination of visual inspection, protective conductor testing, voltage measurement, phase sequence testing, loop impedance, short-circuit current assessment, RCD testing and documentation provides a reliable picture.
| Test aspect | Why important? | Typical fault |
|---|---|---|
| Protective conductor | Basis for protection in the event of fault currents | PE interrupted, high contact resistance or incorrect connection. |
| Phase conductors | Supply the three-phase load | Phase missing, conductors swapped or contact loose. |
| Neutral conductor | Important for 230 V loads inside a machine | N missing, incorrectly connected or overloaded. |
| Phase sequence | Determines the direction of rotation of motors | Incorrect phase sequence leads to wrong motor direction. |
| Loop impedance | Influences disconnection conditions in the event of a fault | Value too high, short-circuit current too low. |
Safety: testing only by qualified specialists
CEE sockets and three-phase connections are not test points for laypersons. With 400 V connections, high short-circuit currents, dangerous touch voltages and significant arc flash risks can occur. Tests on electrical installations may therefore only be carried out by qualified electricians or appropriately competent persons using suitable equipment.
Before every measurement, it must be clear whether work is being carried out de-energized or whether a live measurement is required. Verifying absence of voltage, safe isolation, securing against reconnection and suitable personal protective equipment are part of professional work. Depending on the measurement and the condition of the installation, additional protective measures may be necessary.
The measuring instrument must also match the task. An unsuitable voltage tester, a multimeter with the wrong measurement category or an installation tester without the appropriate measuring function can cause dangerous misinterpretations. Especially with three-phase connections, the measuring equipment must be suitable for the voltage level, measurement category and operating environment.
It is also important that a test does not consist only of reading a measured value. The measured values must match the network type, protective concept, fuse protection, cable length, connected load and applicable standards. An installation tester helps with the measurement, but it does not replace professional assessment.
Visual inspection: detecting mechanical damage and thermal traces
Visual inspection is particularly important for CEE sockets because many faults begin mechanically or thermally. CEE connections are often used in harsh environments: workshops, construction sites, outdoor areas, machine halls or mobile systems. Tensile stress, moisture, dust, oil, vibrations, impacts and frequent plugging affect the socket and plug there.
Typical visual findings include damaged housings, broken hinged covers, missing strain relief, loose screws, damaged seals, discolored contacts, melted plastic parts or traces of corrosion. Such signs should be taken seriously even if the socket initially still works.
Thermal damage is particularly critical. A discolored contact point or scorched socket insert can indicate loose contacts, overload, poor terminals or frequent high loads. If such a connection continues to be used, the fault can worsen and lead to failure or fire risk.
The visual inspection should also include the environment. A CEE connection in a damp environment, near chips, chemicals, oil mist or mechanical stress has different requirements than a protected socket in a dry technical room. Degree of protection, installation location and cable entry must match the application.
| Visual finding | Possible cause | Assessment |
|---|---|---|
| Discolored contacts | Heating due to poor contact or overload | Inspect socket and plug more closely, replace if necessary. |
| Broken housing | Mechanical stress or aging | Protection against touch and environmental influences may be impaired. |
| Loose cable entry | Strain relief or cable gland damaged | Check mechanical safety and degree of protection. |
| Corrosion | Moisture, aggressive environment or unsuitable degree of protection | Assess contact quality and environmental conditions. |
| Melted plastic parts | Overheating, arc or contact problem | Do not continue using the connection without testing. |
Clarifying network type, protective measure and connection type
Before measured values can be assessed, the network type must be known. A CEE connection can be located in a TN, TT or IT system. Depending on the network type, the protective concept, measurement, fault current path and assessment of disconnection conditions differ. The network type is therefore not a side issue, but a basis for testing.
In TN systems, the connection to the protective conductor or PEN system plays a central role. In TT systems, the earthing system is particularly important, and RCDs are often an essential part of the protective measure. In IT systems, the assessment can be different again because the first fault does not necessarily trigger the same disconnection mechanism as in TN or TT systems.
The connection type of the CEE socket must also be clear. A five-pole CEE socket provides L1, L2, L3, N and PE. A four-pole version has no neutral conductor. If a machine requires a neutral conductor, it must not be operated on a connection without N. Conversely, the presence of N must not simply be assumed just because a socket externally looks like a three-phase connection.
The protection of the circuit is also part of the assessment. Rated current of the socket, cable, fuse protection, RCD, cable length and connected load must match. A 32 A CEE socket in an installation alone says nothing about whether the protective measure and disconnection conditions are actually fulfilled.
Protective conductor testing: why PE is decisive for CEE connections
The protective conductor is particularly important for CEE and three-phase connections. Many connected machines have metal housings, motors, control cabinet parts or conductive components. In the event of a fault, the protective conductor must ensure that dangerous touch voltages are avoided and protective devices operate reliably.
Protective conductor testing checks whether the protective conductor is continuous and sufficiently low-resistance. This is not just about whether a PE is present somewhere. The decisive point is whether the connection is robust enough in the event of a fault and whether there are no loose, corroded or incorrectly connected points.
Especially with CEE sockets, protective conductor problems can occur due to mechanical stress. Frequent plugging, pulling on cables, vibrations, loose terminals or moisture can increase contact resistance. A visual inspection alone is not sufficient here.
Protective conductor testing is also important before connecting machines. A machine can appear externally flawless, but with a missing or high-resistance PE it can become dangerous in the event of a fault. The protective conductor connection of the socket should therefore always be assessed before a powerful machine is connected or recommissioned.
Checking voltages between L1, L2, L3, N and PE
In a CEE socket, the voltages between the relevant conductors must be plausible. In a typical 400/230 V three-phase connection, around 400 V is expected between the phase conductors, around 230 V between phase conductor and neutral conductor and also around 230 V between phase conductor and protective conductor. The exact assessment must match the network type and installation.
The voltage measurement shows whether all phase conductors are present and whether the neutral conductor is correctly connected. A missing phase conductor can cause three-phase motors not to start, hum, become overloaded or trip protective devices. A missing or interrupted neutral conductor can become particularly critical if 230 V controls, power supplies or auxiliary circuits are supplied inside a machine.
A measurement between N and PE can provide additional indications. Depending on the network type and operating state, small voltages can occur there. Noticeable values must be professionally assessed because they can indicate load currents in the neutral conductor, poor connections, wiring errors or potential differences.
A suitable two-pole voltage tester is helpful for a quick voltage check. However, an installation tester is required for the complete assessment of the installation, because voltage testing, protective conductor assessment, loop impedance, RCD testing and documentation must be considered together.
| Measurement | Typical expectation at 400/230 V | Possible indication in case of deviation |
|---|---|---|
| L1-L2, L2-L3, L3-L1 | Line-to-line voltage around 400 V | Phase missing, conductor fault or network problem. |
| L1-N, L2-N, L3-N | Voltage around 230 V | Neutral conductor missing, incorrect assignment or interruption. |
| L1-PE, L2-PE, L3-PE | Voltage around 230 V | Check PE problem or unusual network situation. |
| N-PE | Usually low, depending on network type and load state | Potential differences, load currents or wiring errors possible. |
| All conductor combinations | Plausible overall picture | A single value is not sufficient for complete assessment. |
Phase sequence: why phase order is important before connecting machines
The phase sequence is a central point for CEE sockets because many connected loads contain three-phase motors. Pumps, fans, compressors, saws, conveyors or machine tools can run in the wrong direction if the phase sequence is incorrect. This can immediately lead to damage or dangerous situations.
Correct voltage between the phase conductors does not automatically mean that the phase sequence is correct. L1, L2 and L3 can be swapped. The phase sequence should therefore be checked before connection or after work on the installation.
An installation tester or a suitable two-pole voltage tester with phase sequence indication can display the phase order. It is important that the measurement is carried out correctly and interpreted clearly. Phase sequence testing is particularly relevant in practice for mobile distributors, construction site power, extension cables or modified sockets.
If the phase sequence is incorrect, improvisation at any arbitrary point is not acceptable. The correction must be carried out professionally and then tested again. For machines with their own direction reversal or frequency inverter, the machine concept must also be considered.
Neutral conductor: when a missing or incorrect N becomes critical
The neutral conductor is underestimated in many three-phase loads. Pure three-phase motors often do not require N, but many modern machines do. Controls, power supplies, lighting, operating panels, auxiliary circuits or internal 230 V loads may depend on a correct neutral conductor.
In a five-pole CEE socket, the neutral conductor should be present and correctly connected. If N is missing, interrupted or incorrectly wired, failures, overvoltages in partial circuits, malfunctions or damage to control components can occur. Unbalanced loads are particularly critical.
In four-pole CEE connections, no neutral conductor is provided. This is not automatically technically wrong as long as the connected load does not require N. It becomes problematic when a load that requires a neutral conductor is operated via adapters or incorrect connecting cables.
The test must therefore match the specific socket and the planned load. It is not enough to merely detect that a phase sequence is present. For many machines, it must also be clear whether N and PE are correctly available.
Evaluating loop impedance and short-circuit current
Loop impedance is an important measured value for assessing whether a sufficiently high fault current can flow in the event of a fault. Only if the fault current is large enough can fuses or circuit breakers disconnect within the intended conditions. This point can be particularly relevant for CEE sockets with long cables, mobile distributors or sub-distributions.
High cable resistance, poor terminals, long supply cables or unfavorable network conditions can cause the short-circuit current to be too low. Then a protective device may trip too slowly or not as expected in the event of a fault. Loop impedance should therefore be considered not only for standard sockets, but also for powerful three-phase connections.
The assessment depends on network type, protective device, cable protection, fuse protection and protective measure. An installation tester can provide measured values such as loop impedance and the derived short-circuit current. However, the professional assessment must be made for the specific installation.
This point is especially important for newly installed CEE sockets, after modifications to sub-distributions, with long cable routes, mobile construction site distribution boards or when high-power machines are to be connected. A functioning load does not prove that the protective conditions are fulfilled.
| Measured variable | Meaning | Why important for CEE? |
|---|---|---|
| Loop impedance | Resistance of the fault current loop | Influences disconnection conditions in the event of a fault. |
| Short-circuit current | Prospective current in the event of short circuit or fault | Must match fuse protection and protective measure. |
| Cable length | Increases resistance and voltage drop | Long supply cables can influence protective conditions. |
| Terminals | Can cause contact resistances | Loose contacts increase heating and fault impedance. |
| Fuse protection | Determines required disconnection conditions | Measured value must be assessed according to fuse and network type. |
RCD testing on CEE sockets
Many CEE sockets are protected by residual current devices. Whether an RCD must be present and which type is required depends on the installation, area of use, protective concept, socket rated current, usage and applicable requirements. In practice, it should therefore always be checked which protective measure is intended.
RCD testing is not only about whether an RCD trips. Tripping current, tripping time, RCD type, network type and connected loads must also be considered. With three-phase loads containing electronic drives, frequency inverters or DC residual current components, selecting the correct RCD type can be particularly important.
A CEE socket can appear externally correct but be protected by an unsuitable or faulty RCD. Conversely, an RCD can trip during a test even though the other measured values have not yet been sufficiently assessed. RCD testing is therefore one component of the overall installation test.
Installation testers such as COMBI519, COMBI521 or EASYTEST can support RCD testing depending on the version. It is important to select the correct measuring function and interpret the results in the context of protective measure, network type and load.
Loose contacts, heating and recurring faults
CEE sockets are often operated under high load. As a result, contact problems often carry more weight than with small loads. A loose terminal, a damaged plug contact or a poor connection can lead to heating, voltage drop, failure or fire risk.
Such faults are not always immediately visible. Sometimes the connection works without problems at low load, but becomes warm or fails sporadically under high load. Especially with machines that draw high currents only intermittently, the fault can be difficult to detect.
Indications can include discolored contacts, melted plugs, smell, deformation, repeated tripping of protective devices, voltage drops or irregular machine behavior. In such cases, not only the socket, but also the plug of the load, the connecting cable and the distribution board should be checked.
A recurring test should therefore not only record measured values, but also consider the mechanical condition. Especially in workshops, production areas and construction site environments, CEE sockets can age more quickly due to frequent use and mechanical stress.
Testing before machine connection and commissioning
Before connecting a machine to a CEE socket, it should be checked whether the connection matches the machine. This includes rated current, voltage, phase sequence, neutral conductor requirement, protective conductor, fuse protection, RCD concept and connection type. A matching plug shape alone is not sufficient evidence.
The first connection after a new installation, after a repair, after relocating the machine or after modifications to the sub-distribution is particularly critical. In these cases, conductors may have been swapped, terminals incorrectly installed or protective measures incompletely tested.
For machines with motors, the phase sequence is important. For machines with electronic controls, the neutral conductor is often relevant. For machines with frequency inverters, mains filters or leakage currents, the protective concept must also be assessed. A simple voltage test does not fully cover these points.
A structured test is therefore useful for commissioning. First the socket is assessed, then the connection to the machine and finally operation under controlled conditions. Measured results and observations should be documented so that later faults can be classified more easily.
| Check before machine connection | Why important? | Possible consequence of faults |
|---|---|---|
| Rated current and fuse protection | Socket, cable and machine must match | Overload, tripping or thermal damage. |
| Phase sequence | Motors must run correctly | Wrong direction of rotation, machine damage or process fault. |
| Neutral conductor requirement | Controls or auxiliary circuits may require N | Failure or damage to 230 V components. |
| Protective conductor | Personal protection in the event of faults | Dangerous touch voltages possible. |
| RCD and leakage currents | Electronic loads may have special requirements | Nuisance tripping or unsuitable protection. |
Documentation and recurring testing
For commercial and industrial CEE sockets, documentation is an important part of testing. Measured values should be assigned traceably: which socket was tested, which circuit, which fuse protection, which network type, which RCD, which measured values and which condition were found?
Good documentation helps not only as proof, but also for maintenance. If a recurring test shows that loop impedance, voltage drop or contact condition has worsened, action can be taken early. Without reference values, gradual changes often remain unnoticed.
Changes to the installation should also be documented. If a CEE socket is changed from 16 A to 32 A, a supply cable is extended, a distributor is relocated or an RCD is replaced, the test must be reassessed accordingly.
Installation testers with memory and documentation functions can simplify this process. However, clear measuring point designation remains important. A stored measurement without clear assignment to the socket is of limited use later.
Typical faults in CEE and three-phase connections
A common mistake is assuming that a CEE socket is OK if the connected machine runs. A machine can initially function despite incorrect phase sequence, poor protective conductor, excessive loop impedance or thermally damaged contacts. Safety-related assessment requires more than a functional test.
Another common fault is swapped phase conductors. This often only becomes apparent when a motor runs in the wrong direction. The phase sequence should be checked especially with extension cables, mobile distributors or repaired plugs.
Missing neutral conductors are also relevant in practice. A five-pole socket can be incorrectly wired, and a four-pole connection can accidentally be used for loads that require a neutral conductor. Such faults can damage controls, power supplies or auxiliary circuits.
Loose contacts are also problematic. They cause heating and can lead to voltage drop, failure or fire damage under load. A test should therefore include not only electrical measured values, but also visual findings and mechanical condition.
| Fault pattern | Possible cause | Test approach |
|---|---|---|
| Machine runs in the wrong direction | Phase conductors swapped, incorrect phase sequence | Check phase sequence and correct professionally. |
| Control system fails | Neutral conductor missing or faulty | Check N against phase conductors and PE, assess connection type. |
| Protective device does not trip as expected | Loop impedance too high or protective measure unsuitable | Assess loop impedance, short-circuit current and fuse protection. |
| Plug or socket becomes warm | Contact problem, overload or poor terminal | Assess visual inspection, contact condition and load situation. |
| RCD trips unintentionally | Leakage currents, wrong RCD type or load problem | Check RCD type, load and residual current situation. |
Practical example: testing a CEE socket in a workshop
In a workshop, a new machine is to be connected to an existing 32 A CEE socket. The socket has only been used occasionally so far. Before commissioning, it is checked with an installation tester and a suitable voltage tester.
During visual inspection, slight discoloration on the socket insert is noticed first. Mechanical inspection shows that the socket is firmly mounted, but the cover is damaged. Since the machine will be operated in a working area with dust and oil mist, the degree of protection and the condition of the socket are assessed more closely.
The measurement shows that all three phase conductors are present and the voltages between the conductors are plausible. The phase sequence is then checked. This reveals an incorrect phase order. For the planned machine, this would be critical because a motor could start in the wrong direction.
In addition, the protective conductor, loop impedance, short-circuit current and the installed RCD are checked. Only after the wiring has been professionally corrected, the socket repaired and the measured values documented is the machine connected. The example shows: before connecting a machine, a CEE socket should not only be checked for voltage, but assessed completely.
Which measuring instruments / products are suitable?
The COMBI519 installation tester is a suitable solution for structured testing of electrical systems and installations. It is suitable for typical VDE test tasks where protective conductor, insulation measurement, loop impedance, RCD testing, phase sequence and documentation play a role.
The COMBI521 installation tester is particularly interesting when, in addition to classic installation tests, extended test sequences, modern documentation or additional applications such as charging infrastructure are relevant. For companies that test many different electrical connections, this can be an important advantage.
The EASYTEST installation tester is a practical solution for recurring installation tests and structured test sequences. It is particularly suitable when measurements in the field need to be carried out efficiently and traceably.
Two-pole voltage testers are useful as supplementary test instruments. They are suitable for quick voltage checks, detecting voltage and, depending on the version, also for phase sequence indication, continuity testing or load switching. However, they do not replace a complete test with an installation tester.
| Product / area | Typical use | Particularly relevant for |
|---|---|---|
| COMBI519 installation tester | Testing electrical systems and installations | CEE sockets, three-phase connections, RCD, loop impedance, phase sequence and documentation |
| COMBI521 installation tester | Extended installation testing and modern test sequences | Industry, maintenance, charging infrastructure, extensive documentation |
| EASYTEST installation tester | Structured and recurring tests in the field | Fast installation testing, service, maintenance and simple documentation |
| Two-pole voltage testers | Quick voltage testing and supplementary checking | Absence of voltage, voltage check, phase sequence indication and plausibility check |
| CEE test adapters and test leads | Safe connection between tester and socket | Recurring testing of many CEE connections and clear measuring point assignment |
Conclusion: do not test CEE sockets only for voltage
CEE sockets and three-phase connections must be assessed more carefully than simple functional connections. Present voltage or a running machine does not automatically mean that the connection is safe and compliant. Protective conductor, phase conductors, neutral conductor, phase sequence, loop impedance, short-circuit current, RCD and mechanical condition must be considered together.
Testing before machine connection, after modifications to the installation, with mobile distributors, on construction sites, in workshops and during recurring tests is particularly important. Faults such as incorrect phase sequence, missing neutral conductor, loose contacts or excessive loop impedance can have serious consequences.
The most important recommendation is: always treat CEE sockets as complete electrical measuring points. A two-pole voltage tester is an important supplementary test instrument, but for complete assessment, installation testers such as COMBI519, COMBI521 or EASYTEST are the right tools. This reliably combines safety, function and documentation.
FAQ: frequently asked questions about testing CEE sockets
Why does a CEE socket need to be tested?
CEE sockets often supply powerful three-phase loads. Faults in the protective conductor, phase conductors, neutral conductor, phase sequence or contacts can be dangerous or damage machines.
Is it enough if voltage is present at the CEE socket?
No. Voltage alone does not show whether the protective conductor, phase sequence, neutral conductor, loop impedance, RCD and mechanical condition are OK.
What is tested on a CEE socket?
Typical checks include visual inspection, protective conductor testing, voltage measurement, phase sequence testing, loop impedance, short-circuit current assessment, RCD testing and documentation.
Who is allowed to test a CEE socket?
Tests on CEE and three-phase connections may only be carried out by qualified electricians or appropriately competent persons using suitable measuring equipment.
Why is the protective conductor so important?
The protective conductor ensures that dangerous touch voltages are avoided and protective devices can operate in the event of a fault. A missing or high-resistance PE is critical.
Which voltage is expected at a 400 V CEE socket?
In a typical 400/230 V three-phase connection, around 400 V is expected between the phase conductors and around 230 V between phase conductor and neutral conductor. The assessment must match the network type.
What does phase sequence mean?
Phase sequence describes the order of the phase conductors. It determines the direction of rotation of three-phase motors.
Why is an incorrect phase sequence dangerous?
An incorrect phase sequence can cause motors, pumps, fans or machine axes to run in the wrong direction. This can cause damage or dangerous movements.
Can a machine work despite incorrect phase sequence?
It may start, but run in the wrong direction or not operate correctly. The phase sequence should therefore be checked before connection or after modifications.
Why is the neutral conductor important in CEE sockets?
Many modern machines need the neutral conductor for controls, power supplies or 230 V auxiliary circuits. A missing or incorrect N can cause failures or damage.
Is a four-pole CEE socket without neutral conductor wrong?
Not in principle. It is suitable for loads that do not require a neutral conductor. It becomes critical when loads requiring a neutral conductor are operated on it.
What does loop impedance indicate?
It describes the resistance of the fault current loop. It is important for assessing whether a sufficiently high current flows in the event of a fault and protective devices can trip.
Why is the short-circuit current relevant?
The prospective short-circuit current must match the fuse protection and protective measure. If it is too low, disconnection conditions may be at risk.
Does an RCD on CEE sockets have to be tested?
If an RCD is part of the protective measure, its function must be assessed with suitable measurements. Tripping time, tripping current and RCD type must match the application.
What role does the RCD type play?
The RCD type must match the network type and load. Machines with frequency inverters or electronic drives can create special requirements.
Why do CEE sockets sometimes become warm?
Heating can be caused by overload, loose contacts, poor terminals, damaged plugs or worn contact surfaces.
What should be checked before connecting a machine?
Important points are rated current, fuse protection, protective conductor, neutral conductor requirement, phase sequence, RCD concept, loop impedance and mechanical condition of the socket.
Can a two-pole voltage tester replace the test?
No. A two-pole voltage tester is very helpful for quick voltage and plausibility checks, but it does not replace a complete installation test with an installation tester.
Which measuring instruments are suitable for CEE tests?
Installation testers such as COMBI519, COMBI521 or EASYTEST are suitable. Two-pole voltage testers and suitable test adapters are useful supplements.
Why is documentation important?
Documentation makes it traceable which socket was tested, which measured values were present and whether protective measure, condition and function were OK at the time of testing.