Insulation measurement
An insulation measurement is a measurement of the insulation resistance or a test of the insulation in order to determine whether it is safe or free from defects. Proof of the insulation resistance through measurements is required by law for electrical equipment in many contexts and is included in standards. The measurement is used to assess functionality and safety and can reveal defects at an early stage. For example, circuit breakers only react when there is an undoubted danger, for example when thermal power from around 4000 watts occurs at faults in lines for sockets with protective contact . Furthermore, the insulation measurement is used for systematic troubleshooting and is a method for assessing insulation materials and constructive insulation solutions.
Legal requirements
Measurement needs
Insulation measurements must be carried out for new systems, conversions, changes, repairs, electric vehicles in traffic and in the event of malfunctions. If the insulation resistance of electric vehicles is too low, which is measured during the main inspection , there is a risk of losing the general operating license and thus the insurance cover. In addition, regular repetitive tests at regular intervals are prescribed for commercial systems. In the private sector, too, the condition of the electrical system should be checked at certain time intervals, since insulation values can deteriorate due to aging, weathering, moisture, damage, thermal influences, overloading or other causes.
In Germany, the occasion for insulation measurements is regulated by the German statutory accident insurance " DGUV regulation 3 " (formerly BGV A3).
Measurement process
The insulation resistance must be measured between each active conductor and the protective conductor or earth. Since the insulation resistance decreases with the length of the lines, the rule of thumb is that only the part of the system after the last upstream fuse element is used for assessment. In the case of domestic installations, this means that each individual circuit must comply with the minimum insulation resistance, but the entire system can have a lower value.
In DIN VDE 0100-600: 2017-06, the specification was (re) included that the active conductors (L1-3 & N) should also be measured against each other. However, this only makes sense if it is impossible to measure through (switched on) consumers. These would strongly falsify the measurement result or could even be damaged.
Measurement run
The measurement takes place in individual steps in the sequence protective conductor (PE) against neutral conductor (N) and then protective conductor against all external conductors (L1, L2, L3), as well as the active conductors among each other. However, a fixed order is not required. Measurements are made by feeding in a high measuring voltage with a low measuring current (~ 1mA) in order to avoid the development of a fire or other damage. A breakdown of the measurement voltage or a measurement value that is too low is then an indication of an insulation fault or flashover.
Qualified measuring methods are described nationally and internationally in norms and standards such as EN , VDE , ÖVE , NIN .
Limit values for insulation resistance
In the past, national standards led to different measurement conditions without any specific physical justification. The introduction of European standards led to standardization.
Germany
Until 1987, the rule of thumb was 1000 Ω per volt of the measurement voltage. The initial test of new systems is carried out in accordance with DIN VDE 0100-600: 2017-06, which replaces DIN VDE 0100-610: 2008-06 with a transition period of March 17, 2020.
| Nominal voltage of the system | Measuring voltage | Insulation resistance |
|---|---|---|
| Voltages at SELV / PELV | 250 V DC | ≥ 0.50 MΩ |
| up to 500 V, except SELV / PELV | 500 V DC 1 | ≥ 1 MΩ |
| over 500 V to 1000 V | 1000 V DC | ≥ 1 MΩ |
| Type | resistance | tension |
|---|---|---|
| Protection class I. |
> 0.3 MΩ (with heating elements) 1 |
500 V DC |
| Protection class II | > 2.0 MΩ | 500 V DC |
| Protection class III | > 0.25 MΩ | 500 V DC |
| Circuit | standard | Measuring voltage | Insulation resistance |
|---|---|---|---|
| Main circuits against protective conductor system (up to 1 kV) 1 | DIN EN 60204-1 | 500 V DC | ≥ 1 MΩ |
| Main circuits against protective conductor system (over 1 kV to 36 kV) 2 | DIN EN 60204-11 | the lower value of a voltage, equal to the rated voltage or 5 kV | ≥ 1 MΩ |
Periodic testing of electrical systems
DIN VDE 0105-100 defines requirements for the operation of electrical systems. In order to maintain the proper condition, it may be necessary to a. the recurring test by measuring prescribed. For systems with nominal voltages up to AC 1000 V and DC 1500 V, the following information applies to the insulation resistance. For electrical systems with nominal voltages above 1 kV, reference is made to DIN VDE 0101 without direct information on an insulation measurement.
The measuring methods and limit values correspond to DIN VDE 0100-600 . All active conductors may be connected to one another for the measurement in non-fire-endangered locations or explosion-proof areas. The measurement of the insulation resistance is to be carried out with direct voltage. When the measuring device is loaded with 1 mA, the measuring voltage must be at least equal to the nominal voltage of the system. The voltage specification in Ω / V relates to the nominal voltage of the electrical system.
The frequency of the measurements is determined independently taking into account various risks, but it can also be specified by legal or national regulations.
| boundary conditions | Insulation resistance |
|---|---|
| General | Values according to DIN VDE 0100-600 (≥ 1 MΩ) |
| with connected and switched on consumers | R iso ≥ 300 Ω per volt of the nominal voltage (e.g. U N = 230 V → R iso = 69 kΩ) |
| without connected consumers, but with closed switching devices | R iso ≥ 1000 Ω per volt of the nominal voltage (e.g. U N = 230 V → R iso = 230 kΩ) |
| for systems outdoors, in rooms or areas that are hosed down for cleaning purposes with consumers connected and switched on |
R iso ≥ 150 Ω per volt of the nominal voltage (e.g. U N = 230 V → R iso = 34.5 kΩ) |
| in systems outdoors, in rooms or areas that are hosed down for cleaning purposes without connected consumers, but with closed switching devices |
R iso ≥ 500 Ω per volt of the nominal voltage (e.g. U N = 230 V → R iso = 115 kΩ) |
| in IT systems | R iso ≥ 50 Ω per volt of the nominal voltage (e.g. U N = 230 V → R iso = 11.5 kΩ) |
| Conductor lines and slip rings | Lower values if other measures ensure that the leakage current does not lead to dangerous contact voltages or fires |
| at low voltage ( SELV / PELV ) | ≥ 0.25 MΩ at a measuring voltage of 250 V. |
| for low voltage without safe separation ( FELV ) | Measure whether the body is properly connected to the protective conductor / equipotential bonding conductor |
Austria
The insulation resistance of the system components without consumer devices between two overcurrent protective devices or after the last overcurrent protective device must at least correspond to the values in the table.
| Nominal voltage of the system | Measuring voltage | Insulation resistance |
|---|---|---|
| Voltages at SELV / PELV | 250 V | ≥ 0.25 MΩ |
| up to 500 V | 500 V 1 | ≥ 0.5 MΩ |
| over 500 V to 1000 V | 1000 V | ≥ 1 MΩ |
| Type | resistance | tension |
|---|---|---|
| Protection class I. |
> 0.3 MΩ (with heating elements) ¹ |
500 V |
| Protection class II | > 2.0 MΩ | 500 V |
| Protection class III | > 0.25 MΩ | 500 V |
Measurement
Measuring voltage
In order to exclude capacitive and inductive influences, direct voltage is used for measurement. The level of the measuring voltage must at least correspond to the operating voltage of the system. Common values are 250 V for telecommunications and extra-low voltage systems , 500 V for house installations and household appliances and 1000 V for low-voltage motors . According to DIN VDE 0100-600, the test voltage can be reduced to 250 V if a surge arrester / protection is available.
Direct and alternating voltages are used in measurements for analysis and fault diagnosis; the voltages depend on the target.
Measuring device
For the insulation measurement according to the standard, crank inductors are used, among other things , in which the required high measuring voltage is generated by a small generator driven by a gearbox and hand crank. There are also measuring devices with built-in electronic vibration generators that step up the battery voltage via a transformer and then rectify and smooth it again. The measurement results are displayed using pointer instruments or digitally.
In principle, measurements to assess an insulation for research and development or technological implementation can also be carried out with the certified devices for standard-compliant testing. However, any other high voltage sources and measuring devices for current and voltage measurement can also be used.
Significance of an insulation measurement
Insulation measurements can reveal damage, aging, the influence of moisture, faults within or the unsuitability of an insulating material before it experiences a voltage breakdown. In addition to the absolute value of the leakage current, an important variable for DC voltage is its time course:
- Effect of polarization .
the measured leakage current falls with an intact insulating material in the first time (seconds to minutes) and asymptotically approaches a value - Effects of previous damage or moisture:
the leakage current increases over time and / or shows discontinuous courses - a sign of the beginning of a discharge channel or of the temperature-related increase in the conductivity of an already existing bridge made of foreign substances or decomposition / pyrolysis products
AC voltage tests can indicate that there are inhomogeneities, holes or cracks in the insulating material when pre-discharges occur. On the other hand, freedom from pre-discharge indicates, for example, a cavity and bubble-free casting (e.g. resin-encapsulated transducers) or perfect vacuum impregnation. The technical and constructive structure of an insulation with regard to its reliability can also be verified in this way, because pre-discharges always lead to permanent damage to the insulating material barrier. Pre-discharges are carried out in AC voltage tests at 50 Hz by analyzing the higher-frequency components of the leakage current.
In DC voltage tests, one tries to distinguish between different effects on the basis of the time curves and the voltage dependency:
- the polarization index is the ratio of the leakage currents after 1 minute and after 10 minutes: if it is 2…> 4, it is assumed that the system is in order - the course is then almost entirely the result of polarization
- the absorption index (dielectric absorption ratio DAR) is to be evaluated similarly as before, but has measurement times of 30 and 60 seconds
- the dielectric discharge test makes statements about self-discharge and is also used for capacitors or given as their specification
- The voltage dependency of the insulation resistance (step voltage test) indicates poor insulation: the value then drops with increasing voltage and indicates holes, cracks or voltage-dependent layers of dirt
literature
- Hans-Günter Boy, Uwe Dunkhase: Electrical installation technology The master's examination . 12th edition, Vogel Buchverlag, Oldenburg and Würzburg 2007, ISBN 978-3-8343-3079-6 .
- Dieter Vogt, Herbert Schmolke: Electrical installation in residential buildings. 6th edition, VDE Verlag GmbH, Berlin and Offenbach 2005, ISBN 3-8007-2820-6 .
Web links
- technical guidelines for operational safety
- According to which standard do I have to check WHAT on the Gossen Metrawatt website
- Measurements and tests on electrical devices on the Gossen Metrawatt website
- Repetitive testing of protective measures for stationary electrical equipment on the Gossen Metrawatt website
- Glossary of measurement technology and electronic terms on the Sourcetronic GmbH website
- https://www.weka.de/elektrosicherheit/
- ↑ mebedo.de THE NEW VDE 0100-600: 2017-06
- ^ Company publication of GMC-Instruments Schweiz AG
- ↑ Company publication of the Chauvin Arnoux Group