Description:
Damage to the electrical insulation (reduction in insulation resistance) can repeatedly result in personal injury. For this reason, residual current devices (RCDs) are installed in many systems. These have a defined tripping range of approx. 15 to 30 mA. In addition to this residual current circuit breaker, another measure is to measure the insulation resistance of the system as a repeat measurement in accordance with DIN VDE 0105 Part 100. This ensures that the system complies with the safety regulations and installation standards. This measurement can only be carried out on a de-energised system. The standard value is four years. For financial reasons, shorter test intervals are not favoured. However, in order to recognise signs of ageing in the insulation system of the installation at an early stage and thus counteract an unplanned shutdown, a residual current measurement is recommended.
In addition to the insulation test, rising residual currents can be detected. However, there are other phenomena that complicate the situation in addition to these residual currents that are dangerous to people. For example, in addition to the residual current or ohmic leakage current, various capacitive leakage currents are measured that cannot provide any information about the insulation status of the system. These leakage currents are often generated by machines driven by electric motors. Capacitive leakage currents can flow in the large winding capacitance of the motor to the laminated core and thus to the housing, which can increase considerably during operation with frequency inverters and even lead to damage to the motor ball bearings. The capacitance of long motor cables also leads to leakage currents via the shield.
As a result of these leakage currents, a residual current of more than 30 mA is often measured. Despite these difficulties, it is often possible to recognise a certain trend in the residual current. This should then be interpreted as a sign for an early repeat test in accordance with DIN VDE 0105 Part 100. The DACT residual current transformers can be connected directly to some universal measuring devices or provide a 4-20 mA output with an additional device. There is also the option of connecting a relay.
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