Residual current monitoring offers an alternative to insulation resistance measurement on production equipment
Author : Roland Bürger | Business Development Engineer | Danisense
01 November 2022
Figure 1. The new residual current monitor from Dansisense SRCMH070IB+ with 4-20 mA DC output signal & extended Windows software
Often, conventional insulation measurements in production systems can be quite costly. In some cases, systems must be shut down completely, or components such as frequency converters or switching power supplies disconnected, because the sensitive semiconductor components can be damaged by the test voltages.
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Continuous residual current measurement or RCM, on the other hand, offers several advantages, Roland Bürger, Business Development Engineer at current measurement expert, Danisense explains here – and is therefore now being used more widely, even becoming mandatory in some cases...
Plant testing was already required in the first German VDE regulation from 1896. In the meantime, plant testing for new and extended plants is described in the international standard IEC 60364-6:2016. This standard is the basis for many national regulations. The intention of this regulation is to ensure that electrical equipment is in proper condition. This minimises the risk of personal accidents, electrically ignited fires or damage to equipment.
Figure 2. IEC 60364-6:2016 - Low voltage electrical installations - Part 6: Verification
Mandatory testing
Because these national guidelines are a legal requirement, companies are obligated to carry out inspection tests. In the event of damage, insurance companies regularly refuse to pay benefits if there is no proof that the equipment causing the damage has been inspected in due time. Even the cancellation of policies is not uncommon in serious cases. In addition to the losses and costs, the company may also be threatened with lawsuits for damages that could endanger its existence.
As understandable as this regulation may seem, it causes considerable costs for the operators of electrical systems and stationary machines. For the test, the systems and machines are often switched off in order to carry out the insulation measurement or to test the protective devices (such as residual current devices or RCDs). The time intervals for the recurring test is listed in the national directives. For electrical systems and fixed equipment that are not specified in more detail, the test interval is specified as four years in many countries.
Predictive maintenance
Figure 3. Electrical equivalent circuit of a speed-controlled motor
Regardless of the inspection period for the repeat inspection, the plant or machine can fail due to unforeseeable circumstances. To counteract this scenario, predictive maintenance is already being practiced in some innovative industrial sectors or critical properties. Based on condition data obtained via sensors, the inspection or maintenance times of a machine or system can be precisely predicted.
In the field of electrical installations, unforeseen damage to the insulation can be detected in good time by means of continuous residual current measurement (RCM), so that the repeat test can be brought forward in the event of increased residual current values. In most cases, this can prevent an uncontrolled shutdown of the installation.
Continuous differential current measurement is already mandatory for new data centre construction in Germany and other countries in Europe (EN 50600-2-2). The residual current is used here as a characteristic value for the condition of the system’s insulation. In a current project, the operator was able to detect defective switching power supplies at an early stage via increased residual current levels and provide for replacements in good time. An uncontrolled shutdown of various servers could thus be prevented.
Figure 4. Software user interface
Insulation tests can be omitted
A welcome side effect is that an inspection of the insulation according to IEC 60364-6 and many national regulations can be omitted.
Often, a conventional insulation measurement is quite costly. In some cases, system components such as frequency converters or switch-mode power supplies must be disconnected, because the sensitive semiconductor components can be damaged by the test voltages.
Some of the testing institutes are already using residual current monitoring in various industrial sectors. By analysing the measured values of the residual current recorded in databases, the test period of the repeat test according to the risk assessment can be shortened if necessary. Different alarm thresholds trigger notifications via text message or e-mail. The system is thus exposed to continuous monitoring, whereas the conventional insulation resistance test is merely a snapshot. If the plant has no protective device to be tested, the shutdown can even be avoided completely in some cases.
Table 1. Typical causes of system-related residual currents regarding their frequency
Residual current monitoring in detail
When selecting the residual current monitor, it should be noted that today, the speed-controlled, three-phase motor is a standard element in all automated process plants and commercial buildings. High-efficiency asynchronous motors, but especially motor technologies such as permanent magnet motors, EC (electronically commutated brushless DC) motors and synchronous reluctance motors, require control via frequency converters; in fact, for many motor types, direct operation via a standard 3-phase power supply is no longer possible.
Due to the use of frequency converters, a system-related leakage current is present in most cases, which can cause problems for commercially available Residual Current protective Devices (RCDs). While fault currents mostly consist of a high resistive component, system-related leakage currents are predominantly capacitive. However, the RCD cannot distinguish between the different leakage currents. Therefore, it can already trip if the sum of all system-related leakage currents is above the tripping threshold. This is also possible during normal operation.
Figure 5. Signal shape of the measured residual current
As shown in Figure 3, different frequency components can occur in the residual current from DC up to several kHz. For this reason, RCD or RCM devices with specification B+ are recommended, which cover the frequency range DC to 20 kHz. When analysing the measured residual current, the system-related residual current must always be taken into account, because this is present despite perfect insulation and cannot be technically separated. Also, due to inductances (for instance, motors), high current peaks can be generated during the switch-on processes, which can lead to relay tripping at the RCDs and RCMs.
In general, the different frequency components can be interpreted as follows.
When installing a residual current monitor, it is important to know the actual system-related leakage current. Only then it is possible to set appropriate warning thresholds and relay trip thresholds in a reasonable way.
Figure 6. FFT of the residual current
The residual current monitor from Danisense (SRCMH070IB+) processes the signal up to 100 kHz and can be read out via a USB socket, using specially developed software for Windows systems. With this setup, we now move on to a production machine with a wide variety of robot systems and speed-controlled electric motors. Due to the installed frequency converters, different frequency components of the system-related leakage current should be detectable.
The user interface of the software provides the following overview.
A true RMS value of 290.1 mA is detected over the integration interval of 1000 ms. We start with the maximum trigger threshold of the integrated relay of 1000 mA and look at the signal of the differential current via the FFT tab.
Figure 7. The weighting of different frequency components relevant for the relay
The signal is plotted over the time interval of 0.1 seconds. Over an interval of 20 ms (one sine wave @ 50 Hz) we detect 3 sine waves. A fundamental of 150 Hz thus forms the largest amplitude in our signal. The FFT analysis confirms our assumption.
Relay function
It should be noted that not all frequency components of the residual current are weighted equally for the relay output, and therefore, a smaller true RMS value (210.6 mA) is displayed in the user interface for the relay function. This is due to the normative regulation of RCDs, which also applies to RCMs according to IEC 62020.
Here you can see the relay function of the relay of an RCD type B+, which can detect a differential current from DC to 20 kHz. As shown in Figure 7, only the frequency components between 50 and 100 Hz are included 1:1 in the current value relevant for the relay. Lower and higher frequency components are weighted weaker. The tripping value of 30 mA is given in the range of the mains frequency of 50 Hz, since the possibility of a fault current is greatest there. The permissible tripping value increases with increasing frequency. This means that the high-frequency residual currents of the drive are already partially taken into account. This weighting is also applied in the relay output of residual current monitors. For this reason, higher-frequency current components are significantly attenuated in the relevant waveform for the relay output and the true RMS value is smaller than the conventionally determined true RMS value.
Figure 8. Damping of the higher frequency components for the internal relay function of the RCM
Measurement analysis
In order to generate stable monitoring, and at the same time, be protected against false alarms, we now look at the different values of the differential current generated by the machine during different operating modes.
The values in Figure 9 were exported from the Danisense software as a .csv file. The machine has been previously subjected to an insulation measurement. No defects could be found. Due to the integration interval over 1000 ms, the possible current peaks during switch-on and switch-off processes are smoothed out, so that no significantly increased values can be detected via the 4-20 mA DC interface. The differential current oscillates between 236.5 and 333.7 mA. Two alarm thresholds at 450 or 550 mA can now be defined via the 4-20 mA interface in the PLC (programmable logic controller) or in the universal measuring device. The relay output can be set to 1000 mA. According to the relevant standards, a triggering between 50% and 100% (500 to 1000 mA) is defined here. The system should therefore be reasonably monitored with these parameters. In principle, however, the use of the relay output is not mandatory. Over a period of two months, no false alarms could be detected.
Figure 9. TRMS values of the residual current
In some projects, the measured values are already linked to the respective machine states via the PLC. An evaluation of the measured values can thus be carried out even more precisely.
The differential current monitor also has an intelligent algorithm that can be triggered by simultaneously activating two pressure switches in the operator terminal. This analyses the differential current signal within only 10 seconds and automatically selects the appropriate parameters in the user interface.
Conclusion
In addition to increased safety and availability of technical equipment, residual current monitoring is an attractive aid for testing institutes and self-testing industrial companies to increase the effectiveness and efficiency of the required tests.
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