Reasonable alarm thresholds for residual current monitoring

Protection targets

Currently, more and more production plants in the industrial sector are equipped with residual current measurements. This monitoring measure is mostly used to target the protection goals of fault and plant protection. In some special cases and conditions, fire protection can also be mapped by residual current monitors in accordance with IEC 62020.

Defined protection levels for residual current monitors

In addition to these protection targets, according to the international standardization, the insulation measurement, which is part of the periodic inspection for fixed installations, can be avoided.

Residual current values of a production plant with 270 A rated current

Production plants are mostly complex electrical systems, which are realized by a combination of different electrical equipment. In most cases, a PLC takes over the control. While the individual electrical equipment is regulated by standards with regard to its system-related leakage current, larger system-related leakage currents can occur in complex systems. Normally, these are capacitive filter currents that dissipate harmonic components to the protective earth conductor.

Residual current signal of a production plant

The following oscillogram was detected on production plant in an industrial environment. A fundamental oscillation can be detected, which oscillates three times in the time interval of 20 ms. In the FFT analysis, the largest amplitude can thus be expected at 150 Hz.

Fast Fourier Transform

Due to these often high values, it was not possible to protect the system with conventional RCDs for personal protection or fire protection. Freely accessible sockets and the associated RCD for personal protection were simply omitted for this reason.

The question now arises how to deal with higher system-related residual currents in the residual current measurement, especially since a stable amplitude is rather seldom established over the various operating states of the system. The measured values shown in the following diagram originate from a large manufacturing plant with a rated current of 270 A.

TRMS values for residual current over an integration interval of 400 ms

How to find reasonable alarm thresholds

The relatively large current values are mainly due to the capacitive filter currents of the individual equipment. Frequency converters are the main cause of this. If these measured residual current values are now linked internally in the PLC with the respective operating states of the plant, a normal state of the plant can be defined.

Link of the measured residual current and machine status

This procedure allows intelligent monitoring of the plant. Switch-on peaks can also be declared as a normal condition in this way. Costly individual monitoring of the individual equipment for plant protection is no longer necessary.

In general, however, it should also be noted here that this solution is not approved for personal protection. Due to the addition of the capacitive and the resistive current vector, levels of 15 to 30 mA are hardly detectable at high capacitive levels in the TRMS signal, as the following figure makes clear.

Diagram of a production plant with several types of operating equipment

Without the 30 mA resistive component (IR), the residual current is 250 mA (IRC). If the resistive component increases to 30 mA, the total residual current is only 251.8 mA.

In the following, the problem will be illustrated in an oscillogram with the same RMS values.

Residual current consisting of resistive and capacitive current

The capacitive current lags the resistive component by 90° or 5 ms. If the capacitive leakage currents are higher-frequency and integer multiples of the 50 Hz fundamental frequency prevailing in the conductors, as in the above example at 150 Hz, the problem is hardly changed even with different shifts between the 50 Hz and 150 Hz signals.

Oscillations of a resistive current (30 mA) and a capacitive 150 Hz current (250 mA) and the summation of both oscillations

For all capacitive current signals with a phase offset of 90° or 5 ms and all integer multiples of 50 Hz, the percentage increase in TRMS with increasing resistive current is given by the following formula.

the percentage increase in TRMS with increasing resistive current
Percentage increase of the TRMS value in relation to the ratio between capacitive and resistive current

These relationships should be taken into account when selecting the alarm thresholds in the PLC control system. In our table the values have to be changed as follows if the same protection targets are still valid.

Protection targets with new calculated values due to the ratio between capacitive and resistive current

In several projects, the measured residual current values are additionally connected to the PLC with the respective phase currents via energy measurement modules.

In some applications, frequency components beyond 100 Hz or 2 kHz are deliberately omitted. This setting can be made at the operating terminal of the Danisense residual current monitor. In this way, capacitive frequency components with a higher frequency are only taken into account in the TRMS value in a strongly damped form.

User terminal of the residual current monitoring device SRCMH070IB+

In this way, the system-related leakage current can often be reduced and changes in the resistive fault current are easier to detect. This procedure appears legitimate, as the relay function of conventional RCDs and RCMs also provides for a general damping of higher frequency components in accordance with the standard.

Diagram of The weighting of different frequency components relevant for the relay


Individual monitoring of the operating equipment of an industrial manufacturing plant is often not possible in terms of cost and retrofitting. A good alternative is to measure the main connection and to link the output of the 4-20 mA output with the PLC. In this way, inrush current peaks and higher levels can be linked to the operating status of each controlled device. Dangerous resistive current components at 50 Hz can thus be detected more easily. It may be necessary to damp large system-related residual currents in order to obtain a better ratio between capacitive and resistive currents. To be able to analyze the given differential current sufficiently, the SRCMH070IB+ variant with free analysis software is recommended.

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