RCM – Residual Current Monitoring

  • General information

    Residual currents caused by the failure of insulation can constitute a significant risk to safety in electrical systems. Using an appropriate protective concept it is possible to detect residual currents, eliminate insulation faults in good time and therefore ensure the availability of the system.

    RCM stands for Residual Current Monitoring and means the monitoring of residual currents in electrical systems. This current is calculated as the sum of the currents of all conductors, apart from the protective earth (PE), which feed into the system. Residual currents are typically the result of insulation faults, leakage currents or EMC filter leakage currents for example.

  • Fig.: Report prior to switching off - an aim of re- sidual current monitoring

Whilst RCD devices (residual current circuit breakers) switch off the power supply in the event of a certain residual current being exceeded, RCM measuring devices indicate the actual value, record the long-term development and report the exceeding of a critical value. This message can also be used in order to switch off the power supply via external switching devices (contactors, relays). Through the use of residual current measuring devices (Residual Current Monitoring, RCM) it is possible to detect and report residual currents in a timely manner. It is possible to initiate counter measures in good time, so that it is not necessary to switch the system off.This facilitates the implementation of measures in the event of slowly deteriorating insulation values or steadily rising residual currents – caused for example by ageing insulation – before the system is switched off. For example:

  • Insulation faults in pumps
  • Residual currents from electrical loads
  • Defective PP power capacitors for the PFC
  • Defective components in switched mode power supplies, e.g. in computers • Correctness of TNS systems (Terra Neutral Separate)
  • Disclosure of impermissible PEN connections
  • Avoidance of neutral conductor reverse currents to grounded equipment

Residual current monitoring in conjunction with energy measurement in combined energy / RCM measuring devices in electrical systems constitutes a measure for fire protection and maintenance prevention. Down times and the associated costs are thereby reduced. Timely and preventative maintenance – facilitated through the information additionally gained from an RCM measuring device – also significantly enhances the efficiency and availability of a system.

Constant RCM monitoring is of particular significance in preventing unwanted surprises in ongoing operation, and provides consistent information regarding the actual status of the electrical system.

  • Fig.: Fault current to ground through high ohmic ground fault

  • Fundamental measuring process with RCM

    The functionality of RCM measuring devices is based on the differential current principle. This requires that all phases be guided through a residual current transformer at the measuring point (outlet to be protected), with the exception of the protective earth. If there is no failure in the system then the sum of all currents will be nil. If, however, residual current is flowing away to ground then the difference will result in the current at the residual current transformer being evaluated by the electronics in the RCM measuring device.

    The measurement process is described in IEC/TR 60755. Differentiation is made here between type A and type B.

DIN EN 62020 / VDE 0663 / IEC 62020 standard:

The standard applies to residual current monitoring devices for domestic installations and similar applications with a rated voltage of < 440 V AC and a rated current of < 125 A.

  • Modern, highly integrated measuring devices facilitate the combined measurement of

    • Electrical parameters (V, A, Hz, kW ...)
    • Power quality parameters (harmonics,THD, SIs ...)
    • Energy loads (kWh, kvarh ...)
    • RCM residual current in just one measuring device. The following example shows a measuring device with 6 current inputs for this purpose:
  • Fig.: Fault current to ground due to an insulation ageing of the motor windings. Minor current through high ohmic fault can be captured with RCM in time and remedial measures initiated to avoid a solid short circuit over time. Thus a production stop can b