Residual current measurement
General
Residual currents caused by insulation faults can pose a considerable safety risk in electrical installations. An appropriate protection concept can detect residual currents, eliminate insulation faults in good time and thus ensure system availability. RCM stands for Residual Current Monitoring and means monitoring the residual current in electrical systems. This current is calculated from the sum of the currents of all conductors except the protective conductor (PE) that lead into the system. Residual currents are typically the result of insulation faults, leakage currents or e.g. EMC filter leakage currents.
While RCD devices (residual current circuit breakers) switch off the power supply when a certain residual current is exceeded, RCM measurement devices display the current value, record the long-term trend, and report when a critical value is exceeded. This message can also be used to switch off the power supply via external switching devices (contactors, relays). Residual current monitoring (RCM) devices are used to detect and report residual currents at an early stage. Countermeasures can be initiated in good time so that the system does not have to be shut down. This allows measures to be taken in the event of slowly deteriorating insulation values or gradually increasing residual currents, for example due to aging insulation, even before the system is switched off, e.g.:
- Insulation faults on cables and electrical equipment
- Leakage currents on the electrical consumers • Defective PP power capacitors for the PFC
- Defective components in switching power supplies, e.g. in computers
- Correctness of TN-S systems (Terra Neutral Separate) • Detection of impermissible PEN connections
- Avoidance of neutral conductor return currents on earthed equipment
Residual current measurement in connection with energy measurement in combined energy/RCM measurement devices in electrical systems is a preventive measure for fire protection and maintenance. Downtimes and the associated costs are reduced. Timely and preventive maintenance also significantly improves the efficiency and availability of a system thanks to the additional information gained from an RCM measurement device.
In particular, permanent RCM monitoring is essential in order to avoid any unwanted surprises during operation and to be constantly informed about the current status of the system.
Basic RCM measuring method
The functionality of RCM measurement devices is based on the residual current principle. All conductors at the measurement point (outgoing circuit to be protected), with the exception of the protective conductor, are routed through a residual current transformer. In the absence of any faults, the sum of all currents is zero. If, on the other hand, a residual current flows via ground, the current difference in the residual current transformer causes a current that is evaluated by the electronics of the RCM measurement device.
The measuring method is described in IEC/TR 60755. A distinction is made here between type A and type B.
Standard DIN EN 62020 / VDE 0663 / IEC 62020:
The standard applies to residual current monitoring devices for domestic installations and similar applications.
Optimal monitoring thanks to 6 current measuring channels
Modern, highly integrated measurement devices allow the combined measurement of
- electrical parameters (V, A, Hz, kW ...)
- Power quality parameters (harmonics current, THD, KUs ...)
- Energy consumption (kWh, kvarh ...)
- RCM residual current in just one measurement device. The following example shows a measurement device with 6 current inputs for this purpose: