Application Example - Beckhoff SCT Series Documentation

Application example

5 Application example
Residual current monitoring of a production machine with different speed-controlled motors
Today, the speed-controlled three-phase motor is a standard element in all automated process systems and
commercial buildings. Highly efficient asynchronous motors, but especially motor technologies such as
permanent magnet motors, EC motors and synchronous reluctance motors, require control via frequency
converters; for many motor types, direct operation via a 3-phase standard power supply is even no longer
possible at all.
This development is contrasted by decades of safety guidelines designed to guarantee personal, fire and
plant protection. For example, periodic inspection of low-voltage installations must be performed in
accordance with IEC 60364-6 (Edition 2.0 2016-04). Section 6.5.1.2 requires, among other things, a check of
the insulation resistance, in which a test voltage is applied between the respective conductor and the PE
protection potential. Many manufacturers of frequency converters explicitly prohibit this test on their devices.
Therefore, the frequency converter must be disconnected during this measurement to prevent possible
damage. IEC 60364-6 also offers us a way out under point 6.5.1.2. Here explains the standard:
"Where a circuit is permanently monitored by an RCM in accordance with IEC 62020 ... it is not necessary to
measure the insulation resistance if the function of the ... RCM is correct."
IEC 62020, which is mentioned in connection with the SCT5xxx, describes the technical boundary conditions
that a differential current monitor must fulfill in order to be recognized as a complete substitute for
conventional insulation resistance measurement.
Fig. 24: Beckhoff SCT5 series
An increase in the measured levels with the residual current monitor may indicate a fault in the insulation of
the system. A subsequent check of the plant can then be timed to avoid an uncontrolled shutdown of the
plant and an unintentional interruption of the production processes. In contrast to conventional insulation
measurement, the system is monitored seamlessly by differential current monitoring and faults in the
insulation can be detected immediately.
It is therefore a procedure that can be classified as a predictive maintenance solution. When commissioning
a differential current monitor, a number of boundary conditions must often be observed to ensure correct
functioning.
Due to the use of frequency converters in production machines, in the vast majority of cases a system-
related leakage current is present, which can cause problems for the commercially available Residual
Current Protective Devices (RCDs). While residual currents mostly consist of a high ohmic component, the
36 Version: 1.0 SCT5xxx