Control circuits and auxiliary circuits

The increasing demands on machines and installations in terms of operational safety and accident prevention require comprehensive measures to protect both man and machine. For example, insulation faults in a control system must not lead to hazardous machine motions. Therefore, when choosing a system type (TN, TT or IT systems) and the associated protective measures, the following aspects are of particular importance:

• Probability of insulation faults
• Necessary basic protection and fault protection
• Continuity of the power supply
• Technical and commercial aspects
• Experience regarding secondary effects (fire, downtime costs, etc.)

In the case of electrical machines, this applies to the control circuits and auxiliary circuits as well as to the main circuits. In main circuits, the most important aspects are protection against electric shock, protection of persons and fire prevention. In control circuits and auxiliary circuits, especially those with voltages below the permissible touch voltage of AC 50 V/DC 120 V, the focus is on operational safety.

Due to dirt, dust or moisture e.g. on limit switches, symmetrical insulation faults may occur in control circuits, which may then prevent the installation from being shut down or cause uncontrolled start-ups.

Symmetrical insulation faults are resistive faults of the insulation of an electrical installation or device where all line conductors have approximately the same resistance to earth.

In this case, two high-resistance insulation faults on sensitive control inputs are enough to "set" the input. These faults do not cause the overcurrent protective device to trip since the fault current is noticeably below the required short-circuit current. Residual current devices also can't help since the forward and reverse currents are identical and therefore no residual current is generated.

When selecting the corresponding insulation monitoring device it should be ensured that it is capable of detecting symmetrical insulation faults. Earth fault relays that measure the displacement voltage to earth as an evaluation criterion do not fulfil this requirement from a measuring principle point of view.