Ground-fault monitoring: The concept

• A symmetrical deterioration in insulation can be said to occur when the insulation resistance of all conductors in the system to be monitored declines to (approximately) the same extent. An asymmetrical insulation deterioration can be said to occur when the insulation resistance, e.g. of a conductor, declines to a significantly greater extent than that of the other conductor(s) in the system.
• Devices known as earth fault monitors, which use the voltage unbalance (voltage shift) occurring in the event of an earth fault as a sole measuring criterion, are not insulation monitoring devices under the terms of this standard.
• Under certain system conditions, a combination of various measuring techniques, including unbalance monitoring, can be used for monitoring.

Notes:

Symmetrical insulation faults are common in DC voltage systems and control circuits. If the resistance of both insulation faults is approximately equal, insulation monitoring devices ope-rating on the basis of the principle of superimposed voltage measurement will not be able to detect these insulation faults. It is for this reason that IEC 61557-8 requires the use of continuous insulation monitoring devices.

The AMP measurement method patented for Bender is based on a special clocked measuring voltage which is controlled by a micro-controller and adapts automatically to the prevailing system con-ditions. Software-based evaluation enables system leakage currents causing interference on the evaluation circuit to be differentiated from the measured variable proportionate to the insulation resistance in ohms. This means that broadband interferences as they occur, for example, during converter operation, do not adversely affect the precise determination of the insulation resistance.

The AMP Plus measurement method takes interference suppression to the next level. Devices supporting this measurement method can be used universally in AC, DC and AC/DC systems, e.g. systems with varying voltages or frequencies, high system leakage capacitances or DC voltage components. This makes them ideal for use in today's state-of-the-art distribution systems, which are usually subject to this type of interference (converters, EMC).