- 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.
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.