25 1/2018 | MONITOR | In low voltage systems, the offset voltage is deter- mined by direct measurement of voltage to earth. All ‘earth fault monitor’ variants share the following common features. • The aim is rapid detection of asymmetric, very low-impedance insulation faults (close to 0 Ohm) on an active conductor to earth. • When a first asymmetric fault is detected, a load or a defective branch must be shut down rapidly. • Symmetrical insulation faults, caused for example by ageing and a vast array of electrical and physical stress factors and by environmental factors normally associated with operations are not detected. • The activation threshold sensitivity depends to a very large extent on system parameters. – Insulation resistance level – System leakage capacitance – Mains voltage • High-impedance insulation faults - we are talking here about a few kOhm – are for the most part not detected. All of these properties have led to the fact that the most commonly used measuring technology methods used for earth fault monitors are prohibited for use in insula- tion monitoring devices, as defined in product standard DIN EN 61557-8 (VDE0413-8) or IEC 61557-8. Also, the function of rapid automatic shutdown, when this occurs in the context of a first asymmetric and very low-impedance insulation fault, more closely resembles the functionality of a residual current device (RCD) than that of an insulation monitoring device. However, unearthed systems (IT systems) are char- acterised by the fact that, when a first insulation fault occurs, they do not need to be shut down. Concealed earth fault monitor function The concealed earth fault monitor function of the isoGEN423 is only for use in IT systems and is intended here for the low voltage sector for use up to a maximum of DC 500 V (400 V + 25%). This function is suitable for use in systems with a maximum of 5 μF system leakage capacitances. The earth fault monitor function of the isoGEN423 can be used by setting the appropriate operating mode in the ‘Set’ device menu. If the ‘DC’ operating mode is set, a rapid tripping becomes possible in DC systems in response to asymmetric insulation faults. The maximum response time here is ≤ 1 s for system leakage capaci- tances of C e ≤ 2 μF. Compared to the earth fault monitors described above, a rapid shutdown is also possible if the insulation resis- tance value drops below a limit value for symmetrical insulation faults. Here, response times of ≤ 10 s are maintained for system leakage capacitances levels C e ≤ 5 μF und R F ≤ R an /2. In this mode, leakage capacitance C e is also measured. Dipl.-Ing. Dieter Hackl, Standards & Innovation CONCLUSION The rapid shut-down of a power supply based on an offset voltage value is comparable in functional terms to the FU protection switch function from the Fifties. Since this kind of protective device is now prohibited in public power supply systems, for specific applications where the earth fault monitor function is abso- lutely essential, Bender also offers the option of implementing this safely and to real benefit with the new ISOMETER ® isoGEN423.