MAGAZINE FOR ELECTRICAL SAFETY 20 1 CC613 charge controllers: Safer charging with the new generation Use in induction furnaces and hardening installations: Insulation monitoring in installations with low insulation levels Additional page "Practical Expertise" at the back of the booklet for collection! Operating TT systems safely and with high availability | MONITOR |1/2016 Operating TT systems safely and with high availability In everyday electrotechnical life it is often attributed to the TT system that it is old and outdated and inferior to the TN system.But this isn't the case. TT systems have a simple earthing design giving them a decisive advantage over TN systems, especiallywhen supplying power to remote, stand-alone installations (e.g., remotely located farms and agricultural sites). The challenge of TN systems TN systems have a protective earth conductor routed from themainoperating earth to the customer's instal- lation. The conductor's potential to a remote earth (0V)must not exceed AC 50 V which could happen if there is an earth fault outside of the customer's installation due to a high-current cable experiencing an insulation fault from very low-ohmic earthing con- tact resistance (e.g. a power cable earth fault from the drinking-water supply system). In unfavourable circumstances, high residual currentsmay occur here whichmight (just)mean theoverloadprotectivedevice does not respond. If the total earthing resistance of the TN system is higher than that of the large-surface drinking-water supply systemwith earthing contact, a dangerouspotential can occur at thePE. Practical PageA Expertise The IEC 60364-4-41:2017-03 standard therefore states that: 411.4.1 InTN systems the integrity of the earthing of the electrical installationdepends on the reliable and effective connection of thePEN orPE conductors to earth. [..] • R B / R E ≤50V/( U 0 –50V) where • R B is theearthelectrode resistance, inohms, of all earth electrodes in parallel; • R E is the lowest contact resistance in ohms of extrane- ous-conductive-parts that are in contactwith earth and are not connected to aprotective conductor andwhere a fault canoccurbetween theouter conductor andearth; • U 0 is the nominal a.c. r.m.s. voltage to earth, in volts. NOTE 2 InGermany compliancewith condition R B / R E ≤ 50/( U 0 – 50) is compulsory for the supply network operator. 1|2020 | MONITOR | 56 When signs of the times become problems Updating instead of purchasing new