TN-S System And Resudial Current Monitor
Unwanted power-interruptions and disturbances always cause high costs. No matter if it´s a fault of a simple room lighting or if there are disturbances in computer systems. Reasons for that are insulation faults, stray currents, overloads on the neutral conductor caused by harmonics, interruptions of PE- and N-conductors and as well EMC-influences. On the other hand there are effects like unwanted power-interruptions, damage caused by fire, impacts on protection systems, inexplicable malfunctions and damage to telecommunication-, fire alarm and computer systems, corrosion on pipes and lightning-protection systems. Depending on the location of the damage, costs that can easily reach several thousand or even hundred thousands of dollars can be caused. In this report the specific risks and reasons for damage will be explained and measures to avoid damages in modern electrical systems with Residual Current Monitors (RCMs) are stipulated.
Insulation faults occur e. g. as a result of mechanical, thermal or chemical damage in electric insulations. But also pollution, moisture or damage caused by the environment (animals and plants) can damage the insulation so that an unwanted leakage current is flowing via the fault location.
The magnitude of the current is defined by the system voltage, the earthing resistance and the insulation fault RF.
This fault current IF can flow between the active conductors via the insulation fault RF and/or via conductive parts to earth. If the current is high enough (short-circuit or earth fault) the connected protective device trips and the faulty equipment or part of the sys- tem is disconnected from the system. If the fault current IF is not sufficient to trip the protection system (incomplete short-circuit or earth fault), there´s an immediate fire risk, when the fault energy exceeds a value of about 60 W at the fault location (about 260 mA/ 230 V). For safe and reliable protection purposes, a residual current protective device (RCD) can be used that provides a reliable disconnection in dangerous situations e.g. with a rated residual current below 300 mA.
Especially in computer systems an interruption causes severe con- sequences. Therefore RCDs are usually not used in this area. Further: Frequently UPS-systems are used, which can only withstand a limited short-circuit current; so they cannot trip fuses or MCBs, because the conditions for disconnection cannot be met. As a result high fault currents with critical values concerning the protection of people and fire risk can be observed.
In addition to common protective devices it´s recommended to use RCMs (Residual Current Monitors) acc. IEC 62020 These devices can provide selective monitoring of single devices or monitoring of parts of the system and optionally pre-alarm information before the response value of the protective device is reached. In combination with a circuit-breaker disconnection can be carried out under defined conditions.
Although the TN-S system has been promoted (because of EMC- reasons) for some time now (IEC 60364-5-548), Electrical installations of buildings – Part 5: Selection and erection of electrical equipment – Section 548: Earthing arrangements and equipotential bonding for information technology installations“; IEC 60364-4-444, Electrical installations of buildings – Part 4: Protection for safety – Chapter 44: Protection against electromagnetic interferences (EMI) in installations of buildings), practice is frequently different. The systems are mainly designed with regard to the protection of people and optimized costs, so that the N-conductor – from a special cross section (10 mm2 copper) – is allowed to be joined together with the PE to a PEN-conductor. That´s why the current in the return path (N-conductor) can split via all the earth connections and equipotential bonding conductors, because the N-conductor in every floor distribution is connected to the PE system. As a result of that there are high balancing currents flowing through the entire building via all conductive (metal) parts (e. g. water pipes, heating pipes, conduits), which can partly lead to high electro-magnetic fields, cause indefinite faults which are difficult to locate in electronic circuits. Further, corrosions on water pipes of the fire fighting systems can develop. This effect is worsened by harmonics in the N/PEN conductor.
Therefore the TN-S system should always be given preference when electronic data processing systems are used. Return currents from many individual electrical devices can be carefully directed towards the power supply source and stray currents cannot creep back to the transformer neutral via earth connections. The N-conductor is only allowed to have one connection with the earthing system (preferably in the low-voltage main distribution).
This connection should be equipped with a current transformer and an RCM, which permanently monitors the connection and gives an alarm in case a preset threshold is exceeded. Additionally the PE system should be monitored by a RCM in order to check the load situation of the system.
The requirements for the TN-S system can be found in IEC 60364- 5-548: 1996-02 – Section 548: Earthing arrangements and equipotential bonding for information technology installations and IEC 60364-4-444: 1996-04 – Chapter 44: Protection against electromagnetic interferences (EMI) in installations of buildings), a properly installed TN-S system is required for a building where the use of information-technology is expected.
Electrical safety is indispensable. Safe and reliable use of electricity in various sectors calls for a high degree of electrical safety. In addition to personnel protection, our primary objective is to increase the protection against material damage and fire, and to increase the reliability of operation.
References W. Hofheinz: Fault Current Monitoring in Electrical Installations
 Bender Technical information No: 03
Harun Öndül- Sales Manager – Aktif Mühendislik