Low voltage DC systems (LV DC systems) play an important role in an ever-increasing number of applications. The increasing importance of renewable energies is one of the reasons. Photovoltaic installations generate DC that can be stored in decentral battery installations. In addition, there are a large number of loads that can be operated in an energy efficient manner with DC, such as electric vehicles and railway rolling stock or lighting installations. The DIN VDE 0100 series of standards is also definitive here for the electrical safety in the DC installation, however the specific requirements for DC systems must be observed

I. INTRODUCTION

To operate DC systems safely, these systems are often designed as an unearthed power supply (IT system) with insulation monitoring

Protection Against Electric Shock

Low voltage DC systems (LV DC systems) play an important role in an ever-increasing number of applications. The increasing importance of renewable energies is one of the reasons. Photovoltaic installations generate DC that can be stored in decentral battery installations. In addition, there are a large number of loads that can be operated in an energy efficient manner with DC, such as electric vehicles and railway rolling stock or lighting installations. The DIN VDE 0100 series of standards is also definitive here for the electrical safety in the DC installation, however the specific requirements for DC systems must be observed.

Basic Standards

From the standards point of view, in accordance with DIN VDE 0100-100 (VDE 0100-100):2009-06, the types of system familiar from the AC area, that is TN, TT and IT systems, are initially also applicable to LV DC systems and therefore also the protective measures described in DIN VDE 0100-410 (VDE 0100-410):2007-06. However, it is to be noted that, in accordance with DIN VDE 0100-410 (VDE 0100-410):2007-06, the permissible touch voltage UB DC is ≤ 120 V (AC ≤ 50 V) and the required shut down times also have different values (see Table 1). The exception here is the IT system for which shut down on a first fault is not necessary in accordance with section 411.6.1. Only on a second fault on a different active conductor are shut down times to be observed.

Protective Measure in Earthed DC System Residual Current Devices (RCD)

In AC systems, residual current devices (RCD) are a proven protective device of which millions are in use. However, the residual current devices (RCD type A, B etc.) currently defined in accordance with the DIN VDE 0660 series of standards are not suitable for DC systems, as the integrated switching element is not suitable for DC load currents. In addition, the measuring technology in type B for the acquisition of DC residual currents is voltage-dependent and the necessary auxiliary volt- age is drawn from the AC distribution network. So-called

MRCDs (Modular Residual Current Device) can be used for this task. These devices comprise a separate detection device (RCM) resp. a circuit breaker and are used in industrial applications. Here residual current monitoring devices (RCM) resp. corresponding circuit breakers suitable for DC applications could be used

The DC switches or isolating devices must meet special requirements, as on opening the contact an arc may be produced. This arc will result in contact burning that reduces the service life of switchgear. The higher the load current and/or the voltage, the greater the energy in the arc. While with AC the arc is interrupted in a natural manner when the voltage crosses zero, with DC the arc must be extinguished by other design measures, e.g. by means of the high opening velocity of the contacts, adequately large contact spacing, plasma cooling and acceleration of the arc into the extinguishing chamber, e.g. by a permanent magnetic field.

Unearthed Power Supplies (IT System) with Insulation Monitoring

To operate DC systems safely, these systems are often designed as an unearthed power supply (IT system) with insulation monitoring. Unlike an earthed system (TN, TT system) there is no direct connection between the active conductors and the protective earth conductor (earthing system). At this point it is appropriate to note that even an unearthed system (IT system) has, in accordance with sec. 411.3.1.1 DIN VDE 0100-410 (VDE 0100-410):2007, an earthing system that must meet the requirements of DIN VDE 0100-540 (VDE 0100-540):2012-06 and is therefore identical to an earthing system for TN or TT systems. If the conductive parts in an IT system are connected together and earthed via the same earthing system, in the event of a second fault comparable conditions to the TN system apply. If the conductive parts are earthed in groups or individually, comparable requirements to the TT system apply

In principle the IT system offers two crucial advantages

• In the event of a first fault there is no shut down
• The possible touch voltage is significantly less than 50 V.

The reduced risk of a parallel arc is to be considered an additional advantage of IT systems in DC applications. In IT systems the insulation resistance is monitored continuously during operation by insulation monitoring devices in accordance with DIN EN 61557-8 and an alarm triggered if the resistance drops below the limit set (in accordance with DIN VDE 0100-410 (VDE 0100-410):2007-06 sec. 411.6.3.2).

Modern Insulation Monitoring Devices (IMDs) monitor the insulation resistance of the entire IT system including the loads connected and during this process measure the sum of the individual insulation resistances in the form of a parallel circuit.

In addition to monitoring the entire system, the section of the system with a fault can be indicated and located using insulation fault location systems (IFLS) in accordance with DIN EN 61557-9:2009-11. Insulation monitoring and insulation fault location are undertaken without interrupting the supply and do not affect system operation or the loads connected.

RESULT

To realise protection against electric shock in accordance with the standards in low voltage DC systems (LV DC systems), IT systems with insulation monitoring devices in accordance with DIN EN 61557-8 (VDE 0413-8):2007-12 are the first choice.

Particularly in applications in which the failure of the power supply on the occurrence of the first fault must be avoided, e.g. computer centres, railway power supply systems etc., the IT system offers the clear advantage of higher availability. Also high protection against electric shock is ensured by very low touch voltages in the event of the first fault.

Harun Öndül, Sales Manager Aktif Mühendislik

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