In electrical installations and on electrical equipment, the insulation resistance is the determining variable with respect to personal safety, equipment safety and fire safety. If insulation resistance is insufficient; protection against direct and indirect contact cannot be ensured, fault currents will lead to service interruptions, short-circuit and earth fault currents can cause fires and explosions, and damage equipment beyond repair, the lives of individuals and farm animals are put at risks.

Electrical safety for power supplies

A reliable EMC-friendly electrical installation ensuring the highest degree of safety for persons and equipment is of primary importance for all the persons responsible for industrial systems, hospitals and office buildings. The safety concept for electrical installations therefore must

• Ensure the safety of both personnel and equipment
• İmprove the operating continuity
• Contribute to the efficiency of the electrical system.

In order to achieve this, power supply systems and the associated protective and monitoring equipment should be carefully selected to allow

• Comprehensive protection of people and the electrical system against hazards caused by electric current
• Fast indication and response to critical operating conditions in electrical installations
• the reduction of maintenance costs and expenses due to failures
• Equipment data management according to customer requirements.

How to choose the right power supply system

The following aspects should be considered when selecting the appropriate power supply system and the associated protective measures in accordance with IEC 60364-4-41:

• Likelihood of insulation faults;
• Basic protection and fault protection;
• Continuity of power supply;
• Technical and economic possibilities;
• Values that proved to be successful in practice.

A detailed description of the power supply systems with earth connection is given in IEC 60364-3. The three main types of distribution systems are the TN system, the TT system and the IT system. In TN systems, one point is directly earthed; the exposed conductive parts of the installation are connected to that point by protective conductors. In TT systems, also one point is directly earthed; the exposed conductive parts of the installation are connected to earth electrode which is electrically independent of the earth electrodes of the system. In IT systems (be careful not to mix up the term IT systems which means isolated systems with information technology systems which are also called IT systems) all active parts either are insulated from earth or one point is connected to earth through a sufficiently high impedance. The exposed conductive parts of the electrical installation either are earthed individually or collectively.

In order to guarantee sufficient protection of persons and property, always the coordination of the earth connection and the characteristics of protective conductors and the type of distribution sys- tem is required. The permissible protective measures are specified in IEC 60364-4-41. In earthed systems (TN, TT systems), usually residual current devices (RCD) and overcurrent protective device are used, whereas in IT systems insulation monitoring devices are used almost everywhere.

On principle, in electrical installations, the protection of persons must be high on the priority list. But more and more importance also is attached to the availability of electrical energy. The following aspects should be considered when the electrical installation is disconnected due to an insulation fault:

Persons are at risk, for example, due to

• A sudden failure of the lighting system;
• Disconnection of equipment which is intended to guarantee operating safety.

Risk of economic loss due to

• High costs incurred due to interruption to production, especially in areas where restarting is tedious and expensive;
• Data loss
• Increased expenses due to disturbances and destruction in installations or equipment.

Moreover, sensitive loads can be disturbed by high fault currents. Disconnection may cause overvoltages and/ or electromagnetic influences which may lead to functional disturbances or even to damage to sensitive devices. Therefore, with regard to the availability of the power supply, in particular, the behavior of power supply systems in case of a first fault is of importance.

Comparison between earthed and unearthed power supply systems

IT systems (unearthed systems) either are supplied from an isolating transformer or an independent voltage source, such as a battery or a generator. The peculiarity of these systems is that no active conductor is directly connected to earth. In the event of a short- circuit to exposed conductive parts or to earth, only a small fault current will flow, owing to the lack of a return path, dependent on the value of the insulation resistances RF and the capacitance Ce of the conductors to earth. The difference between the earthed system and the unearthed system in the case of an insulation fault becomes clear by comparing the figures 2 and 3.

On the occurrence of a direct earth fault RF, an earth leakage cur- rent Id equal to the short circuit current IK will flow in earthed systems (TN/TT systems). The series-connected fuse blows, and the power supply is cut off (figure 3). By way of contrast, the unearthed system (IT system) (figure 2). It is obvious that in the event of an insulation fault 0 < RF < ∞ only a limited capacitive current flows through the line to earth capacitances. The series-connected fuse will not react in this case, which means that the power supply is guaranteed in case of a single fault to earth.

As far as the safety of the power supply is concerned, the IT system offers the most advantages. That is the reason why it is used in a lot of areas where a maximum of reliability and safety of the power supply is required. These are for example control circuits according to EN 60204: 1998-11, power supply systems for medically used rooms according to IEC 60364-7-710, mobile generators according to IEC 60364-7-717, IEC 60364-5-551 and other application fields. Apart from this, IT systems with insulation monitoring are increasingly used in industrial systems with variable-speed drives, in complex production systems or electronic data processing systems, because an unexpected failure of the power supply may cause considerable costs.

When operating an IT system, it has to be considered that „a first fault“ changes an initially unearthed system (IT system) into an earthed system (TN or TT system) and that a further fault may lead to the tripping of circuit breakers and hence lead to disconnection. Experience has shown that the single-pole fault (first fault) is the most likely type of fault to occur (> 90%) and the risk of hazards due to a second fault are regarded to be very unlikely. This fact is taken into account in the standard IEC 60364-4-41 where it is re- commended that a first insulation fault should be eliminated with the shortest practical delay

Result

Nowadays, the increasing complexity of industrial systems places extremely high demands on the reliability of power supply, where even a short power failure may be expensive due to production stoppage and malady function. With the application of IT systems a tool is available that effectively helps to solve this problem

References

Harun Öndül- Sales Manager – Aktif Mühendislik