The increasing health and safety requirements on machines and installations require comprehensive measures to protect man and machine against possible hazards. For instance, insulation faults in a controller must not result in hazardous movements of a machine. If there is a high availability requirement, it is also appropriate to consider this aspect correspondingly on the selection of the type of system and the protective measures. Both the related normative and practical requirements are explained in the following article.

As far as machines, electrical systems and people are concerned, the IT System offers the greatest benefits. IT systems are increasingly being used in many different energy systems.

Safety Objectives

An insulation fault can, independent of the cause, represent a life-threatening hazard for personnel and a hazard for the integrity of assets and the availability of electrical power. For this reason the following aspects in particular should be taken into account on the selection of the type of system (TN, TT and IT systems) and the related protective measures:

• Probability of insulation faults
• Necessary basic protection and fault protection
• Continuity of the supply of electrical power
• Technical and commercial aspects
• Experience available on secondary effects (fire, downtime costs etc.)

On electrical machines these aspects relate to the control and auxiliary circuits, as well as the main circuits. While in the main circuits the issue is more the protection against electric shock, the protection of personnel as well as the prevention of fire, reliability plays a significant role on control and auxiliary circuits, in particular those with voltages below the permissible touch voltage of AC 50 V/DC 120 V.

Auxiliary and control circuits

In DIN VDE 0100-557 (VDE 0100-557) a differentiation is made between control circuits and auxiliary circuits. Accordingly, this standard is always to be applied if there is no other independent standard. As such DIN VDE 0100-557 (VDE 0100-557) is not to be applied, for example, to

• Electrical equipment of machines:
  DIN EN 60204 (VDE 0113) series of standards;
• Installations and equipment for the transmission and processing of information for the public electricity supply:
  DIN VDE 0800-1 (VDE 0800-1) and DIN VDE 0804 (VDE 0804);
• Electrical equipment for furnaces:
  DIN EN 50156-1 (VDE 0116-1);
• Auxiliary equipment and control systems in high power installations exceeding rated AC voltages of 1 kV:
  DIN VDE 0101 (VDE 0101);
• Alarm systems for fire, intrusion and hold-up
  DIN VDE 0833 (VDE 0833) series of standards

In DIN VDE 0100-557 (VDE 0100-557):2007-06 and DIN EN 60204-1 (VDE 0113-1):2007-06 both earthed and unearthed control circuits are addressed. Therefore the key differences are given in the following.

Earthed or unearthed control circuits?

On the occurrence of an insulation fault RF a fault current IF, flows in the earthed system (TN system); in the event of a very low-resistance insulation fault this current corresponds to the short circuit current IK. This situation then triggers the overcurrent protective device and the supply of power is interrupted. Residual current devices (RCD) also shut down.

By contrast, in the IT system the active conductors are not connected to earth and such a small fault current flows on the first insulation fault that disconnection is not necessary (DIN VDE 0100- 410 paragraph 411.6).

To prevent the triggering of the overcurrent protective device on a second fault on one of the other conductors, the first fault is detected and signaled by the insulation monitoring device (IMD). The system operator is therefore informed and can undertake the maintenance measures at a suitable time. The other key advantage: the supply of power is maintained and operation is not interrupted. Particularly in industrial installations that require a high level of continuity (e.g. automotive, glass production etc.), this is a significant advantage and avoids high costs associated with failures.

RESULT

The usage of earthed control circuits is not stipulated for the design of control circuits according to the standards. The usage of unearthed control circuits with insulation monitoring is often the better choice, particularly in relation to the aspects of reliability, prevention of failures and cost reduction. Systems for insulation fault location, for instance according to DIN EN 61557-9 (VDE 0413-9), can be used in addition to indicate the location of the insulation fault while the installation is in operation. Which measures are taken at the end of the day is to be determined as part of a risk analysis.

REFERENCES

• Wolfgang Hofheinz:
  Schutztechnimit Isolationsuberwachung, 3. Aufl age, VDE-Schriftenreihe Band 114, VDE Verlag Berlin
• DIN EN 61557-8 (VDE 0413-8):2014-10
  Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. – Equipment for testing,
• DIN VDE 0100-410 (VDE 0100-410):2007-06
  Low-voltage electrical installations – Part 4-41: Protection for safety – Protection against electric shock

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