The new office block of the HUK Coburg Insurance Group, completed in 1998 in the town of Coburg in Upper Franconia, is reached along a path lined with trees and stretches of water. What is particularly striking about the building is its open, generous and bright architecture. This is particularly true of the fully-glazed foyer. It links two wings of the building via glazed bridges, giving the impression that the people on them are floating in the room. This impression is reinforced by clever lighting, suggesting that there is a great deal of technology hidden behind such a facade. A 700 kW block-type power plant is available for power supply.

The electrical system was planned by b.i.g. bechthold Ingenieurgesellschaft mbH, Karlsruhe, and installed by Cegelec AEG GmbH's Nuremberg branch. 80,000 installation spring cage terminal blocks from PHOENIX CONTACT were used in distribution boards, and another 120,000 in underfloor and ceiling jumpering terminal blocks. The surge voltage protection is also from PHOENIX CONTACT. The external lighting was equipped with arresters from the Trabtech coarse and medium protection, whilst office technology, such as phone, fax and computer, was equipped with 2,500 fine protection modules of type PRT-S/A 230/FM.

Such a plant requires good planning and clear execution. Complications during initial startup and later maintenance work are undesirable. This is why Cegelec AEG chose to supply each circuit via three individual modular terminal blocks.

For conductor cross sections up to 4 mm², installation modular terminal blocks with spring cage technology were chosen. For the main supply and distribution of cross-sections from 6 mm², the classic screw terminal blocks were chosen.

The demand for special installation terminal blocks is laid down in the DIN VDE 0108 standard. This requires that in the case of "Power installation and safety power supply in communal facilities" distributors be so designed that it is possible to measure the insulation resistance without disconnecting individual neutral conductors. This is valid up to a conductor cross-section of 10 mm² and requires so-called neutral disconnect terminal blocks. The construction of such a neutral disconnect terminal block should be characterized in particular by clear and reliable handling. It should be possible to replace each terminal block separately without obstruction from a neighboring terminal block or the neutral busbar.

Neutral isolation is required in the standard only up to 10 mm²; such terminal blocks are still used above this cross-section range. If they are used for the incoming supply, it is particularly important that the disconnect slide can be locked with a locking mechanism. Accidental release of the mechanism leads to the loads linked via busbar being connected to two phase conductors and thus at worst to 400 V. Damage, in particular to electronic devices, is the result. Another possibility of avoiding unintentional releasing is by using a pick-off terminal block directly on the busbar.

HUK Coburg's advertizing slogan is: "I'm in safe hands!" And this is certainly true of the system designed by Cegelec AEG. In order for this to remain so, the insulation resistances are regularly checked during routine maintenance work. In DIN VDE 0105, part 1, "Regulations for the operation of power intallations" the contents is roughly: "Power installations with nominal voltages up to 1,000 V must remain in a state of insulation that meets the minimum requirements in acc. with VDE 100 for the individual types of systems!" This implies a frequent volume of work that can be minimized by foresighted structuring of the distributor.

Insulation resistance is measured between each active conductor and earth. The earthed grounding wire of a system or the PEN conductor in a TN-C network can be considered as earth. In order to measure the insulation resistance, the neutral conductor must therefore be disconnected from earth and then connected to the phase conductor. This connection is required when electrical equipment is to be found in the circuit to be tested. It is better to first disconnect operating equipment and components that may be damaged during testing from the network.

Surge voltage protection in a circuit would impair measurement or make it impossible. It is therefore absolutely necessary to use pluggable protection modules. The additional advantage of this is that a protection module that has been damaged by surge voltage can easily be replaced. In underfloor installation, as with HUK Coburg, it is also advisable to use protection modules that have not only a visual, but also an acoustic fault display. Only then can it be guaranteed that a damaged module is recognized immediately, even if it is hidden by the cover of the casing or anything else.

As we can see, emphasis was placed not only on outward appearance, but also on technical details in the new HUK Coburg building. The result is certainly something to be proud of.