29 Bilthoven, long cable routes and excessive circuit resistance

A new car park was built in Bilthoven near the end of the central platform. The power supply of the car park was connected to the platform. The street cabinet with the switch board was on the other side of the central platform. In the event of a short circuit (or earth fault) in the last lampposts, the current will remain too low due to the long cable length and will not trip the circuit breaker or fuse (circuit resistance too high).

This means the last lamppost may be energised without tripping the fuse. To prevent this dangerous situation, the circuit impedance (resistance) is tested annually in accordance with management standard NEN3140.

The circuit impedance/resistance in the most common situations on platforms may not exceed 2.4 Ohm (for an installation circuit breaker with 16 ampere B-characteristics).

In Bilthoven, we shortened the cable lengths by installing an additional street cabinet close to the car park at the end of the central platform. A thick cable was installed to power that cabinet. The cabinet houses the installation circuit breakers for the car park and the cable routes are not too long anymore. The excavation work in the central platform made this an expensive adjustment (around € 30,000).

At some small stations where no cables were replaced, the final NEN3140 inspection showed that the circuit impedance was slightly greater than 2.4 ohm after the LED replacement. In consultation with the inspection authority, 12 amp instead of 16 amp fuses were used to ensure that the fuse would still trip in the event of a short-circuit at the last lamppost. This is possible because the power consumption of the LED lighting is much lower than that of the old lighting.

To prevent short circuits and earth faults in the lampposts, we also replaced the open terminal strips in all lampposts with closed, water resistant, touch-safe lamppost boxes with a local fuse (e.g. by Eleq or Sogexi, see adjacent photo).

Tips and conclusions:

Excessive circuit impedance can be solved with an additional intermediate switch board (shorten cable routes).

– Excessive circuit impedance can also be solved with a lower fuse (if possible in terms of total current consumption of the lighting).

– Prevent short circuits in lampposts with touch-safe lamppost boxes and refrain from using open terminal strips.