A simple scenario: In the staircase of a large residential building, the building management replaces the incandescent lamps with LED retrofit versions. A specialist measures and verifies the prescribed brightness. The potential for savings is promising and in addition to the more favourable energy consumption, the long service life should reduce maintenance costs. But suddenly the staircase light timer switch fails. An examination shows scorched or fused contacts the devices were obviously overloaded although the rated output of the installation was reduced significantly.
How can an LED lamp that only has a few watts of rated output destroy a switching contact that has been manufactured for a multiple thereof? Upon closer inspection, the answer is found in the switching currents: In incandescent lamps the typical switching currents of the cold spiral coil causes a tenfold of the respective rated current. In the case of LED lamps and energy-saving lamps with their capacitive characteristics, one finds switching current pulses in the µs range that could be a 1000-fold and more than the rated current. A measurement in our test laboratory authorised by the VDE has shown, that in a most unfavourable case, a switching current of 19 A from a 1.8 W LED lamp, 1706 times the rated current!
At this point, we also inform you about problems with the switch-off pre-warning (flashing twice or similar according to DIN 18015-2): The flashing is not reliably visible, as the switching-off is buffered by the electronic ballasts or capacitors of the LEDs. Switching like this can also impair the service life of the device.
Two contacts for all switching cases:
High currents require special contacts. Alongside silver tin oxide (AgSnO2), Theben uses a combination of two contacts closing one after the other: tungsten pre-contacts. The leading contact consists of high-ohmic and very resistant tungsten. This captures the switch-on current and limits it at the same time. In this way, the low-ohmic main contact remains unimpaired by switching peaks. Theben uses these relays in the digital time switches TR 609 top2 S and SELEKTA 174 top3, as well as in the performance motion detectors theLuxa P and the presence detector theRonda P.
Switching at precisely the right time:
Zero point switching.
Switching devices that are designed for the C load generally deal with the switching currents better. Theben uses a particularly efficient solution; so-called zero-cross switching. It calculates the zero crossing of the sinus curve of the alternating voltage. At this moment, the inrush current is at its minimum during switching. This protects the relay contact and extends its service life, even at nominally high switching loads. Almost all of the devices of the top2 and top3 series are equipped along with the motion detector theLuxa S and the presence detector PlanoCentro.
Environmentally aware and switching safely:
For a long time, cadmium oxide was thought to have been the ideal contact material for high switching currents. It is now prohibited according to the RoHS guideline – where there are exceptions for electrical switching contacts. Nevertheless, Theben had already shifted to environmentally friendly AgSnO2 materials at the turn of the millennium. These offer comparably good, in some cases even better contact and switching properties, such as higher burn-off resistance, increased resistance against electric wear and a significantly reduced inclination to material migration during direct current operation.