show one of the following aspects: Main signals Fail safe If one of the green lights in signal 22 fails, the indication becomes the lower speed signal 21 – this is fail-safe. Other nearby countries reverse the role of the single green aspect and double green aspect. Distant signals Wrong-side failure If the yellow light in signal 24 fails, the signal displays a higher speed indication, which would be a wrong-side failure. To prevent this, a current transformer in the lamp circuit monitors the current through the yellow lamp. If the yellow lamp fails, a relay will also switch off the green light and the signal becomes totally dark, which is then treated as "expect stop." A capacitor in the relay circuit ensures that the relay operation is a couple of seconds delayed, to prevent the relay operating for every blink. Hence, if the yellow lamp does suddenly fail, the green light will blink alone 1-2 times before it is switched off by the relay. Note: The animated picture of signal 24 is wrongly giving the impression that the yellow light is shown before the green one. This is not correct: The yellow and green lights are always switched on and off simultaneously, for every blink/flash. Warning systems Norway uses the Ericsson ATP warning system, also used on Perth's suburban railway network. References External links Norwegian signalling rulebook