This question was answered by Jonathan Bell, commercial director of Liteplan Limited.
Originally, green charge LEDs were not available and only red LEDs could be used to show that the batteries of an emergency lighting fixture were taking charge.
Following the introduction of green LEDs, the standards were updated as it was felt that ‘green for safe’ was more appropriate. During the transition between red and green charge LEDs, there was a period when both types were acceptable. This occurred in the period between the introduction of the new standard and the date of withdrawal (DOW) of the old standard. This date has now passed and the current requirements under 60598-2-22 are for green to indicate a healthy emergency lighting luminaire.
There are now many self-test and DALI self-test systems, where two LED colours are required. In these systems, green is considered to be safe and red is considered to indicate a fault. As an example, faults could be indicated in the following way:
Luminaires being used in this way will usually feature a bi-colour charge LED which show green for healthy and red for fault indication.
To conclude, all emergency lighting installations should now feature a green charge LED to show a healthy condition.