Role of zener diodes accompanying High power UV Diodes
Series LED strings have the advantages of assuring even current and keeping total currents low .String length is limited by maximum acceptable voltage .     For example you could run 12 leds in series that drop say 3 volts each giving a typical string voltage of 36 .This is safe and allows a simple current limited       boost converter to have cheap parts and easy PCB clearances .LEDs can and do fail .If a LED shorts then the sign still works with the boost converter putting out the same current albeit at 33V .The zener prevents total string failure when there is an open circuit LED failure .The Zener is selected to be just above the led voltage and rated to carry the string current .I  specified 5V1 zeners on the string drivers for signs that I did in a previous life. This meant that any color LED would be OK        .It is said that some LEDs are static sensitive and do not like reverse voltage .I used dummy loads to represent the LED strings when I developed the string drivers so I do not have first hand experience with this static issue .The Zener is a good idea for protection even if you are not running series strings like I did .