XPost: uk.d-i-y   
   From: '''newspam'''@nonad.co.uk   
      
   On 06/10/2021 18:43, gareth evans wrote:   
   > I have been called uppon by SWMBO to repair her Pifco Xmas   
   > tree lights which she purchased over 5 years ago.   
   > (Yes, we have plenty of modern LED versions to   
   > replace them)   
   >   
   > 1. The instructions say that if one bulb of the 20 fails,   
   > then the res will remain illuminated. What's the technology   
   > here because they're all wired in series?   
      
   A special glass that has a low breakdown voltage in the "normal" bulbs.   
   If a filament fails then the glass finds itself with the full 240v ac   
   across it and quickly becomes conducting as a dead short. The remaining   
   N-1 bulbs then remain lit but with N/(N-1) volts across each of them.   
   They burn progressively brighter with each failed bulb in the string.   
      
   Things get hairy when 3 or more bulbs fail which is when the fuse bulb   
   should prevent a cascade failure where they are all quickly destroyed.   
   >   
   > 2. Is the Fuse Bulb a capacitive unit, because I get a momentary   
   > flick of the ohmmeter?   
      
   No. It is has a slightly higher rating and behaves like fuse in series.   
   It should blow before things get out of hand but like all safety devices   
   can sometimes save itself by allowing everything else to fail!   
      
   Test each bulb in turn on resistance.  Dead ones will be almost zero   
   ohms. Spares are usually available from eBay.   
      
   Mains strings of lights are these days considered somewhat dangerous.   
      
   --   
   Regards,   
   Martin Brown   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)