I recently found this old DC distribution board that was once used by finnish state owned telecom company called Tele. It was their own design and it was fitted with this fuse supervisory unit that would indicate alarm when fuse, which has load connected to it, blowed out. It was also manufactured by Tele. I was just wondering if someone could be so kind and explain me how this supervisory unit works. I opened the unit and drew this draft schematics from it. This is -48v dc system so the fuses are on - side and 0v is connected to ground.

http://www.whitenightcape.com/kuv/scan0211.jpg

That is interesting. The only fuse supervisory devices I've seen are certain types of fuseholders for car fuses that has a LED in parallel to the fuse socket so if the fuse blows the LED lights.

That is interesting. The only fuse supervisory devices I've seen are certain types of fuseholders for car fuses that has a LED in parallel to the fuse socket so if the fuse blows the LED lights.

Basically the same idea, only they could use a optical isolater, which is just an LED and phototransistor sealed in black epoxy to keep external light out, and when you turn on the led the phototransistor will conduct power, and wire the output up to your beeper or what ever and if your playing with DC, the optical isolater also comes in an SCR format which will latch on activation so you have to manually reset it if needed...

Some telecom fuses had a spring loaded contact that was held against an auxiliary contact by the fuse wire.  The auxiliary contact going open circuit meant the fuse had blown.

This system detects if there is load connected behind fuse. Auxilary contacts and such require the fuse to be on even though there is no any load to not to make alarm. I've seen some such systems in use.