Oddity with fluorescent tube.

[Nick]

About a month ago I replaced an ageing double 6ft fitting with new. Fitted to a ceiling about 3.5m from ground. It is one of a bank of 4 similar fittings. Today noticed that one tube was not lighting. On further inspection it is glowing dimly with a bright spot about 12" from one end. Armed with new tube, clambered up to have a look. Blow me down, there is a hole in the 1" glass tube. The bright spot surrounded the hole. A hole big enough to stick the end of my little finger into. Of course when I twisted tube to remove the thing just shattered. Never seen the like before and I have replaced hundreds of them over the years. Fit new tube and it works perfectly. Ho hum.

[Andrew Gabriel]

Getting a hole at the end where the filament supports are is not unheard of with older electronic control gear. When the filament coating is all sputtered off, the tube is dead. However, some electronic control gear will provide sufficient extra voltage to run the tube as a cold cathode tube, which causes the remaining filament to burn away and then the support wires take over as the electrodes, and run hot enough to melt the glass if they're near to the glass wall. Usually the tube breaks and drops out of the fitting, but melted holes and even setting light to something combustable nearby have happened in this case. (Cold cathode tubes generate significant extra power in the "cathode fall" area just around electrodes.)

Modern electronic control gear detects the tube filaments wearing out and stops trying to run the tube when this happens.

[Brian Gaff]

Bat quality control by the sound of it. A flaw in the glass?

Brian

[Nick]