End of Lamp Life

When the filament fails in an ordinary incandescent lamp, the high temperatures and high voltage present may be sufficient to allow an electric arc to jump the gap between the broken pieces of filament. This quickly develops into a high current discharge, illustrated in Figure I20. Very high currents in excess of 200A can be passed, creating a dangerous situation that can have destructive consequences before a fuse blows somewhere in the circuit. The offending bulb may well explode, and damage to the lamp holder is possible. House wiring can also be damaged and it may start a fire.

House wiring fuses are heavy duty devices which do not act quickly enough. To ensure consumer safety, it has therefore been customary to build fast-acting fuses into every gas-filled GLS lamp since the 1930s. However price pressures have resulted in an increasing number of lamps being found on the market with poor or inadequate fusing.

The efficiency of a fuse depends in part on its length, but often there is not enough space to fit a fuse of the required dimensions inside a GLS lamp. For this reason two short fuses are generally used. The fuses are located inside the glass stem, and can be seen in Figure I21. Figure I22 illustrates an inferior lamp having only one fuse, and the lamp in Figure I23 is a dangerous design with no fusing at all. Figure I24 illustrates a lamp having the alternative Ballotini type of fuse.

When the fuses fail, a secondary arc can form between the broken fuse wires and the metal shell of the cap. The heat it produces causes the trapped volume of air inside the cap to expand, elevating its pressure to the kind of levels found in a car tyre. If the quality of the capping cement has deteriorated over life, the glass part of the bulb will be forcibly ejected like a bullet, leaving its metal cap hanging in the luminaire. It is accompanied by a shower of sparks, with the possibility for personal injury or fire. In good quality lamps, the bond between the bulb and its cap is strong enough to avoid this kind of violent failure. However the evidence of arcing can be witnessed as small holes melted in the side of the cap, known as cap puncture (Fig. I25). Similar holes occur in the lampholder, sometimes firmly welding the bulb into its socket. Invariably the whole lighting circuit will be tripped due to a failed house-wiring fuse.

Double fusing alone is not enough to protect the consumer, if no measures are taken to prevent this dangerous arcing inside the cap. For instance the lamp shown in Figure I26a is double fused, but still represents a dangerous design. When the fuses fail, any secondary arcing will go for the exposed metal area inside the cap shell. Also the shape of the glass neck mould is poor, indicated by a red line, so the bulb could easily be blown out of its cap. To prevent this arcing, one of the three methods below is required, to provide some kind of insulation between the fuses and the metal cap shell.

Also known as glass-lined caps, the black glass insulator between the contact terminals is extended right up the side walls of the cap, until it meets with the cement. It is clear from Figure I26b that there is no bare metal at all inside this cap. Therefore the secondary arcing cannot occur, and this lamp will fail safely at end of life. Also note the excellent contour lines of the neck moulding of the glass bulb. The cement can easily grip this indented style bulb, and in the rare event that arcing does still occur, the chances of the bulb being ejected from its cap are minimised.

High vitrite caps are quite expensive, and a cheaper option is possible simply by filling the entire space inside the cap with a special foaming organic compound, shown in Figure I26c. When the fuses fail the arc is quickly quenched by the surrounding material, and any destructive arcing to the cap shell is prevented. However these organic foam materials gradually deteriorate at high temperatures. For long-life or high temperature service lamps, the foam cannot be relied upon to work at end of life, and then one of the alternative insulation methods must be employed.

This is the safest and most reliable fuse type - but also the most expensive. The fuse wire is encapsulated in a small glass tube filled with tiny glass beads called Ballotini, named after Potters Ballotini, the Italian producer. Should a secondary arc form between the broken pieces of fuse wire, its heat will melt the glass beads to form an electrically insulating barrier which contains and rapidly quenches the arc. It is so efficient that only one fuse is required, but on account of their higher price they are generally employed only in the more expensive special lamps, or decorative lamps which are too small to be able to accommodate two sufficiently long bare fuses (e.g. candle and globe lamps).