The routine for replacing incandescent lamps has been to replace individual lamps as they burn out. This strategy maximizes the use of each lamp but has some short-comings as well.

Maintenance costs, using a replace-on-burnout strategy can be rather high. Much of the time required to replace a lamp is involved in getting the lamp and other equipment such as tools and ladders on site. A maintenance person can replace several lamps in about the same time that it takes to replace only one lamp. As a group of incandescent lamps reach their rated life they will begin to burn out one by one. With a replace on burnout strategy, a maintenance person can be plagued by numerous service calls, taking time away from other duties. Also, incandescent lumen output declines over the life of a lamp. As a lamp gets closer to the end of its rated life the level of light output may be lower than the space requires.

Group relamping is another name for the systematic replacement of lamps in a lighting installation before they burn out. This is possible and even recommended because incandescent lamps generally have uniform service life. It is scientifically practical to predict when most of the lamps will burn out, and to replace them shortly before that time.

Group relamping ends the old, expensive, haphazard spot relamping system and substitutes modern mass production methods featuring low cost-per-lamp costs. The savings in labor usually more than compensates for the shorter time the lamps are left in service. A group relamping program results in reduced maintenance costs, better lighting, and fewer work interruptions.

Many installations can be systematically group relamped. The time schedule varies for each installation depending on the amount of spot lamp replacement desired and the types of lamps in service. By planning and scheduling group relamping on a calendar basis, maintenance time and costs will be known and can be then budgeted in advance.

Inoperative Lamps

When an unused lamp fails to light it is usually because of a broken filament. When either of the filaments in a three-way lamp do not function, the reason may be a broken filament. However, more often the failure is because the lamp doesn't fit firmly in its socket because the center socket contact has lost its resiliency and has become permanently depressed.

Shock & Vibration

Metals become relatively soft and pliable when heated to incandescence. The tungsten metal used in incandescent lamps is no exception. It is obvious, therefore, that the tungsten coils in low-wattage or small-wire diameter lamps as well as other lamp types are liable to become stretched and distorted or break when the lamp is subjected to shock or vibration. This condition may be readily detected by examining the filament coils; for the lower-wattage lamps, a magnifying glass is often required.

Application of either a general lighting service or a vibration-type lamp to portable extension cord service subjects the lamp to shock and is sure to result in shorter life. For this and other applications where shock is involved, only rough-service lamps should be used.

Vibration is a serious threat to lamp life because it cannot always be readily observed. The life of a lamp is probably being shortened if any evidence of vibration is detected when a hand is placed on the lamp or its fixture. If the vibration is severe or bordering on shock, rough-service lamps should be used to increase the service life.

All other factors being equal, the greater the diameter of the filament wire, the better a lamp is able to withstand shock and vibration. For a given voltage, the wire diameter increases with an increase in wattage. For this reason, vibration type lamps are not made in wattages above 200, while 500 watts is the maximum for Rough Service lamps.

Excessive Operating Temperatures

Excessive operating temperatures will adversely affect a lamp's performance causing the glass to release moisture. This moisture attacks the filament, causes the bulb to soften and bulge. It can also cause the base to loosen. Initially, the bulb temperature may cause moisture release without softening the bulb. However, the moisture may bring on severe blackening, causing the bulb to absorb enough additional radiant heat to soften the glass.

Improper focusing of equipment, particularly fixtures using tubular bulb lamps, may cause blistering on the side of the bulb opposite the filament. This occurs because the image of the filament is focused on the bulb wall instead of on the filament itself.

Under normal conditions, a properly manufactured lamp will not develop excessive temperatures in a fixture as long as the lamp is of the recommended wattage and type for use in that fixture. When a lamp used in conventional equipment develops a blister, check for water cracks, improper focusing, manufacturing defects or mismatch of wattage or lamp type.

Loose Bases

Loose bases generally result from operating lamps at excessive temperatures causing the cement in the base to "powder" and disintegrate. Lamp bases are also likely to oxidize or discolor due to the excessive heat. It is also possible that the solder may show signs of softening or melting. Prolonged storage of lamps in high humidity areas is another cause of loose bases.

Rattlers

Lamps will occasionally have a slight rattle when shaken. This happens when fragments of the basing cement loosen and lodge within the outer stem tube. These fragments do not adversely affect lamp performance, and if the lamps are otherwise satisfactory, they provide perfectly adequate service.

Leakers

If, for any reason, air leaks into a bulb, the lamp will fail. Failure usually occurs immediately, but in some cases, the lamp may last for a considerable period before the filament finally disintegrates.

The air contacts the heated filament, combines with the tungsten and produces a tungsten oxide vapor that is deposited on the inner surface of the bulb. This process continues until the filament wire disintegrates or breaks. The color of the deposit may be yellowish white, bluish white, bluish black or very dense black depending upon the nature of the air leak.

Leaks are often caused by damage to lamps in transit or while they are being handled by the user. Water or other liquids, contacting the hot bulb of a gas-filled lamp will also cause the glass to crack and sometimes result in failure. Vacuum type lamps are not affected in the same way. Glass cracks frequently have a rough crescent shape not more than 3/16" across. This can make them very difficult to detect. The resultant leak is sometimes the "slow" type which causes the bluish-black deposit. Lamps with this appearance should always be carefully examined for bulb cracks. Remember that "water-proof" fixtures often leak. When water cracks a bulb there may be no evidence of the leak as long as the lamp is burning. Once the current is turned off and the gas in the bulb cools, a partial vacuum is created, pulling air in through the crack. When the lamp is again lighted, it may fail violently, burn out immediately, or, if the crack is minute, continue to burn with a resulting bluish-black discoloration.

In contrast to water cracks, those resulting from the bulb's impact with another object may be "star" cracks; cracks radiating in all directions from the point of impact or occasionally long, irregular cracks.

Improper Burning Position

Operating filament lamps in the wrong position may cause the lamp to fail immediately or fail after an abbreviated life. This type of failure can sometimes be detected by the presence of small globules of molten metal adhering to the bulb's inner surface beneath the filament.

For example, if a base-down burning lamp is operated base up, the globules may be found in the bowl area of the bulb. Conversely, the globules would adhere to the base area of the bulb when a base-up burning lamp is burned base down.

If a lamp has burned for an appreciable period, the normal bulb blackening can be used to determine the position the lamp was used. A blackened deposit found in the bowl of a base-up burning lamp indicates that the lamp was burned base down.

If a base-down burning lamp is operated base up and lasts for an appreciable period, the evidence of improper use may be found in a softened glass button, or stem tube, or both. In addition, the base may loosen from the bulb.

Concentrated filament tubular-bulb lamps, designed only for base-up or base-down burning, are liable to develop bulb blisters if burned horizontally.

Operation Of Lamps Near Neon Tubing

When an incandescent lamp, especially 6 or 10 watt, is used in combination with neon tubing, the electromagnetic field around the high-voltage tubing may cause the incandescent lamp's filament to vibrate at high frequency and finally to break. This can be prevented by using a grounded metal shield located between the tubing and the lamps.

Over-Voltage Or Over-Current Operation

Many building operators are not aware of the negative effect on lamp life created by operating a lamp at a voltage or current in excess of the lamp's labeled design. A 120-volt lamp operated on a 125-volt circuit will suffer a 40% loss in life.

Examining burnt-out lamps will not indicate this condition. Spot checks with a voltmeter or ammeter may be enough to identify this problem. A better, more reliable approach is to use a recording meter over a period of several days. The local utility will usually cooperate in checking circuit voltages on its lines. The key point is that the lamp's rating corresponds with actual circuit operating conditions.