nic ballasts generally provide starting voltages higher than their electromagnetic counterparts. You can also find their open circuit voltages on the ballast's label.)
To measure the filament voltage on a single lamp unit, read voltage between Red-Red and Blue-Blue leads. For two lamp units, read voltage between Red-Red, Blue-Blue, and Yellow-Yellow leads. For all 4-ft and longer RS lamps, the filament voltage should be between 3.5V and 4.5V. Sometimes with a two-lamp, RS series ballast only one lamp will light to full brilliance. Refer to Fig 2 again (in the original article). If the lamp between the Red and Yellow leads is lit and the other lamp is out, look for a pinched Yellow lead. If the lamp between the Red and Yellow leads does not light and the other one does, the cause is probably a short.
Why is good contact of lamp pins with their sockets so important? When an RS lamp circuit first energizes, the low-voltage winding of the ballast provides the cathode heating. Shortly afterward, the open-circuit voltage is applied across the lamp, or lamps, to begin arc conduction.
If only one cathode heats up, either the lamp will fail to start or it will be slow in starting. Heavy premature end-darkening, usually at only one end, usually indicates one cathode is not getting proper heat. Therefore, the heater circuit is incomplete. This is not the same as gray or brownish bands occurring about 2 in. from the lamp base, with the edge of each band on the side nearer to the base being sharper. These bands occur when the cathode coating wears out. While they may detract from the appearance of the lamp, moderately dark bands have no significance concerning either life or performance of the lamp. However, you should replace failed RS lamps promptly to avoid any possible ballast damage.
The lack of cathode heating voltage can come from a poor connection, either between the lamp pins and lampholder contacts, or between the ballast leads and lampholder terminals. Improper lampholder spacing in the lengthwise direction, with the lamp end held either too tightly or too loosely, can also cause poor contact at the lamp pins. Measure the voltage at the socket terminals to determine if adequate heater voltage is present.
Sometimes, in high-humidity conditions, RS lamps may start slowly or not at all, even though the cathodes are heated properly. This can result from dirt on the lamps, which makes the silicon coating ineffective. It can also be due to a poor silicon coating. If a new installation experiences random starting under high-humidity conditions, low supply voltage or poor silicon coating on the lamps is generally the cause. If you see the same poor starting in high-humidity conditions on an older installation, the cause is usually an accumulation of dirt on the lamps. In this case, just wash the lamps.
Why is the RS lamp surface condition so important? For the lamp to start within the voltage range of the ballast, it is necessary to excite the gas fill inside the lamp by means of an external voltage; thus the excitation ionizes the gas fill. The capacitance between the lamp and reflector or fixture channel assists this external excitation. Of course, you must bond (connect) the metal fixture to the equipment grounding circuit, and operate the ballast from a grounded electrical system. Any dirt or moisture coating on the lamp can reduce this capacitance.
If you experience random starting of RS lamps, check the fixture for proper grounding. As previously stated, for completely reliable starting in RS circuits, you must have a starting aid, defined as an electrically grounded metal strip at least 1 in. wide extending the full length of the lamp. The lamp should be within half in. of the grounded metal strip for 40W lamps and smaller (three-fourths in. for T8 lamps) and 1 in. for higher output lamps.
The modified RS ballast, a version of the RS ballast, reduces power consumption by a few watts, and operates the lamps similar to the standard RS ballast. The only difference is: You reduce or eliminate filament heating, after the lamps i