Improvements in lamp design and the development of new accessory components for HID sources must be understood to be applied properly in emergency lighting systems.

Are high-intensity-discharge (HID) lamps a viable option to other light sources in emergency lighting applications? Based on performance improvements, lamp modifications, and new variations, the answer seems to be YES. But be careful. Not every new device will work with every emergency system. Understanding how each device works will allow you to design functional emergency lighting systems using HID lamps.

HID lamp operation at power outages

All HID lamps are sensitive to power system fluctuations during normal operations because the arc extinguishes every half cycle of the 60 Hz supply frequency (120 times a sec). As such, there must be sufficient voltage in a pure sine wave to re-ignite the arc every half cycle. If this re-ignition does occur every half cycle, the lamp will not visibly extinguish.

Using HID light sources in an emergency lighting system can be done with a number of methods. However, you must take the time to select the best method for the project at hand because many technical and physical factors complicate the selection process. Most importantly, the lamping system you choose must be able to operate within the transfer time from normal to emergency power (and back to normal power). As such you must understand the method of supplying emergency power to the lighting circuit.

Emergency power supplies

Two types of emergency power supplies are normally used for emergency lighting: the engine generator and the battery-powered inverter.

Engine generator set. An engine generator set is reliable and capable of supplying emergency power for an extended time period. However, power can't be transferred instantaneously from normal to emergency because gensets generally require up to 10 sec to reach rated operating speed and deliver a consistent voltage or current sine wave. The inability to deliver instantaneous power usually rules out the use of standard HID lamps on a engine/generator system. However, equipment employing new technologies are now available that allow HID lamps to operate with generator-supplied emergency power. These equipment are discussed later, under the subhead "Options other than a UPS."

Battery-powered inverter. A battery-powered inverter system converts DC battery power to AC power, which is then supplied to the load through a ferroresonant transformer. An inverter system used to serve HID lamps must have three essential properties.

* Power supplied must be a pure sine wave form.

* Transfer time from normal to emergency must fall within the operating requirements of the lamp. (The transfer time to return to normal utility power and the method in which the transfer occurs are also critical if the HID lamp is to remain ON once normal power is restored.)

* Inverter must be compatible with HID ballast.

Typically, a battery inverter system is available with three different transfer time ratings. The transfer time for a standard unit is from 30 to 100 ms, which is acceptable for incandescent and fluorescent lamps, but not for HID lamps. The transfer time of a fast transfer system is from 4 to 8 ms. In some literature, manufacturers of fast transfer systems will claim that their units are compatible with some HID lamps. However, their guarantees usually state only that the maximum transfer time will not be exceeded. Lamp operation is not guaranteed; this must come from the lamp manufacturer.

While lamp manufacturers agree that standard HID lamps usually will not drop out within an 8 ms transfer time, they can not guarantee operation of their standard HID lamps with any power system that does not supply uninterruptible power supply. That's because the lamp manufacturers have no control over the point in the sine wave that the interruption will occur, regardless of how brief. (As mentioned earlier, the arc must re-ignite every half cycle to keep the lamp lit. If the interruption occurs at the peak of the sine wave, when the arc should be re-ignited, the lamp will extinguish and will have to cool down before it is able to restrike and warm-up.)

For emergency lighting, lamp operation must be guaranteed. Therefore, the third type of battery inverter system, a UPS meeting the three essential system properties mentioned above, is the only assured means of supplying emergency power to standard HID lamps. A UPS (the most expensive inverter system) uses a continuously operating inverter; thus, it has a zero transfer time. Not all UPS systems function properly with HID ballast loading. The UPS manufacturer should be made aware of the loading and should guarantee capability. Some complete systems have recently been introduced; one manufacturer supplies both the battery system and the ballasts. These systems typically limit the type and size of HID lamp that can be used.

Options other than a UPS

Today, various lamps and/or systems that don't rely on an expensive UPS system are available to serve HID lamps in emergency lighting circuits. Eight of the most important methods with their capabilities and limitations are shown in Table 1. These methods, which allow operation from engine/generators and non-UPS inverters, are described in more detail below.

Instant-restrike M-H lamp. An instant re-strike M-H lamp has a lumen output and [TABULAR DATA FOR TABLE 1 OMITTED] rated life similar to a standard M-H lamp. It's available in ratings from 175W to 1650W. As seen in Fig. 1, this specially constructed lamp has an external wire lead opposite the socket that connects to a pulse generator, which must be within 12 to 15 in. of the lamp. The pulse generator, in turn, is wired to an igniter, which is installed adjacent to the ballast. Because of the high voltage produced by the pulse generator, a fixture using an instant restrike lamp must have a safety-interlocked door enclosing the lamp compartment. The fixture also must be able to accommodate the added components.

Fig. 2 shows the initial lumen output level achieved by a 1000W hot-restrike M-H lamp for power interruptions lasting from 5 to 180 sec. Basically, full light output is attained immediately if the power interruption doesn't exceed 10 sec. For interruptions exceeding 10 sec. light output is restored at the lower levels shown.

The hot-restrike lamp is more than three times a s expensive as a standard M-H lamp of the same wattage. However, this added cost is insignificant considering an installation in a stadium, for example, where the loss of field lighting for even a few sec due to a power dip would be unacceptable at a televised professional sporting event.

Double arc lube HPS lamp. A double arc tube HPS lamp has two arc tubes connected in parallel. Only one arc tube is ignited when the lamp is first energized. If a power interruption extinguishes this arc, the second arc tube strikes instantly when the power is reestablished. About 3 to 5% of full light output is restored instantly, with 90% of full output occurring within 2 min, provided the power interruption does not exceed 35 sec. A warm-up time of 3 to 4 min is needed for 80% light output if the power interruption exceeds a 35 sec.

When a second interruption of power occurs (i.e. transfer back to normal power), the cooler of the two arc tubes will re-strike. If the second interruption occurs within 35 to 55 sec of the first interruption, there may be a 5- to 20-sec delay in the restrike.

When this lamp is operated under normal conditions, one arc tube may burn out, causing the other to ignite. To maintenance personnel, the lamp would appear to be functioning properly; however, should a power outage occur, the lamp would not be able to provide emergency lighting. For this reason, this lamp is not a reliable source of emergency lighting.

The candlepower distribution pattern for a fixture using this lamp type may vary slightly from the published distribution pattern, since neither arc tube is centered in the bulb. Understandably, the ballast and the fixture must be compatible with the lamp.

Special note: these HPS lamps may implode if broken and contact with water during lamp operation will cause breakage; thus, external protection is required.

This type of HPS lamp is available in wattage ratings from 70W to 1000W.

Hot restrike HPS starter. As shown in Fig. 3, a hot restrike starter can replace the starter module supplied with the standard HPS ballast and provide an exceptionally high voltage pulse to the arc tube when power is restored following a power interruption. This helps in overcoming the standard cool down time associated with HPS lamps. The restrike time depends on the duration of the power disruption, as shown in Table 2, and the ambient temperature of the fixture. The time it takes to transfer to emergency power is directly proportional to the length of time required to regain full light output.

The lumen output of a lamp served by a hot restrike starter will be less than the rated lumens given in the lamp catalog because of insertion losses. These losses, which can be up to 20%, vary depending on the starter manufacturer and the lamp wattage. There is no adverse effect on lamp life.

Both the lamp and the ballast must be compatible with the restrike starter, which must be mounted within 5 ft (or less) of the lamp for reliable operation. For medium-base lamps above 70W, an accessory device that senses the end of lamp life and takes the fixture off-line should be used. This accessory saves energy and prolongs the ignitor's life.

Auxiliary TH lamp. A separately wired and circuited tungsten-halogen (TH) lamp can be used in most HID fixtures, adding an incandescent lamp socket within the HID lamp compartment. The circuit feeding the TH lamp (which is a separate circuit) is energized when normal power is lost. This system must use a device to delay return to normal power, keeping the TH lamp operational while the HID lamp cools down to a temperature suitable for lamp restrike and then warms up to an acceptable lumen output level.

The TH lamp provides significantly less lumens than the HID lamp; as such, you should use the TH lamp's candela distribution in calculating emergency light level requirements.

TH lamp with instant transfer relay. With [TABULAR DATA FOR TABLE 2 OMITTED] this method, the TH lamp begins operation after power is interrupted and it extinguishes as soon as the HID lamp is able to restrike. This procedure does not guarantee that the HID lamp has reached a lumen output level acceptable for emergency lighting requirements. As noted above, most HID lamps produce negligible lumen output immediately after restriking. Cautionary note: fixture manufacturers often refer to this system as their standard optional emergency relay system.

Any HID light source can use this system and no additional conduit, wiring, or circuits are needed. The fixture manufacturer determines the maximum TH lamp rating. The fixture must allow sufficient heat dissipation so that the HID lamp can cool down while the TH lamp operates. Also, the TH lamp wattage cannot exceed the ballast's maximum auxiliary lamp size.

TH lamp with timed current sensing relay. This method works by sensing current flow on the secondary side of the ballast, then setting a timer that keeps the TH lamp on for a preset time after the arc has struck (2 min is standard) while the HID lamp warms up.

Any HID light source can use this system. The maximum TH lamp size must be determined by the fixture manufacturer. Also, the fixture must be capable of allowing sufficient heat dissipation so that the HID lamp can cool down while the TH lamp operates. In addition, the TH lamp wattage cannot exceed the ballast's maximum auxiliary lamp size.

The branch circuit should be sized for the HID lamp, ballast and TH lamp; however, the emergency light level requirements must be met using only the TH lamp.

Two options are available for normal start-up operation. A hot start option will not energize the TH lamp during a normal start-up condition. A delayed operate option will energize the TH lamp during a normal start-up, the same as it would during a power interruption.

TH lamp with voltage sensing relay. This method is available for MV and M-H lamps ranging from 75W to 400W. Here, the TH lamp is energized after power is interrupted and remains on until nominal operating voltage, corresponding to 60 to 70% of lamp lumen output, is sensed. This break point, when the TH lamp extinguishes, occurs before 90V nominal is reached on the secondary side of the ballast. [ILLUSTRATION FOR FIGURE 4 OMITTED].

(A normal 400W M-H lamp operates at 135V nominal on the secondary of the ballast. At ignition, the voltage may be 20V to 30V and will increase to 135V as the resistance in the lamp decreases.)

During an open circuit condition (i.e. as the HID lamp is cooling down prior to restrike), high voltages occur across the arc tube. Open circuit conditions with a 400W M-H lamp may reach 300V. Higher voltages would damage the device; thus, a 400W M-H lamps is the maximum permitted.

Maximum TH lamp rating must be determined by the fixture manufacturer and the fixture must allow sufficient heat dissipation. Again, emergency light level requirements must be met by the TH lamp.

TH lamp with voltage and current sensing relays. As shown in fig. 5, this method works the same as the voltage sensing relay described above; however, it's able to handle the higher open circuit voltage conditions that occur in M-H lamps above 400W.

Maximum TH lamp rating must be determined by the fixture manufacturer, and the fixture must allow sufficient heat dissipation. As usual, emergency light level requirements must be met by the TH lamp.

SUGGESTED READING

EC&M Articles:

"Practical Guide to HID Lighting-Part 1," April 1994 issue.

"Practical Guide to HID Lighting-Part 2, "July 1994 issue.

For copies, call 913-967-1801.

EC&M Books:

Practical Guide to Modern lighting Techniques To order, call 800-654-6776.

RELATED ARTICLE: TERMS TO KNOW

Ambient temperature: The temperature of the air or other medium in the vicinity of an operating lamp, luminaire, or ballast.

Arc discharge: An electric discharge characterized by high cathode current densities and low voltage drop at the cathode.

Ballast: A device that modifies incoming voltage and current to provide the circuit conditions necessary to start and operate electric-discharge lamps.

Candela: The international unit of luminous intensity. One candela is one lumen per steradian.

Efficacy: The ratio of the total luminous flux of a lamp to the total lamp power input. (Expressed in lumens per watt.)

High-intensity-discharge (HID) lamp: An electric discharge lamp in which the light producing arc is stabilized by wall temperature and the arc tube has a bulb wall loading in excess of 3W per [cm.sup.2].

Lamp: The term used in the electrical industry to indicate the light source itself; often what laymen refer to as the bulb.

Lumen: The quantity of light emitted from a light source.

RELATED ARTICLE: WHAT IS AN HID LAMP?

To generate visible light, an HID lamp creates a plasma arc within a transparent or translucent tube (arc tube) containing a gas vapor at a pressure that is slightly higher than atmospheric pressure. The arc tube itself, which is generally surrounded by an outer glass bulb, emits all or a large portion of the visible light generated.

Like a fluorescent lamp, an HID lamp requires a current limiting device: a ballast. HID ballasts typically consume 10 to 35% additional watts, whereas fluorescent ballasts typically consume 1 to 20% additional watts. Unlike a fluorescent lamp, an HID lamp produces full light output only when the arc tube reaches its operating pressure, which is generally several minutes after starting.

The HID lamp family consists of mercury vapor (MV), metal-halide (M-H), and high-pressure sodium (HPS) lamps.

MV light source. An MV lamp has an outer bulb that encloses an inner arc tube with electrodes at either end of the arc tube. The sealed arc tube (made of quartz glass) contains an argon gas fill and a small quantity of mercury. As the circuit is energized, a starting voltage pulse causes a very short arc to be struck across the starting electrode and the adjacent main electrode. As this shod arc develops heats, mercury atoms are ionized, causing a decrease in the resistance between the main electrodes so that, finally, electron flow across the main electrodes begins. Thus, the arc stream is the electrical circuit through the lamp.

The MV lamp takes about 5 to 7 min to warm up, depending on ambient temperature. If power to the lamp is lost, the total time to cool down and warm back up to full stabilized output is 3 to 6 min, depending on the wattage, ambient temperature, and operating condition of the lamp.

M-H light source. Only single-ended, medium and mogul base, M-H lamps will be discussed here. This is because double-ended M-H lamps always require a high voltage pulse to begin arc conduction. An M-H lamp, although similar to an MV lamp in construction and operation, has a quartz arc tube that is slightly smaller than that of a MV lamp of the same wattage.

In addition to mercury, small amounts of thorium iodide, sodium iodide and scandium iodide vaporize in the arc stream, allowing the M-H lamp to have better color rendering characteristics and higher efficacy than an MV lamp.

The M-H lamp, which operates at a relatively higher temperature than an MV lamp, takes about 5 min to warm up to full light output. If power to the lamp is lost, the total time to cool down and warm back up to full stabilized output is 10 to 15 min, depending on wattage, ambient temperature, and operating condition of the lamp.

HPS light source. An HPS lamp differs from both the MV and M-H lamp in construction and operation. The flow of electrons across the electrodes inside the ceramic translucent arc tube is begun by an electronic starter circuit, which works in conjunction with the magnetic component of the ballast.

Since the arc tube diameter is too narrow to house a starting electrode, all HPS lamps use a high voltage starter circuit.

The starter supplies a short high voltage pulse on each cycle, or half cycle, of the supply voltage. This pulse has sufficient amplitude and duration to ionize the xenon gas and begin arc conduction.

The HPS lamps takes about 4 min to warm up to full light output. Since the operating pressure of an HPS lamp is less than either an MV or M-H lamp, the restrike time is much shorter: about 60 to 90 sec.

Karen Pharaoh-Murphy and Reynold Territo are senior staff engineers with CUH2A, Inc., Architects and Engineers, Princeton, N.J.