Special Considerations When Using
Kino Flo Image 85s
on Studio Electrical Systems



A popular lighting fixture in stage bound effects production is the Kino Flo Image 85 because it offers many benefits in this type of production. But, the Image 85 fixture also exhibits electrical characteristics that must be taken into consideration when used in quantity.


Image 85 fluorescent light fixtures are commonly used by the hundreds to light Chroma Key and Ultimatte stages because they offer many benefits in this application. Effects stages need to be lit evenly and with the best possible color saturation. Evenness is easily achieved using Kino Flo Image 85s because of the soft quality of the light and the wide beam spread. In effects work, it’s not as much about how much light is used to light a stage, but rather what produces the best saturation of reflected blue or green light. For film or high end HD shooting the best saturation of color is achieved by using Kino Flo’s 420nm blue-spike lamps on blue screens and 525nm green-spike lamps on green screens. Finally the use of Image 85 fixtures offer tremendous energy savings as well.


Capable of ten times more light per watt than tungsten lighting, fluorescent light sources like the Image 85 offer tremendous energy savings. Not only do the lights consume less power, but they also generate less heat, which results in energy savings in the amount of air conditioning required to keep the stage comfortable.


They also offer both energy and cost savings because the color specific spike tubes for both green screen and blue screen applications mentioned above eliminate the cost of gels to reach the desired color saturation - the energy savings comes in not having to go to a larger fixture to compensate for the loss of output that results from putting gels on the fixtures in order to obtain the desired color saturation.



When lighting blue or green screens, Image 85 fixtures are usually rigged in rows on 7ft (2m) centers. Typically there is a row at the top and bottom of a screen in order to make the reflected light more even over the screen and to increase brightness of the blue or green reflected light. As pictured above, 18 Image 85 fixtures are typically required to light a 40x40 backdrop. Which means that upwards of 175 Image 85s are required to light the 390 linear feet of a 3 wall temporary cyc (120+120+150=390) of a 120’ x 150’ stage.


And, if in addition to lighting the cyc walls, Image 85s are used for overall top illumination (as seen is some of these production stills) an additional 180 Image 85 fixtures may be used. The problem from an electrical distribution standpoint is that Image 85s exhibit electrical characteristics that must be taken into consideration when used in this quantity.


The electrical characteristics of the Image 85 fixture that should be taken into consideration are many. An Image 85 is a fluorescent light fixture that uses qty eight 75W T-12 fluorescent tubes for a total of 600W per light. But, because the ballasts in the Image 85 head are non-power factor corrected Switch Mode Power Supplies with large smoothing capacitors, the Image 85 has a Power Factor (PF) of .64, which means that each head has an Apparent Power of 938VA for it’s 600W output. While its RMS load is nominally 8.4A, its peak load is 20A with a crest factor of about 2.25. Started cold, it has an inrush current of 70A peak for a period of about 24 cycles while its smoothing capacitors charge. And, finally it is harmonically rich with a Total Harmonic Distortion (THD) of 77%.


Before looking at the effect on the electrical service head of a stage of using this non-linear load in the quantities pictured in the production stills above , let’s first look at the effect on just one of the stage's individual company switches.

Current on System Neutrals:

As can be seen in the Power Quality Meter reading of a single Image 85 fixture below, with a THD of 77%, Image 85 heads generate a lot of harmonic currents. The distribution of harmonic currents generated by a single fixture is as follows:


The high content of the 3rd and 9th Harmonics are of particular concern when using these fixtures by the hundreds on stages because the 3rd harmonic, and odd multiples of the 3rd (9th, 15th, etc - called the triplen harmonics) can lead to unusually high current levels on the system neutral, which is a potential fire hazard and can cause the neutral conductor and connectors to deteriorate or fail catastrophically. Here's why: AC power is delivered throughout the distribution system at a fundamental frequency of 60 Hz. (50 Hz in Europe.) The harmonic currents generated by Image 85 fixtures are integral multiples of this fundamental frequency. For instance, the 3rd harmonic frequency is 180 Hz, the 5th is 300 Hz, etc. In the US, the standard configuration of service transformers providing power to motion picture stages is 208/120 wye. Hence there are three phase wires and a neutral wire. The voltage between any two phase wires is 208, and the voltage between any single phase wire and the neutral wire is 120. All 120 volt loads, including Image 85s, are connected between a phase and neutral. When the loads on all three phases are balanced (the same fundamental current is flowing in each phase), the fundamental currents in the neutral cancel and the neutral wire carries no current. Phase cancellation of the fundementals currents generated by incandescent lamps is illustrated below.

When the loads are non-linear (Image 85s in this case) the situation changes. Since the front end components of an Image 85 ballast uses smoothing capacitors to flattened out rectified AC current, they draw current in spikes and generate harmonic currents. As can be seen on the power quality meter screen above, the largest harmonic current generated is the 3rd. Also generated, in smaller amounts, are the 5th, 7th, and all other odd harmonic currents. Like the fundamental currents, most harmonic currents cancel out on the neutral wire. The triplen harmonics (the 3rd, 9th, 15th, etc) however are troublesome when dumped back into the neutral of the distribution system.

The reason triplen harmonics are troublsome is that, as can be seen in the illustration above, they are in phase with each other. For this reason, rather than cancel each other out on the neutral conductor, as the out of phase fundamentals normally do, they instead add up (as illustrated below.)

By generating harmonic currents that stack one upon another, and shifting the phase angle of the primary currents so that they don't entirely cancel, the non-linear electronic ballasts of Image 85 heads can create unusually high returns on the neutral of the distribution system. For example, if each phase wire were carrying, in addition to fundamental current, 100 amps of 3rd harmonic current, the neutral wire could be carrying 300 amps of 3rd harmonic current. This extra current provides no useful power to the loads. It simply reduces the capacity of the system to power more loads, and produces waste heat in all the wiring and switchgear. When the 3rd harmonic current returns to the transformer it is reflected into the transformer primary where it circulates in the delta winding until it is dissipated as heat. The result is overheated neutral wires, switchgear, and transformers. This can lead to failure of some part of the distribution system and, in the worst case, fires.


To determine the magnitude of the current we might expect on the system neutral of effects productions using hundreds of Image 85 fixtures, we conducted the following test: we metered the power drawn by an Image 85 on each phase of a 3-phase distribution system and the current returning on the system neutral. The results are pictured below:


A neutral current of 13.62 when the average of the phase currents is only 7.77A clearly shows that the triplen harmonics generated by the Image 85 fixtures are accumulating on the neutral, rather than cancelling each other out as the out of phase fundamentals normally do. In fact, the neutral is carrying 176% of the average load on the individual fundamental phase legs.


For this reason, on their website Kino Flo cautions that "On large installations it may be necessary to double your neutral run so as not to exceed your cable capacity." What effect will such high neutral returns have on an individual 400A 3-Phase company switch powering a number of Image 85s? If we take the rig pictured below for example, we see that there is the potential to severely overload the neutral bus of a individual company switch.


The 180 lamp heads pictured in this rig would require a service of 1’407A or 469A/leg on a 3-phase service. And since these ballasted lamp heads should not be operated on a dimmed channel, there is no reason to bring the power for these heads back to a central dimmer room. Instead rigging crews will patch them into company switches in the elevated catwalks (called perms), before bringing more power up from another company switch below. For this reason, company switches in the perms tend to be loaded to capacity before the rigging crews bring power up from below.


Typically a rigging crew will not load distribution equipment beyond 80% of its' rated load. Which means that in this situation, they will load each phase leg of a 400A/3-phase company switch in the perm to 320A before bringing more power up from a company switch below. If each phase leg is loaded as such, according to the results of our test we can expect a return on the neutral of 563A (320A x 1.76 = 563.20A.) Since the neutral return of a distribution system has no over current protection, return currents of this magnitude can cause sufficient heat to overload the neutral wire, and the neutral bus of switchboards, unless special precautions are taken.

Purpose built to accommodate the high preponderance of non-linear lighting instruments – HMIs, LEDs, as well as Fluorescents - used in motion picture productions, the system neutrals of motion picture stages are oversized. But, unfortunately, it is not possible to build a stage that can accommodate every conceivable type of production that could take place in it. And, since the 176% returns of effects shows using Image 85s by the hundreds is outside the norm, the electrical distribution systems of motion picture stages like ours are designed to carry neutral currents that are no more than 120% of the current that can be carried on an individual phase leg. Since, neutrals oversized by a factor of 1.2 are still not sufficient for effects shows using Image 85s by the hundreds; what means are there for accommodating the extremely high neutral currents generated by Image 85 fixtures?

On location effects productions, it is a common practice to use company switches equipped with double neutral cams and feed them with “super neutrals” capable of carrying 200% of the load on an individual phase leg. Generators are also oversized to accommodate the higher than normal current level on the system neutral without overheating. Since the cost in man-hours and equipment to run an independent distribution system with super neutrals throughout a stage where a distribution system already exists, to deliver generated power to a large number of Image 85 fixtures, the more cost effective approach from the stage client's standpoint is to instead de-rate the current carrying capacity of the existing stage distribution equipment. Accordingly, a 400A company switches is not loaded beyond 217A per phase leg (400A/1.76 de-rating factor = 227A/leg x 1.2 present over sizing of neutral = 272A x .8 safety margin factor typically applied by crews = 217A.)

High Inrush Currents:

As can be seen in the Power Quality Meter reading of a single Image 85 fixture below, Image 85 fixtures exhibit a high inrush current (70A) when struck cold.


The reason the inrush current is 350% of the steady-state current of 20A peak is that the electronic ballasts of Image 85 fixtures use large smoothing capacitors to convert rectified AC to DC before switching to a high frequency output (greater than 25kw Hz.) If we scale up this characteristic of Image 85 fixtures to the potential load placed on a company switch by a special effects show, we see there is a potential inrush current on an individual company switch of 2’660A on each phase leg (320A/8.4A per fixture = 38 fixtures per phase leg)(38 fixtures x 70A Inrush Current per fixture = 2660A Inrush Current per phase.) Since the trip curve of the electronic breakers in our company switches are adjustable, to avoid nuisance tripping when all the lights are turned on at once, they should be set for an inverse time curve that accommodates a high inrush current that lasts for a duration of 24 cycles.

Transformer Sizing:

The effect of using a large number of non-linear loads, like Image 85s, on facility service transformers have been well documented in recent years. Not only do triplen harmonics add to create unusually high current levels in the neutral, but the heat they generate also significantly increases core loss in transformers. For this reason, motion picture stages use special transformers, called K-rated transformers, that are designed to withstand the extra heat produced by normal loading with non-linear lighting loads. A typical studio service transformer will have a K-rating of 20 which means that it has sufficient steel, neutral connections, and neutral wire size to carry 120% of the phase current for which it is rated.


Though purpose built to accommodate the high preponderance of non-linear lighting instruments used in motion picture productions, the service transformers of stages are not equipped to handle the unusually high return currents generated by hundreds of Image 85 fixtures. For this reason they should be de-rated on effects productions using predominantly Image 85s to light virtual sets.

By what factor should studio service transformers be de-rated? CBEMA (Computer & Business Equipment Manufacturers Association) recommends the following de-rating factor (dF) be used on standard dry type transformers:

dF = 1.414 x Irms / Ip

Where: dF = transformer de-rating factor, Irms = current rms value, and Ip = current peak value When applied to situations where we are powering several hundred Image 85 fixtures Irms = 2’520A (8.4A/per fixture x 300 fixtures = 2’520A) and Ip = 6’000A (20A/per fixture x 300 fixtures = 6’000A)

Then,

dF = 1.414 x 2’520 / 6’000 = 0.59

Therefore, using this method, a 750kVA dry type transformer should be de-rated 41% and not be loaded to more than 442kVA (750kVA x 0.59 = 442kVA). In effect, rather than having a capacity of 2500A on each phase of the stage service, there would be only 1’475A after de-rating the transformer according to the recommendations of the CBEMA.

I don't know how much of a concern this is to us, since our service transformers are of the wet type which I understand are much better at mitigating the heat generated by the triplen harmonics.

Voltage distortion and neutral-ground voltage:

In addition to high neutral currents and transformer loss, there are other, less well documented, but potentially more serious problems associated with harmonic rich loads like the Image 85 fixtures. They include the heavy voltage distortion and high neutral-ground voltages (common mode noise) that are common where high densities of non-linear loads exist.

Voltage waveform distortion is the result of the interaction of the harmonic currents with the impedance of the electrical distribution system. That is, as the harmonic currents circulate through the electrical distribution, they produce voltage drops at each harmonic frequency in relation to Ohm's law - Vh = Ih x Zh. The combined effect of the voltage drops at each harmonic frequency creates a form of voltage waveform distortion referred to as “Flat Topping.”


Since, voltage waveform distortion levels are higher when system impedance is higher; and since system impedance is generally high when fairly long cable runs are serviced from a supply with high source impedance (such as a diesel generator), voltage Flat Topping is another reason to not run an independent distribution system with super neutrals to deliver generated power to a large number of Image 85 fixtures.

The same Ohm's Law relationship that results in voltage waveform distortion also creates high neutral-ground common mode noise voltages. Heavy neutral currents, resulting from the additive effect of the triplen harmonic currents (discussed above) in the neutral will produce a voltage drop along the neutral conductor if not adequately sized. This voltage drop will appear as a potential difference between neutral and ground near the harmonic generating loads (ie. at the company switches and distro box receptacles). Commonly referred to as common mode noise, this voltage can have an adverse affect on the operation of equipment which is subjected to it. This is another reason for devisng a strategy to mitigate the adverse effects of harmonic currents in the stage distribution system.

Ground Currents:

Like most electronic equipment which utilize switch-mode power supplies around audio systems, Image 85 fixtures are equipped with filters to reduce their high frequency EMI emissions. While these filters are effective at reducing EMI, they allow low frequency harmonic leakage currents (ie. 180 Hz) to pass through to the ground wire. This can result in troublesome ground currents circulating through the power system and need to be addressed as well.