Electrical fundamentals for set lighting

Electricity is the lifeblood of a film set. Every light, monitor, camera charger, video village and catering urn depends on it, and the amounts involved are large. A modest commercial lighting package can draw more current than several houses, and a feature film's night exterior may run more power than a small neighbourhood. Electricity is also invisible, silent and unforgiving. It can kill through electric shock, injure through arc flash and burns, and start fires through overheated cables and connections. This lesson explains the basic quantities, voltage, current, resistance and power, and how they apply to set lighting. It covers single-phase and three-phase systems in Canada and the United States, how to calculate the load of a lighting package, how electricity harms the body and why only qualified people should work on electrical systems. Lessons 3 and 4 build on these ideas with distribution, generators, cables and grounding.

Voltage, current, resistance and power

Four quantities describe most of what happens in an electrical circuit.

• Voltage (V), measured in volts, is the electrical "pressure" that pushes charge around a circuit.
• Current (I), measured in amperes or amps (A), is the rate at which charge flows.
• Resistance (R), measured in ohms (Ω), opposes the flow of current. Conductors such as copper have low resistance; insulators such as rubber have very high resistance.
• Power (P), measured in watts (W) or kilowatts (kW), is the rate at which energy is used or delivered.

Ohm's law links voltage, current and resistance, and the power law links power, voltage and current.

For alternating current with resistive loads such as tungsten lamps, these formulas work directly. For loads such as LED drivers and HMI ballasts, the power factor and harmonics also matter, as explained below.

Electricians on set often use the "West Virginia" rule, \( W = V \times A \), to estimate current from the wattage written on a fixture. A 1,000 W tungsten fixture on 120 V draws about \( 1000 \div 120 \approx 8.3 \) A.

Alternating current and frequency

North American power is alternating current (AC). The voltage swings positive and negative 60 times a second, a frequency of 60 hertz (Hz). Light output from tungsten lamps barely changes over a cycle, because the filament stays hot. Discharge lamps and some LED drivers can flicker at twice the line frequency, 120 Hz, which interacts with camera frame rates and shutter angles. That is why HMI ballasts and LED drivers for film are designed to be flicker-free, and why camera crews pay attention to frame rates when shooting under ordinary practical lights (Lesson 5).

The voltages quoted for AC, such as 120 V, are root mean square (RMS) values, which give the same heating effect as the same DC voltage. The peak voltage of a 120 V supply is about 170 V.

Single-phase and three-phase systems

Power in North America is usually delivered as split-phase or three-phase systems.

• Split-phase 120/240 V is common in houses. Two "hot" legs are each 120 V to neutral and 240 V to each other. A house panel with a 200 A main can supply about 200 A on each leg.
• Three-phase 120/208 V is common in commercial buildings, stages and film distribution. Three hot legs are each 120 V to neutral and 208 V to each other, 120° apart in phase.
• Higher-voltage systems power large buildings: 347/600 V in Canada and 277/480 V in the United States. Lighting distribution on set uses 120/208 V, so connecting to these systems requires transformers and specialists.

Three-phase systems deliver more power with less copper and allow large loads to be shared across three legs. Most film generators produce 120/208 V three-phase, with outlets for single-phase 120 V and 240 V or 208 V equipment.

Canadian commercial and industrial buildings commonly use 347/600 V services, with 347 V lighting circuits and transformers stepping down to 120/208 V for receptacles. A film tie-in to a 600 V service is high-voltage work that only licensed electricians can perform, with the right transformers and permits from the provincial electrical safety authority.

Power in three-phase systems

For a balanced three-phase load, the total power relates to the line-to-line voltage and the current in each leg:

Here \( V_{LL} \) is the line-to-line voltage (208 V), \( I \) the current per leg and PF the power factor, which is 1 for resistive loads such as tungsten.

Another way to estimate is to divide the total load by three and calculate each leg at 120 V: \( 16000 \div 120 \approx 133 \) A. Both methods agree for balanced loads.

Power factor, LED drivers and harmonics

Tungsten lamps are simple resistive loads: voltage and current rise and fall together. Modern film lighting is dominated by LED fixtures and HMI ballasts, which are electronic loads.

• Power factor (PF) is the ratio of real power, which does work, to apparent power, which the cables and generator must carry. A PF of 0.9 means the circuit carries about 11 % more current than the real power alone suggests.
• Good electronic ballasts and LED drivers have power-factor correction, raising PF close to 1. Older magnetic HMI ballasts and cheap drivers can have poor power factor.
• Electronic loads draw current in pulses, creating harmonics, currents at multiples of the 60 Hz line frequency. The third harmonic and its multiples add together in the neutral of a three-phase system instead of cancelling.
• Large LED and electronic-ballast loads can overheat neutrals sized only for balanced resistive loads. Many film distribution systems use "double neutral" feeders, with a neutral conductor twice the size of each hot leg, for this reason.
• Harmonics can also make some generators run unevenly, so generators for film use are chosen and sized with electronic loads in mind.
• Inrush current, a brief surge when equipment switches on, can trip breakers when many LED fixtures are switched at once. Staggering switch-on or using a dimmer or DMX console to bring lights up in groups helps.

How electricity harms people

Electric shock happens when current passes through the body. Current, not voltage, causes injury, but voltage drives current through the body's resistance, so higher voltages are more dangerous.

• Around 1 mA can be felt as a tingle.
• Around 10 to 20 mA can make muscles clamp, so a person cannot let go of a conductor.
• Around 50 to 100 mA through the chest can cause ventricular fibrillation, where the heart quivers instead of pumping, which is fatal without rapid CPR and defibrillation.
• Higher currents cause severe burns and tissue damage, often deep inside the body.

Body resistance varies hugely. Dry skin may have thousands of ohms; wet skin, sweat, cuts or standing in water may reduce resistance to a few hundred ohms. That is why wet locations, rain effects and water scenes are so dangerous for electricity (Lesson 4).

Other electrical injuries include arc flash, an explosive release of energy when current jumps through the air between conductors. Arc flash can cause severe burns, blindness, hearing damage and injuries from flying molten metal. It can happen when someone drops a tool across live busbars in a distribution box or connects equipment under load.

Electrical fires

Electricity causes fires when conductors or connections overheat.

• Overloaded cables heat up, softening insulation and eventually causing short circuits.
• Loose or dirty connections, such as a worn pin connector or a damaged Cam-Lok, create resistance and local heating.
• Coiled cable carrying heavy current cannot shed heat and can overheat, so feeder cable is laid out rather than left coiled when loaded.
• Damaged insulation, crushed by vehicles, cut by doors or stapled, can cause shorts and arcing.
• Hot fixtures placed near curtains, paper, set dressing or dry grass can ignite them (Lesson 5).

Overcurrent protection, fuses and circuit breakers, protects cables and equipment by cutting power when current exceeds safe levels. Breakers protect wiring, not people; it takes ground fault protection to protect people from shock.

Qualified people and electrical law

The law restricts electrical work to people with the right training and, for permanent installations and connections to building power, the right licences.

In Canada, the Canadian Electrical Code, Part I (CSA C22.1), adopted with amendments by each province, governs electrical installations. Section 44 covers theatre installations, including buildings used for motion picture and other shows, with rules for portable stage switchboards and equipment. Equipment must be certified by an accredited body such as CSA Group or another certification organization accredited by the Standards Council of Canada. Connecting to a building's electrical system is regulated electrical work that requires a licensed electrician and, in most provinces, a permit or notification to the electrical safety authority, such as the Electrical Safety Authority in Ontario or Technical Safety BC. Provincial OHS rules also require that only competent workers work on electrical equipment and that equipment be locked out before work on it. CSA Z462 sets out workplace electrical safety practices, including arc flash protection.

Set lighting technicians learn safe distribution practice through union training, apprenticeship and experience. Other crew members should never open distribution boxes, make connections or attempt repairs; their role is to recognize hazards and report them.