What are PTC Heaters?

What are 3 Phase Electric Duct Heaters? 

A three-phase electrical circuit can supply three times the power as a single-phase circuit at the same voltage with only one additional wire.

Commercial and Industrial duct heaters utilize 3 phase electrical power to produce more output wattage per square inch than single-phase electric heaters. Higher voltages result in higher currents and more heat output. With limited cross-section area in heating ducts, higher 3-phase voltage connections are more efficient.

An etched foil heater is built by etching foil tracks that conduct electricity laminated between layers of silicone rubber and nylon for strength and durability.  Etched foil as a resistive element is much thinner than wire resistive elements with better heat distribution across the heater's surface.