Are lamps connected in series on airports?
EVERYONE who is not aware of how series-fed constant current runway lighting systems work should read the excellent article cited by Kevin White. 


Series Lighting Circuitry: Why Use It?


Many airport lighting systems did and still do use series feed - but, ...


They are vastly more sophisticated systems than are used in eg Christmas Tree lights.


10's of kW, 
Up to 5 kV 
Constant current AC feed, 
Transformer coupled lights, 
Immune to blown bulbs and various other faults. 



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  A famous example of lamps connecting in series was at an airport, especially to indicate the landing strip. The purpose was to guarantee that all lamps would work together. If one lamp would be bad, all lamps would be off and no distorted signal would be visible.


Airport lighting systems did and still do use series fed lighting systems. (Parallel lighting systems are also used). The system does NOT cause all lights to extinguish when one (or more) bulb fails - see below for technical details. 


  ... it is hard to believe that airport lamps (not LED) were really connected in series. 


But, this was and still is a common method of connection.
The reason it works is that equipment is provided at each light to transform the constant current AC feed into a standardised lamp supply voltage and to isolate the lamp if it fails. 

A major feature of the systems is that in systems with 10's of kilowatts of lights spread over miles of circuit, they replace relatively low voltage high current feeds with much higher voltages and much lower currents - leading to "much much" lower wiring losses. eg a smallish sized [!] 30 kW system would require 220 A at 135V (!!!) but only 6A at 5 kV. As resistive losses are proportional to current squared, either losses can be (6/220)^2 = 0.07% as great for the same wiring size OR, more likely a wire size / power loss tradeoff can be implemented. eg using wire 4% the area of that used at 135V will result in 2% of the power losses.
(6A x 6A x 25R)/(220A x 220A x 1R) ~= 2% !!!


  The only answer I do not appreciate is lamps for the Christmas tree, because these were virtually always defective because of the reason stated above.


Christmas tree lights are a very very simple version of the same principle - and "better quality versions" had a bimetallic shorting mechanism that bypassed failed bubs so that a bulb failure did not extinguish the whole string.

A good outline is provided in the above article, but


Airport series lighting systems operate at up to 5 kV feed voltages !
Total lamp powers of up to 30 KW [!] uses a standard 6.6A constant current AC feed and
Lamp powers over 30 kW use a 20A constant current AC feed. 
Use of up to 5 kV series connection allows typically "light weight" 8 gauge cables to be used :-) 
The constant current loop feeds transformer primaries, powering lamps at 6.6A and up to 135 V.
Standard cable size for 6.6 A is 8 AWG (8 mm2) and for 20 A is 6 AWG (13 mm2)


They note that


" ... Series lighting systems can operate for long periods under adverse conditions. In fact, they can operate with a large number of lamps burned out, with badly degraded cable systems, and even with shorts and grounds. ... " 


and 


" ... Using 5kV cable as the standard will limit single circuit load blocks to a maximum of 90kW. But don't be concerned about this limitation, since ..." :-) .


The series transformer primaries is "interesting".
A bulb presents a load to the constant current system - no problem. An open circuit secondary (dead bulb) presents a high impedance primary choke.
They MAY use bulb shorters if bulb is blown to present low impedance primaries to current flow. 

I did not know ANYTHING about these systems 2 hours ago !!!

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References:

An Introduction to Airport
Airfield Lighting - Powerpoint. 12 slides. Series and parallel systems

Lotsa links Googlabet search

Historical - Street Light Information Sheet - Understanding Series Circuits   

Note: This document is a continuation of Understanding Early Street Light Circuits