The Traffic Signal Network
Flashing Operation Tip
by Arthur J. Dock
Just how should flashing be wired in an 8-phase traffic signal cabinet? All of the phase assignments used in this article are based on NEMA standard phasing:
|Southbound Thru 4||Southbound Left 7|
|Eastbound Left 1||Westbound Thru 2|
|Eastbound Thru 6||Westbound Left 5|
|Northbound Left 3||Northbound Thru 8|
The typical traffic signal cabinet will use a two-circuit flasher. Three of the many possible ways to wire the output of the flasher are:
|Circuit 1 Phases||Circuit 2 Phases|
|Option 1||1, 3, 5, 7||2, 4, 6, 8|
|Option 2||1, 2, 5, 6||3, 4, 7, 8|
|Option 3||1, 2, 3, 4||5, 6, 7, 8|
In option one, all of the left turns are on one circuit and all of the through movements are on the other. Through movements may have more indications than the left turns. In this case this is probably not a good layout because the load on the two circuits of the flasher would not be balanced. Where this becomes critical is large intersections where there may be 4 heads per through movement. In that case, there would be 16 heads on one flasher circuit and 8 (assuming two indications per left turn approach) on the other. With 135 Watt incandescent lamps, this would present a load of 2160 Watts (18 Amps @ 120 Volts) into a 15A rated flasher circuit.
In option two, the through and left-turn phases for a given direction flash together. In this arrangment, if one circuit of the flasher fails, two approaches to the intersection will have no indications: not a good option for fail-safe operation.
Option three is better than one or two. In this option the load is split such that wattage is balanced. In the event of a single flasher circuit failure, an approach will still have either a through or left turn still flashing. This option (along with Option 2) have the added benefit of a "wig-wag" display where the flashing alternates between the through and left-turn indications on an approach (assuming protected left turn indications - of course, 5-section protected/permitted left turn heads don't have a red so this would be a moot point in that case).
In summary, the following considerations can be used to help determine how to wire the flasher circuits in a traffic signal cabinet:
Present a balanced load to the two flasher circuits
Assure flashing indications even in the event of a single flasher circuit failure
Present an attention-getting "wig-wag" display
One final word, it is always a good idea to test your flashers as part of routine maintenance.
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