What are 6.6A and 20A circuits?
The two common loop currents in airfield series circuits. The regulator drives either 6.6 A or 20 A round the loop, and the isolation transformer ratio makes sure the lamp gets 6.6 A either way.
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In a series airfield lighting circuit the constant current regulator (CCR) holds one current round the whole loop, and brightness follows that current. At its top step the loop commonly runs at 6.6 A or at 20 A, which is why people speak of 6.6 A circuits and 20 A circuits.
In the common case the lamp is designed for 6.6 A, whichever loop current is used. The isolation transformer bridges the gap: a 6.6 : 6.6 A unit is 1:1, and a 20 : 6.6 A unit divides the current by about three. The point to remember is that the transformer must match the circuit. A wrong ratio gives a very dim lamp in one direction and a burnt-out lamp in the other. As with all work on the loop, isolate the circuit under site procedures first.
Frequently Asked Questions
Q1What is a 6.6 A circuit?
A series circuit in which the constant current regulator drives 6.6 A at its top step round the loop. A 6.6 A : 6.6 A transformer, a 1:1 ratio, passes the same current to the lamp.
Q2What is a 20 A circuit?
A series circuit in which the regulator drives 20 A round the loop. A 20 A : 6.6 A transformer, a 3:1 ratio, steps the current down so the lamp still receives 6.6 A.
Q3Why is the lamp current 6.6 A in both?
The transformer ratio sets the secondary current. In a 20 A circuit the 3:1 ratio divides 20 A by about three, which brings the lamp back to 6.6 A, so the same fittings can be used.
Q4Why would a designer choose 20 A?
For the same total power, a higher current means a lower loop voltage, so a heavily loaded circuit can be run at about one-third of the voltage. Cable, regulator and transformer ratings must then suit the higher current. The choice is made in the design.
Q5What happens if the wrong transformer is fitted?
A 20 : 6.6 A unit in a 6.6 A circuit gives the lamp about one-third of its current, so it is very dim. A 6.6 : 6.6 A unit in a 20 A circuit gives the lamp about three times its current, and the lamp can burn out quickly.
Q6How do I know which circuit I have?
Check the circuit drawings, the regulator nameplate and the transformer markings, and never assume. Isolate the circuit under site procedures before working on it.
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