voltage transistor loop to limit the current

The other transistor (T2) is controlled by the current flowing through the resistor (R1). If a DALI slave unit is connected, this current will be the same as that flowing through the power transistor. The value of the resistor is chosen in such a way that when the current exceeds 250 mA the voltage level across the resistor will open the transisto
voltage transistor loop to limit the current - schematic

r (T2) which in its turn closes the power transistor (T1). In this way the current is maximized to 250 mA. Could anyone explain me how does the resistor R1 (in the first picture) value 2. 7 ohm opens the transistor when the current exceeds 250mA On that design, the signal is transmitted as 12V (logic high) and 0V (logic low).

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