Residual current circuit breaker (FI) for an electrical circuit, having a summation current transformer (SW) for ascertaining a differential current of the electrical circuit, a breaker mechanism (M) comprising contacts for interrupting the electrical circuit, a tripping circuit (A) that is connected to the summation current transformer (SW) and the breaker mechanism (M) and is configured in such a way that the breaker mechanism (M) is activated, and so the electrical circuit is interrupted, if a differential current limit value is exceeded, a test circuit comprising a test key (T) for testing the functionality of the residual current circuit breaker (FI), wherein the test circuit has a current source (IQ), wherein the test circuit has a series connection comprising the test key (T) and the current source (IQ), which is firstly connected to an input-side conductor upstream of the summation current transformer (SW), and secondly is connected to another, output-side conductor downstream of the summation current transformer (SW), and wherein the current source (IQ) is a two-terminal network, characterized in that the current source (IQ) has a series connection comprising two transistors (T1, T2), wherein the drain terminal (D) of the first or second transistor (T1, T2) is connected to a first terminal of a first resistor (R1), and in that the source terminals (S) of the two transistors (T1, T2) are connected to one another via a second resistor (R2), wherein the gate terminal (G) of the first transistor (T1) is connected to the source terminal (S) of the second transistor (T2), and the gate terminal (G) of the second transistor (T2) is connected to the source terminal (S) of the first transistor (T1). a summation current transformer (SW) for ascertaining a differential current of the electrical circuit, a breaker mechanism (M) comprising contacts for interrupting the electrical circuit, a tripping circuit (A) that is connected to the summation current transformer (SW) and the breaker mechanism (M) and is configured in such a way that the breaker mechanism (M) is activated, and so the electrical circuit is interrupted, if a differential current limit value is exceeded, a test circuit comprising a test key (T) for testing the functionality of the residual current circuit breaker (FI), wherein the test circuit has a current source (IQ), wherein the test circuit has a series connection comprising the test key (T) and the current source (IQ), which is firstly connected to an input-side conductor upstream of the summation current transformer (SW), and secondly is connected to another, output-side conductor downstream of the summation current transformer (SW), and wherein the current source (IQ) is a two-terminal network, characterized in that the current source (IQ) has a series connection comprising two transistors (T1, T2), wherein the drain terminal (D) of the first or second transistor (T1, T2) is connected to a first terminal of a first resistor (R1), and in that the source terminals (S) of the two transistors (T1, T2) are connected to one another via a second resistor (R2), wherein the gate terminal (G) of the first transistor (T1) is connected to the source terminal (S) of the second transistor (T2), and the gate terminal (G) of the second transistor (T2) is connected to the source terminal (S) of the first transistor (T1). <EOS>