An electro magnetic ballast (110) for a gas discharge lamp (2) comprises: input terminals (3), for receiving a mains voltage; lamp connector terminals (4), for receiving a lamp; a controllable semiconductor switch (26) coupled in parallel to the lamp connector terminals; a current sensor (127) connected in series with the controllable switch (26); and a control circuit (28) for controlling the controllable switch (26) and responsive to the current sensor (127). When operating in a normal mode, the control circuit (28) is responsive to a current sense signal received from the current sensor to switch the controllable switch (26) ON if said current sense signal indicates a current flowing in the controllable switch (26) and to switch the controllable switch (26) OFF if said current sense signal indicates that no current is flowing in the controllable switch (26).