A control system for sourcing a system load, comprising: a variable speed device, comprising a pump, and having an operable element which is a variably controllable motor in order to source the system load; one or more controllers configured to determine control curve information for variable operation of the operable element of the variable speed device, the control curve information being dependent on at least a first parameter and a second parameter, the first parameter and the second parameter being correlated, wherein the first parameter comprises a pressure parameter and the second parameter comprises a flow parameter, the control curve information including: a) a first setpoint of rated total value of the system load for the first parameter and rated total value of the system load for the second parameter, b) a second setpoint of a minimum partial percentage of the rated total value of the first parameter at an over-percentage of the rated total value of the second parameter; c) a path which maintains the rated total value of the first parameter for all values of the second parameter up to the first setpoint; and d) a path between the first setpoint and the second setpoint; the one or more controllers being configured to receive one or more inputs in relation to at least one of the parameters, and operate the variable speed device based on the received one or more inputs according to the control curve information; characterized in that the control curve information further comprises: e) a further setpoint of over-percentage of the rated total value of the first parameter for shutoff or zero value of the second parameter, and f) a path from the further setpoint to the path which maintains the rated total value of the first parameter. a variable speed device, comprising a pump, and having an operable element which is a variably controllable motor in order to source the system load; one or more controllers configured to determine control curve information for variable operation of the operable element of the variable speed device, the control curve information being dependent on at least a first parameter and a second parameter, the first parameter and the second parameter being correlated, wherein the first parameter comprises a pressure parameter and the second parameter comprises a flow parameter, the control curve information including: a) a first setpoint of rated total value of the system load for the first parameter and rated total value of the system load for the second parameter, b) a second setpoint of a minimum partial percentage of the rated total value of the first parameter at an over-percentage of the rated total value of the second parameter; c) a path which maintains the rated total value of the first parameter for all values of the second parameter up to the first setpoint; and d) a path between the first setpoint and the second setpoint; a) a first setpoint of rated total value of the system load for the first parameter and rated total value of the system load for the second parameter, b) a second setpoint of a minimum partial percentage of the rated total value of the first parameter at an over-percentage of the rated total value of the second parameter; c) a path which maintains the rated total value of the first parameter for all values of the second parameter up to the first setpoint; and d) a path between the first setpoint and the second setpoint; the one or more controllers being configured to receive one or more inputs in relation to at least one of the parameters, and operate the variable speed device based on the received one or more inputs according to the control curve information; characterized in that the control curve information further comprises: e) a further setpoint of over-percentage of the rated total value of the first parameter for shutoff or zero value of the second parameter, and f) e) a further setpoint of over-percentage of the rated total value of the first parameter for shutoff or zero value of the second parameter, and f) a path from the further setpoint to the path which maintains the rated total value of the first parameter.