A size-adjustable headset for monitoring bioelectrical signals, the headset comprising: a headband comprising a first connector element; at least one side band comprising a second connector element distant from a first end of the at least one side band; and at least one electrode contact configured to be electrically connected to a measurement electrode, wherein the first connector element and the second connector element are configured to be connected to one another such that a length of the at least one side band between the first end and the first connector element can be adjusted, characterized in that: the headset further comprising a protection unit configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a first state, block a disconnection of the first connector element and the second connector element, the protection unit is configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a second state, allow the disconnection of the first connector element and the second connector element, the protection unit comprises a key mechanism configured to, when actuated by a key, shift the protection unit from the first state to the second state and/or from the second state to the first state. a headband comprising a first connector element; at least one side band comprising a second connector element distant from a first end of the at least one side band; and at least one electrode contact configured to be electrically connected to a measurement electrode, wherein the first connector element and the second connector element are configured to be connected to one another such that a length of the at least one side band between the first end and the first connector element can be adjusted, characterized in that: the headset further comprising a protection unit configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a first state, block a disconnection of the first connector element and the second connector element, the protection unit is configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a second state, allow the disconnection of the first connector element and the second connector element, the protection unit comprises a key mechanism configured to, when actuated by a key, shift the protection unit from the first state to the second state and/or from the second state to the first state. the headset further comprising a protection unit configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a first state, block a disconnection of the first connector element and the second connector element, the protection unit is configured to, when the first connector element and the second connector element are connected to one another and when the protection unit is in a second state, allow the disconnection of the first connector element and the second connector element, the protection unit comprises a key mechanism configured to, when actuated by a key, shift the protection unit from the first state to the second state and/or from the second state to the first state.