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. <EOS>