In renal haemodialysis there is a condition whereby the patient can by at serious risk of harm if the venous line becomes disconnected or the venous needle becomes dislodged and the haemodialysis machine does not detect this condition. The invented device is designed to safeguard haemodialysis patients from serious losses of blood through venous line disconnection or venous needle dislodgement. The invented device is designed to clamp the venous line automatically before venous needle dislodgement or venous line disconnection becomes a serious problem and to make the haemodialysis machine stop drawing blood from the patient This will cause the haemodialysis machine to alarm in the normal way so as to alert the patient or healthcare worker to the problem. The invented device will have to be manually reset before therapy can continue.