Though they are not installed these days, there are still many houses fitted with secondary glazing, mainly aluminium. Because of the narrow profile and intrinsic weakness of the frames, there is very little additional security which can be added. Such glazing has generally been fitted to wooden framed windows, which can be secured by standard window locks.
However, sliding secondary glazing can offer additional security with the fitting of a key operated clamp on the bottom rail of the frame. Particularly in hot conditions windows are left open to allow the circulation of air. This, inevitably, reduces the level of security. By fitting a sliding window lock to the bottom rail you can restrict the slide to a few inches.
Alternatively, drilling a small hole through the bottom rail at the point at which you wish to restrict the slider and inserting an appropriate sized bolt will suffice. Additionally, to prevent the slider being lifted, fit a wooden block of a suitable depth into the track above the slider. When used in conjunction with stay locks or child safety locks on the primary glazed window, security is enhanced further.