A multi-layer Printed Circuit Board (PCB) holds a number of sections of serpentine track on two or more internal layers of a multi-layer PCB. The sections are connected in series with resistors between each section. One end of each of the resistors a connection to an analogue to digital converter is made. In its simplest form, each of the serpentine track is produced on a separate internal layer of the PCB. The outer layers are typically ground planes. Attempts to drill through the PCB are likely to short the ground plane to one or more of the internal serpentine tracks, which will alter the voltages on the analogue to digital connections. Shorting out sections of serpentine track with a view to then drilling through undetected are made very difficult in this scheme since shorting any tack to the ground plane or any track to any other track or breaking any track will be detected by changes to the voltages on the connections to the analogue to digital converter inputs.