Many metal control panels connect 0 Volt of the 12VDC supply to the metal box which in turn is connected to mains Earth. This literally means that the 0 Volt to your PIR's, Keypads and LIM's are directly connected to all water pipes and electrical appliances in the street! This allows RFI interference (and nearby lightning) to feed back from mains Earth onto the 0 Volt of your 12 Volt supply. Thankfully, this problem can be overcome by fitting an ACT 1166 High Frequency Earth Line Choke in series with the incoming mains Earth supply at the panel.
An essential alarm troubleshooting solution.
