The wiring standard indicates that earth leakage units should trip strictly between 15 - 30 mA. As a quality manufacturer, CBI maintains a stringent safety margin from these limits i.e. it will be a few milliamps less than 30 mA.  Under normal circumstances, a leakage current is already present in most systems. Therefore when testing an installation and it trips well below 30 mA on your tester, that indicates that there could already be some leakage in the system.