Why is the First LIVE Electrical Test so Important.. In the video UK electrician Marcus is carrying out the first live test according to BS7671 re-check supply polarity. Using an approved voltage indicator conforming to GS38 Marcus identifies a problem. On investigation the issue is in the cable supplying the consumers unit. Once the fault is corrected the supply polarity is checked again and confirmed before carrying out further live tests. Videos are training aids for City and Guilds (C and G)