Brian Scaddan, IENG (Incorporated Engineer), MIET (Member of the Institution of Engineering and Technology), is a consultant and an Honorary Member of City and Guilds. He has over 35 years’ experience in Further Education, and is now Director of Brian Scaddan Associates, Engineering Training Consultants, in Waterlooville, Hampshire, England. He is a leading author of books on electrical installation, inspection and testing, including IEE Wiring Regulations: Explained and Illustrated and Electrical Installation Work. Brian also runs PAT training courses for the City & Guilds 2377 exam.
The thin layer of material which covers our planet—rock, clay, chalk or whatever—is what we in the world of electricity refer to as earth. So, why do we need to connect anything to it? After all, it is not as if earth is a good conductor. It might be wise at this stage to investigate potential difference (PD). A PD is exactly what it says it is: a difference in potential (volts). In this way, two conductors having PDs of, say, 20 and 26 V have a PD between them of 26 and 20 V. The original PDs (i.e., 20 and 26 V) are the PDs between 20 V and 0 V and 26 V and 0 V.