Earthing is one of the most complex and poorly understood aspects of power system engineering, with many engineers regarding it as a ‘black art’ or ‘alchemy’. In truth it is not an easy subjective, and one that is not overly intuitive, but there are some key ideas that are worth taking the time to understand. This blog post is only a very brief overview of the main areas of earthing studies, and there will be many more posts too follow on the subject!
One of the key things to first understand is that the HV and LV earthing studies have very different focuses. LV earthing is primarily concerned in providing clear paths for earth fault current to flow in, correctly sizing the conductors to handle the fault current and deciding which earthing strategy to use i.e. TN-S, TN-C-S, TN-C, IT or TT (Wikipedia actually explains them fairly well). In practice, in the UK, most earthing systems are TN-S or TN-C-S, but there are a few important exceptions. This is a complex area, and an incorrect specification can put personnel at risk and lead to nuisance tripping of circuit breakers.
HV earthing is subtler, and much more complex – typically requiring the use of complex modelling packages such as CDEGS. HV earthing is based around is how the system and surrounding area behaves should a phase-earth fault occur, and how the fault current will return to the source. In HV systems the ‘source’ substations are usually far remote, and an earth fault current may need to ‘travel’ many km to reach it. This fault current will flow into the mass of ground the earth rods and if available through cable sheaths and ground wires. In HV systems it is the earth fault current that flows into the ground that is of most concern, as this can create at Earth Potential Rise (EPR) that can adversely affect nearby livestock, personnel and buildings.
An issue important to all earthing studies, is how and where the neutral and earth are bonded together and if neutral earthing resistors or insulation monitors should be used. These decisions are of key importance for the safety, continuity and reliability of the electrical network and a frequent source of confusion amongst even experienced engineers. Due to the complexity of earthing studies and the differences between HV and LV earthing, there will be several dedicated blog posts on each subject later. If you have any questions or would like to know more, please do not hesitate to get in touch.