Safety and the design of earth fault protection scheme

Designing the Insulation of electrical equipment protection schemes and ensuring safety of personnel& equipment is dependent on the type neutral grounding in electrical power systems. Appropriate earth fault protection schemes that is best suited for the earth fault current, which depends on the type of neutral earthing, is critical to ensure safety of equipment and personnel.

Grounding (or Earthing) in the Electrical Power System is important from the point of view of designing the insulation of electrical equipment, designing of protection schemes and ensuring safety of personnel and equipment. A breakdown of insulation to earth, results in flow of fault currents, the magnitude of which depends upon the type of system neutral grounding. The type of grounding includes high resistance or reactance grounding for generators, Medium resistance grounding for industrial power system and solid/effective grounding for transmission and distribution systems.

Grounding in the Electrical Power System is important from the point of view of designing the insulation of electrical equipment, designing of protection schemes and ensuring safety of personnel and equipment. A breakdown of insulation to earth, results in flow of fault currents, the magnitude of which depends upon the type of system neutral grounding. The type of grounding includes high resistance or reactance grounding for generators, Medium resistance grounding for industrial power system and solid/effective grounding for transmission and distribution systems.

Medium resistance grounding is favored in medium voltage (11kV, 6.6kV and 3.3kV) application in the industrial systems. In the medium resistance grounded systems the earth fault currents are limited to between less than 100A to full load current of the transformer but typically around 400A. The actual permissible earth fault current must be chosen with care to limit the damage to the electrical equipment while minimizing the risk of transient over voltages. The minimum current required to ensure graded earth fault protection must also be borne in mind while selecting the magnitude of earth fault current.

There are several approaches to designing the earth fault protection scheme for the medium voltage systems such as earth fault protection using core balance CTs, residually connected earth fault protection, sensitive earth fault protection etc. Proper care has to be exercised in selecting appropriate earth fault protection scheme depending on the magnitude of the earth fault current.