Equipment Grounding vs. System Grounding

Grounding or earthing an electrical system is the process of connecting all metalwork/frame of electrical equipment i.e. the non-current carrying part or some electrical component of the system such as the neutral point in a star-connected system, one conductor of the secondary of a transformer, and so forth to the main body of earth. An earthing system has two distinct but related parts: (a) a low resistance conductor bonding the metalwork, connected to (b) an electrode or array of electrodes buried in the ground. The main purpose of grounding is to convey to earth any leakage of electrical energy to the metalwork without hazard to personnel or equipment.

Grounding/earthing may be classified as:

• Equipment grounding
• System grounding

Equipment grounding deals with earthing the non-current carrying metal parts of the electrical equipment whereas, system grounding implies earthing some section of the electrical system such as grounding of neutral point of star-connected system in generating stations and substations.

Equipment Grounding

As aforementioned, equipment grounding is the process of connecting non-current carrying metal parts such as metallic enclosure of the electrical equipment to the main body of the earth/soil in such a way that in case of insulation failure, the enclosure effectively remains at earth potential.

The leakage current I_L flows from the motor, through the enclosure and to the ground conductor, thus the enclosure remains at earth potential. As a result, the operator would not experience an electric shock.

System Grounding

System grounding is whereby some electrical part of the power system such as the neutral point of a star-connected system, one conductor of the secondary of a transformer, and so forth is connected to earth.

Importance of System Grounding

To help us illustrate the importance of system grounding, consider the following diagram showing the primary winding of a distribution transformer connected between the line and neutral of an 11 kV line.

If the secondary conductors are ungrounded, it would appear that a person could touch either secondary conductor without harm since there is no ground return. However, this is not true. With reference to figure 1.1 above, there is actually capacitance C₁ between primary and secondary and capacitance C₂ between the secondary and ground. This capacitance coupling can produce a high voltage between the secondary lines and the ground and depending on the relative magnitude of C₁ and C₂ it may be as high as 20% to 40% of the primary voltage. If a person touches either one of the secondary conductors, the resulting capacitive current I_C flowing through the body could be harmful even in case of small transformers.

If one of the secondary conductors is grounded, the coupling reduces almost to zero and so is the capacitive current I_C. Thus, the person wouldn’t experience an electric shock.

Consider another scenario where the secondary conductors of a distribution transformer are ungrounded and the high voltage line (11 kV) touches the 230 V conductor as illustrated in the figure below. This could be caused by an internal fault in the transformer or by a falling tree across the 11 kV and 230 V conductors.

Under these conditions, a very high voltage is imposed between the secondary conductors and ground. This would immediately break the 230 V insulation, causing a huge flashover. This flashover could happen anywhere on the secondary network, perhaps inside a plant or a home. Therefore, ungrounded secondary in this situation is a potential fire hazard and may produce grave accidents under abnormal circumstances.

If one of the secondary conductors is grounded, the accidental contact between an 11 kV conductor and a 230 V conductor produces a dead short. The short circuit current follows the dotted path as shown in the figure below. This large current will blow the fuse on the 11 kV side, hence disconnecting the transformer and secondary distribution system from the 11 kV line.

Related articles:

• Residential Electrical Wiring: Key Safety Considerations
• Main Types of Earthing Systems
• Electric Wiring Testing & Inspection Instruments Requirements
• Testing Power and Distribution Transformers
• AC Transmission and Distribution Systems
• Overhead vs. Underground Power Distribution Systems
• Instrument Grounding: General Guidelines