What is a ground?

2016-03-18 00:02:30 MEWOI Electronics Co.,Ltd

What is a ground?

The US National Electrical Code (NEC) Article 100 defines a ground as: “a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth”.

Grounding actually encompasses two different subjects: earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Equipment grounding ensures that operating equipment within a structure is grounded properly.

These two grounding systems must be kept separate except for connections between the two systems. This prevents differences in voltage potential from a possible flashover from lightning strikes. The purpose of a ground is to provide a safe path for the dissipation of fault currents, lightning strikes, static discharges, EMI and RFI signals and interference.

The US National Fire Protection Agency (NFPA) and Institute of Electrical and Electronics Engineers (IEEE) recommend a ground resistance value of 5 or less. The goal in ground resistance is to achieve the lowest ground resistance value possible that makes sense economically and physically.

What affects the grounding resistance?

Four variables affect the ground resistance of a ground system: length or depth of the ground electrode; the diameter of the ground electrode; the number of ground electrodes and ground system design.

Length/depth of the ground electrode

Driving ground electrodes deeper is a very effective way to lower ground resistance. Soil is not consistent in its resistivity and can be unpredictable. The resistance level can generally be reduced by an additional 40% by doubling the length of the ground electrode. It is sometimes impossible to drive ground rods deeper – in areas composed of rock, for instance. In these cases, alternative methods including grounding cement are viable.

Diameter of the ground electrode

Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 10%.

Number of ground electrodes

Using multiple ground electrodes provides another way to lower ground resistance. More than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods must be at least equal to the depth of the driven rod.

The ground electrodes’ spheres of influence will intersect and the resistance will not be lowered without proper spacing. Table 1 provides various ground resistances which can be used as a rule of thumb.

Table 1: Ground resistances for use as a rule of thumb.

Type of soil

Soil resistivity RE

Earthing resistance

Ground electrode depth (metre)

Earthing strip (metre)

ΩM

3

6

10

5

10

20

Very moist soil,
swamplike

30

10

5

3

12

6

3

Farming soil loamy
and clay soils

100

33

17

10

40

20

10

Sandy clay soil

150

50

25

15

60

30

15

Moist sandy soil

300

66

33

20

80

40

20

Concrete 1:5

400

160

80

40

Moist gravel

500

160

80

48

200

100

50

Dry sandy soil

1000

330

165

100

400

200

100

Dry gravel

1000

330

165

100

400

200

100

Stoney soil

30 000

1000

500

300

1200

600

300

Rock

107

Ground system design

Simple grounding systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding. Complex grounding systems consist of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops.

These systems are typically installed at power generating substations, central offices, and cellphone tower sites. Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances.