Prof. D. WiltonECE Dept.

Notes 5

ECE 2317 ECE 2317 Applied Electricity and MagnetismApplied Electricity and Magnetism

Notes prepared by the EM group,

University of Houston.

GroundingGrounding

Grounding: connecting a conductor to the physical earth

• Safety reasons• Noise reasons

equipment

water pipe

Grounding (cont.)Grounding (cont.)

• Ensures that there is no voltage drop between equipment and ground• Ensures that there is no electric field outside of equipment• Ensures that there is no charge build up on equipment

equipment

water pipe

+

-VAB = 0

Grounding (cont.)Grounding (cont.)

equipment

ground

Ground symbol:

Note: grounding is NOT is the same thing as a reference point.

(However, when a circuit is grounded, the ground is OFTEN chosen as a reference point of zero volts.)

ExampleExample

reference point (choose 0 [V])

Find: voltage at the ground VG

12V A V B

+- 100 12 [V]

ground

A

B

0 12V B 12[V]GV

Grounding (cont.)Grounding (cont.)

ground

hot

neutral

(grounded back at junction box)

Grounding (cont.)Grounding (cont.)

ground

hot

neutral

+

-V

I

The neutral wire is grounded back at the junction box.

junction box

Note: The voltage of the neutral wire at the device is not the same as ground, because the neutral wire has a resistance.

wall

Ground ProtectionGround Protection

ground

hot

neutral

The metal casing of an electrical device is normally connected to ground.

GFCI (Ground Fault Circuit Interrupter)GFCI (Ground Fault Circuit Interrupter)

hot

neutral

Ih

In

Ig = Ih - In

GFCI

Circuit trips when Ig > 5 [mA]

Double Insulated ToolDouble Insulated Tool

hot

neutral

Ih

In

The device is surrounded by an insulating plastic case. It does not require a ground.

plastic case