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Tech Tip 14

Proper Equipment Grounding Prevents Shock Hazards


SYMPTOMS

  • Shock injury to humans or animals
  • No equipment grounding conductor
  • No neutral-to-ground bond at main panel

CAUSES

  • Earth is only ground fault return path
  • High-impedance ground fault path

SOLUTION

  • Use equipment grounding conductors to provide low impedance path for fault current

One reason for grounding electrical equipment is to prevent injury and death when equipment insulation fails. Proper grounding increases safety by allowing failed equipment to be quickly de-energized. Common grounding mistakes can be a hazard.

CONDUCTOR GROUND FAULT RETURN

Consider the properly grounded system shown in Figure 1. With a ground fault at the equipment as shown, the only impedances limiting the fault current through the breaker are the impedances of the conductors, splices and terminations. If the total impedance is assumed at one ohm, the 20 ampere breaker will conduct 120 amps and trip the circuit quickly to extinguish the fault.

 

Figure 1: Properly Grounded System

EARTH GROUND FAULT RETURN

Now consider a case where earth is the only return for ground fault current (Figure 2). The total circuit impedance is that of the conductors, splices, terminations and the earth ground impedance. If the total circuit impedance is assumed at 10 ohms, only 12 amps will flow through the breaker. The breaker will not trip and the motor frame will remain energized at near line voltage (120 volts) indefinitely. Anyone contacting this motor frame could suffer severe injuries or even death. Note the assumed impedance may be much lower than reality. Utilities often assume a driven ground rod has an impedance of 25 ohms. This breaker will conduct less than 5 amps with a circuit impedance of 25 ohms.

 

Figure 2: Improperly Grounded System

Since earth grounds may have impedances 10 to 100 times higher than ground conductors, significant shock hazards can occur when relying on an earth return for ground fault current. Using a ground conductor or earth ground as a ground return path has little effect on the normal operation of most equipment. Therefore, hazards due to using the earth as the sole ground return path often go unnoticed for years.

SUMMARY

Maintaining a low impedance for ground fault current is vital to the proper operation of breakers and fuses. Using equipment grounding conductors will help establish this low impedance. Using earth as the only ground return path will severely limit the amount of current the breaker will conduct and inhibit its ability to clear ground faults.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.