Joints grounding conductors

Grounding by attaching the device enclosure to the machine with bolts or other approved means shall be considered satisfactory if all paint and dirt are removed from joint surfaces. Moving machine parts having metal-to-metal bearing surfaces shall not be considered as a grounding conductor. 

Since the three-phase sheath conductors are short-circuited and grounded in every major section as illustrated in Figure 3.6, the sheath voltages of three phases are equal at each earthing joint. Assuming sheath currents are balanced among three conductors, the sheath currents do not flow into the earth at each earthing joint ... 

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