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This is an interpretation of IEEE Std C57.12.20-2005. Interpretations are issued to explain and clarify the intent of a standard and are not intended to constitute an alteration to the original standard or to supply consulting information. Permission is hereby granted to download and print one copy of this document. Individuals seeking permission to reproduce and/or distribute this document in its entirety or portions of this document must contact the IEEE Standards Department for the appropriate license. Use of the information contained in this document is at your own risk.
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Subclause 7.5.4.2 (Tank grounding connector) provides for a tank grounding connector for units with single high-voltage cover bushings (Figure 7, Figure 8 and Figure 9). Is it that a connector is mandatory for these unit types and optional for units with two high-voltage cover bushings?
This interpretation is correct. The tank ground provision and connector is
mandatory on single cover mounted HV bushing units and optional for units
with two HV bushings (whether cover or side-wall mounted). On all units
with two HV bushings, the tank ground provision (threaded boss) is
required, but the connector is not required. Most manufacturers will
furnish the connector as an option in this case.
Subclause 7.5.4.3 (Low-voltage grounding connection) provides for the connection of the low voltage neutral terminal to the tank. Should this connection be via the low voltage grounding provision (7.5.4.4) or via the tank grounding provision (7.5.4.1)? Also, is this provision only mandatory for units with single high-voltage cover bushing and low-voltage tank bushings (Figure 7 and Figure 8) and not for units with two high-voltage cover bushings (Figure 11 and Figure 12)?
Indeed, the LV grounding connection provides the direct connection of the
LV neutral to the tank. The tank is then in turn grounded through the tank
ground provision and connector. The LV neutral ground is normally
accomplished by a copper strap is bolted between the X2 bushing and a
threaded boss located immediately underneath the neutral (X2) bushing.
The LV grounding connection is mandatory for units with one HV bushing, and
is optional for units with two HV bushings.
Can there be a low-voltage grounding provision (7.5.4.4) for units with low-voltage cover bushings (Figure 9 and Figure 12) or is the tank grounding provision (7.5.4.1) sufficient?
A LV neutral grounding provision could be provided as an option for units with cover LV bushings. This would not be a normal requirement and therefore is not included in the standard.
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