Equipment Grounding Connections - Part 2

In the last posting, I noted that the grounding connection within an electrical cabinet needs to be accomplished in such a manner that there is a reliable contact between the ground conductor and the enclosure.  To ensure that a reliable contact is achieved, non-conductive materials (e.g. paint or lacquers) must be removed around the connection.

The next component of the grounding connection of the electrical equipment is to ensure that the connection is sufficient to a reliable connection over the length of use of the equipment.  One part of this is to ensure that screws or bolts that connect the ground conductor to the terminal and the ground terminal to the enclosure are properly tightened.  Component and hardware manufacturers publish proper tightness (torque) specifications in data sheets or other documents.  Terminals that are UL Recognized or UL Listed can be found on the UL website associated with the requirements for the specific component.

If tightness specifications are not available, other references can be used.  Common torque specifications for SAE Grade 6 and Grade 7 Carbon Steel are as follows [1,2]:

Bolt Size (inches)	Number of Threads	SAE Grade 6 Carbon Steel		SAE Grade 7 Carbon Steel
		Tightness (lb ft)	Tightness (Nm)	Tightness (lb ft)	Tightness (Nm)
1/4	20	12.5	16.9	13	17.6
3/8	16	43	58.3	44	59.7
7/16	14	69	93.6	71	96.3
1/2	13	106	143.7	110	149.1
9/16	12	150	203.4	154	208.8
5/8	11	209	283.4	215	291.5
3/4	10	350	474.5	360	488.1
7/8	9	550	745.7	570	772.8
1	8	825	1,118	840	1,139

Carbon steel hardware is typically used to connect the ground terminal to the enclosure.  Copper, brass, and aluminum are common materials used in hardware to secure ground conductors to the ground terminal.  The tightness specifications for hardware made of other grades of steel, copper, brass, aluminum varies depending on its size, the number of threads, and the inherent properties of the material itself. 

To properly tighten the hardware that secures the ground conductor to the ground terminal, and the ground terminal to the enclosure must be done to specifications and using appropriate tools (e.g. torque wrenches).  Relying on the strength or lack of strength of the technician or electrician is not appropriate for creating and maintaining a reliable connection.

References:

1.                  Glover, T.J. (ed) (1993), Pocket Ref.  Sequoia Publishing, Inc: Littleton, CO USA, p. 251

2.                  Unit Conversion, Torque Converter, Retrieved 2011 September 04, Available [on-line] http://www.unitconversion.org/unit_converter/torque.html