Can someone direct me to a good website or briefly explain what the purpose of grounding is? It is not explained in the book I have and I can't seem to get an answer from other lay people. I’d like to learn about this as I only see vague and sometimes inconsistent references to the subject with regard to light fixtures. As far as switches and receptacles, those seem clear cut as the wires in the box are connected, either directly or via pig-tail, to the grounding screw on the light switch or receptacle itself. But in a box where a light fixture goes, I can’t seem to get a consistent response.

For example, I recently took down my three identical outdoor decorative lamps (each with a separate light switch) for cleaning, and this is what I found:

In the first, the ground wire from the power cable was connected to the green of the light fixture and pigtailed to the green screw on the metal mounting bar.

In the second, the ground wire from the power cable was simply just connected to the green wire of the light fixture with a wire nut. No connection to the mounting bar.

In the third, the ground wire from the power cable was first looped around the green grounding screw on the mounting bar and then connected to the green of the light fixture with a wire nut.

Can someone comment on these arrangements? Also, is the rule any different on interior light fixtures? I just purchased a couple of interior light fixtures, and, frustratingly to me, the instructions actually say “ground if necessary.” I thought they were supposed to tell me!

Finally, what are the dangers of improper—or lack of—grounding?

 

The 220/110 voltage for operating your house electrical system is supplied from the "Secondary" of a utility "step-down" transformer". The "Primary" voltage of the transformer could be 7000 volts. If there was an accidental "cross" between the Primary and Secondary at the utility pole your house would be energized with 7000 volts. Contact with a voltage of this value would cause instantaneous death and the house wiring system would be incinerated.To safe-guard against this it's necessary to divert the hi-tension voltage to Ground.This is accomplished by connecting one of the Service conductors that is connected to the transfromer secondary to "electrodes" that are in contact with the earth.The Service conductor so connected is the GROUNDED Service conductor and is always is the "Neutral" conductor of a 220/110 system.The Grounded Service conductor AND the conductors of the interior wiring system connected to the Grounded service condutor at the Service panel are also the IDENTIFIED circuit conductors.The color of an Identified conductor
is ALWAYS White and NO OTHER color.The most effective electrode for "grounding" the Grounded conductor is an underground metal water-service pipe.The connections to Grounding electrodes are the SYSTEM Grounding connections.The EQUIPTMENT Grounding connections protect the "interior" circuits connected to the "Load" side of the Service dis-connect.A 110 volt, 2-wire circuit has a Black(un-grounded) coductor, a White (grounded) conductor, and an Equiptment GROUNDING condutor. THE EQC is parallel to, but insulated from, the White Grounded conductor and forms a low-resistance path for conducting "fault-currents" back to the service panel.If the Black,un-grounded, circuit conductor makes contact with ANY metal surface the EGC will conduct a ground-fault current that will trip the circuit-breaker.Any INSULATED EGC is ALWAYS Green.

 

There is no logical differences in the three examples you cite. All the metal connected to the equipment ground, just with different means.

In your first and third examples, the connections are exactly the same, merely mechanical differences.

In the second example, the manufacturer probably ensured that adequate connection to the mounting bracket was made by the mounting hardware.

Different lamps are designed and built differently. The main rule to follow is to follow the manufacturer's instructions. The manufacturer has designed the product to provide adequate grounding of metal parts with the instructions provided.

 

Section 250.58 states: "Common Grounding Electrode. Where an ac system is connected to a grounding electrode in or at a building as specified in 250.24 and 250.32, the same electrode shall be used to ground conductor enclosures and equipment in or on that building. Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used." Also, section 250.4(A)(4) and (5) also prohibit using a rod to ground equipment. 250.4(A)(4) states "(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path." and (5): "5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path." Note the last sentence that the earth shall not be used an equipment grounding conductor. And last but not least a note to the safety committee: Section "90.1(C) Intention. This Code is not intended as a design specification or an instruction manual for untrained persons."
Hope this helps!
Brian