Understanding Grounding

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Old 12-16-12, 08:05 PM
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Understanding Grounding

While learning the electrical basics, I am stuck on the concept of grounding. Few questions I can't seem to find answers to are:

1. Necessity to ground electrical box separately when grounding a switch or outlet. Since those are already grounded and connected to a box with a metal screw, what purpose does the separate ground serve?

2. Grounding to water pipes. Since all wiring is grounded to the panel and connected to a utility ground, as well as, a ground rod outside, why also ground to cold water pipe? Which leads me to the next question:

3. When pipes are grounded, and there is a short somewhere, wouldn't the person taking a shower or washing dishes get electrocuted? I know it's not advisable taking showers during a thunderstorm.
 
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Old 12-16-12, 09:03 PM
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Old 12-16-12, 09:57 PM
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While learning the electrical basics, I am stuck on the concept of grounding.
If you're interested in learning the basics, read Introduction to Article 250 - Grounding and Bonding |Understanding the Basics of Electrical Systems.

Few questions I can't seem to find answers to are:

1. Necessity to ground electrical box separately when grounding a switch or outlet. Since those are already grounded and connected to a box with a metal screw, what purpose does the separate ground serve?
Which would you prefer to have bonded to ground when the device - the switch or receptacle - is unmounted and the circuit is energized - the device or the enclosure?

Tech note: The enclosure, with a circuit fed into it, is an outlet. The device that receives plugs is a receptacle. A ceiling box is an outlet.

2. Grounding to water pipes. Since all wiring is grounded to the panel and connected to a utility ground, as well as, a ground rod outside, why also ground to cold water pipe?
Grounding to water pipes, or bonding to the cold water supply? With the grounded conductor or the grounding conductor?

Which leads me to the next question:

3. When pipes are grounded, and there is a short somewhere, wouldn't the person taking a shower or washing dishes get electrocuted?
If the cold water supply is a metal pipe that extends at least 10' into the earth outside the structure, isn't it thereby grounded?

I know it's not advisable taking showers during a thunderstorm.
Is that true? How do you know that that is true?

You may be confused, as many are, by the use of "grounding" in the NEC to mean either "connecting to the earth" or "connecting to a conductive body for the purpose of providing a low-impedance path to the source winding." Reading both the reference Furd linked to and the reference linked above should help clarify this area.
 
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Old 12-17-12, 07:37 AM
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The following answers are relatively simplistic. Electrical theory can get quite complicated if you let it.

1. Necessity to ground electrical box separately when grounding a switch or outlet. Since those are already grounded and connected to a box with a metal screw, what purpose does the separate ground serve?
Most (cheaper) switches and receptacles have a small cardboard washer to hold the screw from getting lost. Also, the screw isn't really attached to the receptacle, it's just floating in a large hole (which works nicely for ensuring it's straight). Because the mounting screw isn't securely attached to the receptacle, the electrical connection is marginal, requiring both the metal box and the receptacle/switch to have a secure ground.

There are self-grounding receptacles which have a captive screw mechanism that provides a secure ground and removes the requirement to connect the ground wire to both box and outlet.

2. Grounding to water pipes. Since all wiring is grounded to the panel and connected to a utility ground, as well as, a ground rod outside, why also ground to cold water pipe?
This gets into what's called bonding. All metal utilities in your house need to be bonded together (water, electrical, cable tv, telephone, HVAC ducting, etc). Bonding serves two main purposes:
1) It provides a solid ground for anything metal in your house. If a hot wire accidentally makes contact with your water piping, you want to be sure that the circuit breaker trips immediately. If it didn't have a good ground, the pipe could be 'hot' too, which you'd then likely get electrocuted the next time you touch a faucet.
2) It ensures the potential (voltage) between metal systems in your house are all grounded the same. If your cable TV isn't properly grounded, the difference between the outside wire on your coax cable could be a couple (or a lot) volts different from your electrical ground. If you touched both, you'd get a shock. More often, the cable TV receiver or DVR is the one that gets the 'shock' and gets fried.

3. When pipes are grounded, and there is a short somewhere, wouldn't the person taking a shower or washing dishes get electrocuted?
Electricity follows the easiest path to ground (Note that this concept is overly simplified, but good enough for this conversation). You want that easiest path to be via the grounding/bonding wire back to the neutral/ground at your main panel. Without a good bond, you could be that easiest path.
When you're in the shower, you are still not the easiest path, that wire going back to the main panel is. So you wouldn't feel a thing.

I know it's not advisable taking showers during a thunderstorm.
Your electrical system is designed and built for 100 or 200 amps throughout your entire house. Lightning can be upwards of 10,000 amps, which easily melts wires, jumps between metallic paths, etc. The goal of your grounding system is to protect against static buildup of induced voltages from nearby storms. There's not much that can protect against a direct lightning strike.
 
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