1. How to Use Electricity and Live to Tell the Story
Electrical Safety or
How to Use Electricity and Live to Tell the Story
James W. Bonner
Thomas G. Howard
© 2007
Revised:

2. Chapter I
How do we know when electricity is dangerous?

3. Why is it more dangerous to touch a live wire while standing outside on the ground as opposed to touching a live wire while standing inside your home on the carpet?
It’s all about your body becoming the electrical conductor and the resistance in series with your body.
As resistance in the circuit decreases, the current through your body increases.

4. So, clearly you must avoid contacting an electrical wire when there is a low resistance path through your body.
Let us consider the electrical distribution system. Specifically, how does the electricity get to our home?

5. The center tap of the transformer is connected to earth ground.
Several thousand Volt
230 Volt
Transformer
115 Volt
The center tap of the transformer is connected to earth ground.
Transformer

6. Current through the body is BAD!
Live “Hot” Wire
115 Volt
One side of the electrical system is grounded. So, when you are outside, you are standing on one side of the electrical source.
Therefore, if you touch a live (hot) wire, there will be a difference in potential (i.e., voltage) between your feet and hand which will result in a current flow through your body. This is not good.
Current through the body is BAD!

7. Little resistance to current flow.
Live “Hot” Wire
115 Volt
If your feet make a good connection with earth (dirt), there is a low resistance path through your body which means a high current will flow. Only about 0.01 Ampere through the heart is required for death.
Standing on wet ground is a good example of a low resistance path.
Little resistance to current flow.

8. MUCH resistance to current flow.
Live “Hot” Wire
115 Volt
Carpet / padding
Wood subfloor
Concrete blocks or pad
MUCH resistance to current flow.
However, if you are inside standing on carpet, there is much resistance between your feet and earth ground.
The shock will probably not be fatal.

9. Chapter II
Safe Circuit Design

10. A power system with no secure connection to earth ground is unpredictable from a safety perspective. There's no way to guarantee how much or how little voltage will exist between any point in the circuit and earth ground. Because we stand on the ground, we need to know there will be no difference in potential (i.e., voltage) between our body and the electrical equipment.

11. By connecting one side of the power system's voltage source to earth ground, at least one point in the circuit can be assured to be electrically common with the earth (where you stand) and therefore that side will present no shock hazard. In a simple two-wire electrical power system, the conductor connected to ground is called the neutral, and the other conductor is called the hot:

12. These points are the same; “common”.
As far as the voltage source and load are concerned, grounding makes no difference at all. Grounding exists purely for the sake of personnel safety, by guaranteeing that at least one point in the circuit will be safe to touch (i.e., zero volt potential between the equipment and ground). The "Hot" side of the circuit, named for the possibility of shock hazard, will be dangerous to touch.
These points are the same; “common”.

13. This shock hazard in a simple power circuit is important to understand
This shock hazard in a simple power circuit is important to understand. The following series of illustrations are based on common household wiring systems.

14. If we consider a simple, household electrical appliance such as a toaster with a conductive metal case, we find that there should be no shock hazard when it is operating properly. The wires conducting current to the toaster's heating element are insulated from touching the metal case (and each other) by rubber or plastic.

15. Current through the body.
However, if one of the wires inside the toaster were to accidentally come in contact with the metal case, the case will be electrically connected to the wire, and touching the case will be just as hazardous as touching the wire. Whether or not this presents a shock hazard depends on which wire accidentally touches the case. If the "hot" wire accidentally touches the case, it places the user of the toaster in danger.
Current through the body.

16. On the other hand, if the “neutral” wire contacts the case, there is no danger of shock.
These points are at the same potential (i.e., voltage).

17. To help ensure safety, engineers try to design appliances in such a way as to minimize hot - conductor contact with the case. Ideally, of course, you don't want either wire accidentally coming in contact with the conductive case of the appliance. However, there is still a problem if the plug can be reversed in the outlet. Then, the conductor more likely to contact the case might very well be the "hot" one.
Same-size prongs

18. Appliances designed this way usually come with a "polarized" plug, one prong of the plug being slightly wider than the other. Electrical outlets are also designed like this, one slot being wider than the other. Consequently, the plug cannot be inserted "backwards," and conductor identity inside the appliance can be guaranteed. Remember that this has no effect whatsoever on the basic function of the appliance. It is strictly for the safety of the user.
One prong is larger

19. Some engineers address the safety issue simply by making the outside case of the appliance nonconductive. Such appliances are called double-insulated, since the insulating case serves as a second layer of insulation above and beyond that of the conductors themselves. If a wire inside the appliance accidentally comes in contact with the case, there is no danger presented to the user of the appliance because the case is non-conductive.

20. Other engineers tackle the problem of safety by maintaining a conductive case, but using a third conductor to firmly connect that case to ground. This is the well-known round pin on the power plug.

21. The third prong on the power cord provides a direct electrical connection from the appliance case to earth ground, making the two points electrically common. If they are electrically common, then there can be NO difference in potential (voltage) between them. If the hot conductor accidentally touches the metal appliance case, it will create a direct short-circuit back to the voltage source through the ground wire, tripping any overcurrent protection devices (e.g., fuses, breakers). The user of the appliance will remain safe.

22. This is why it's so important never to cut the round prong off a power plug when trying to connect it to a two-prong receptacle. If this is done, there will be no grounding of the appliance case to keep the user safe. The appliance will still function properly, but if there is an internal fault bringing the hot wire in contact with the case, the results can be deadly.

23. If a two-prong receptacle must be used, a two- to three-prong adapter can be installed with a grounding wire attached to the receptacle's grounded cover screw. This will maintain the safety of the grounded appliance while plugged in to this type of receptacle.
Cutting the ground pin from a power cable or extension cord, ruins its safety feature forever.

24. Additional Safety Measures
Chapter III
Additional Safety Measures

25. Electrically-safe engineering does not necessarily end at the load
Electrically-safe engineering does not necessarily end at the load. A final safeguard against electrical shock can be arranged on the power supply side of the circuit rather than at the appliance itself. This safeguard is called ground-fault detection.
This is especially useful when the safeguards mentioned here have been defeated or they have become ineffective for other reasons.

26. Removing the round, ground pin from the electrical plug or having a frayed, exposed wire would result in a shock hazard. Touching the bare, hot wire or having the hot wire touch the metal case could cause current flow through the body.
Exposed wire here

27. In a properly-functioning appliance (shown above), the current measured in the hot conductor should be exactly equal to the current in the neutral conductor, because there's only one path for current to flow in the circuit. That is:
Good !

28. With no fault inside the appliance, there is no connection between circuit conductors and the person touching the case, and therefore no shock. That is, no current flowing through the body.

29. If, however, the hot wire accidentally contacts the metal case, a current will flow through the person touching the case. The presence of a shock current will be manifested as a difference of current between the two conductors at the outlet:
Bad !

30. This difference in current between the "hot" and "neutral" conductors will only exist if there is current through the ground connection, meaning that there is a fault in the system. Therefore, such a current difference can be used as a way to detect a fault condition.

31. GFCI
If a device is set up to measure this difference of current between the two power conductors, a detection of current imbalance can be used to trigger the opening of a switch, thus stopping current flow and preventing serious shock.

32. GFCI
Such a device is called Ground Fault Circuit Interrupter, or GFCI for short.
When working properly, the GFCI will open the current path before the person can feel the “shock”.
Also, GFI

33. The GFCI is compact enough to be built into a power receptacle
The GFCI is compact enough to be built into a power receptacle. These receptacles are easily identified by their distinctive "Test" and "Reset" buttons.

34. The big advantage with using this approach to ensure safety is that it works regardless of the appliance's design. Of course, using a double-insulated or grounded appliance in addition to a GFCI receptacle would be better yet, but it's comforting to know that something can be done to improve safety above and beyond the design and condition of the appliance.

35. We Should Always Leave Electrical Work to the Experts
Chapter IV
We Should Always Leave Electrical Work to the Experts

36. Connected outside the breaker box.
There is another, very subtle, problem that will pose an extremely dangerous situation. That is when the neutral (usually white) and safety ground (usually bare or green) are connected together outside the breaker/fuse box.
Circuit Breaker box
or Fuse box
Connected outside the breaker box.

37. Connected outside the breaker box.
Now, the neutral and ground wires are in parallel, so the return current has two paths.
Consequently, there will be a potential (voltage) on the ground wire due to resistance in the wire.
This potential may be small, but a person making a good connection to earth ground will conduct appreciable current – it does not take much for injury or death.
Connected outside the breaker box.

38. REVIEW
Power systems often have one side of the voltage supply connected to earth ground to ensure safety at that point.

39. REVIEW
The "grounded" conductor in a power system is called the neutral conductor, while the ungrounded conductor is called the hot conductor.

40. REVIEW
Grounding in power systems exists for the sake of personnel safety, not the operation of the load.

41. REVIEW
Electrical safety of an appliance or other load can be improved by good engineering:
polarized plug
double insulation
three-prong grounding

42. REVIEW
A Ground Fault Circuit Interrupter (GFCI) works by sensing a difference in current between the two conductors supplying power to the load. There should be no difference in current at all. Any difference means that current must be exiting the system by some means other than the two main conductors, which is not good. A significant current difference will automatically open a switch in the GFCI, thereby stopping current flow completely.

43. Improper Connections
Never connect the neutral and ground wires together except in the breaker / fuse box.

44. It’s always about your body becoming the conductor!
Remember:
It’s always about your body becoming the conductor!
“Hot” wire
Path for current

45. The End