THE VITAL IMPORTANCE OF BONDING

UNSAFE OPERATION Here is an example of unsafe operation. The electrical system of the house is properly connected and grounded to the earth at the service entrance. But a metal fence near the house is connected to the earth by the fenceposts, but is not bonded to the electrical system ground. Here is the sorry tale: Our hapless victim is sanding old paint off of the fence with a power tool. An accident far away from the house shorts the hot and neutral wires of the 4000-volt feeder circuit together. Several hundred amperes of current flow instantly. With just a few ohms of resistance in the power lines, the 4 KV is divided along the length of the wires. The ground of the pole transformer, house, and power tool are raised 1000 volts above earth ground by the high current and the voltage dropped on the neutral wire. Because the fence has NOT been raised above ground by the current, it is STILL AT GROUND POTENTIAL! The connection between 1000 volts on the tool, and ground on the fence, is completed by the hapless victim, electrocuting him! The fuse blows farther down the line, ending the current and the voltage rise of the house electrical system. Rescuers arriving at the scene have no idea why the victim was electrocuted, since no faults can be found in the electrical system or in the tool. Many unsolved electrocutions are caused by this effect. This web page, combined with power company records, can be used to identify the cause of such fatalities. Notice that a lightning strike can produce the same effect, also far from the site of the strike.

SAFE OPERATION Here is an example of safe operation. The electrical system of the house is properly connected and grounded to the earth at the service entrance. Here, the metal fence near the house is connected to the earth by the fenceposts, And the fence IS bonded to the electrical system ground. Here is the happier story: Our potential victim is sanding old paint off of the fence with a power tool. An accident far away from the house shorts the hot and neutral wires of the 4000-volt feeder circuit together. Several hundred amperes of current flow instantly. With just a few ohms of resistance in the power lines, the 4 KV is divided along the length of the wires. The ground of the pole transformer, house, and power tool are raised 1000 volts above earth ground by the high current and the voltage dropped on the neutral wire. Because the fence has is bonded to the house electrical system ground, it is also raised to the same potential. The connection between 1000 volts on the tool, and 1000 volts on the fence, is completed by the victim. But since both are at the same potential, no current flows through him! He is not a victim! The fuse blows farther down the line, ending the current and the voltage rise of the house electrical system. Rescuers do not arrive, because nothing happened to the man. He continues to work on the fence, annoyed at the brief power interruption. Because an inexpensive #4 bonding jumper was connected between the electrical service ground and the metal fence, the man's life was saved.

IDENTIFYING HAZARDOUS LOCATIONS SUCH AS THE ABOVE

The following locations are places where such lethal voltages may momentarily develop whenever a short circuit occurs in high voltage power distribution wiring:

1. Wherever a person using a grounded power tool can come in contact with any metal object electrically connected to the earth, but that is not bonded to the power system safety ground.
2. Whenever someone is simultaneously touching a pipe that has not been properly bonded to the power system safety ground, and any object that has been bonded to the safety ground.
3. Wherever a person using grounded electronic equipment can come in contact with any metal object electrically connected to the earth, but not bonded to the power system safety ground.
4. Wherever a person standing on wet earth can come in contact with any metal object electrically connected to the power system safety ground. But if metal objects surrounding the wet earth in all directions are bonded to the power safety ground, then there is no danger.
5. Where the rebars in the building's concrete have not been bonded to the power system safety ground, contact with a concrete floor and a grounded electrical device can cause the hazard.
6. Where the person is in contact simultaneously with the electrical systems of two different buildings, or two different electrical services in the same building, unless they are bonded together.
7. Where there is no grounding electrode system, or where it has been damaged.
8. Wherever a signal cable runs from one building to another.

   When I was on a team that serviced an energy control system that interconnected several tall buildings, we discovered that lightning hits did no damage in the building that was struck, because its safety ground was raised above earth ground by the strike. But we always got control cable interface damage in the buildings on either side of the struck building. Those buildings did not have their safety grounds raised above earth ground. The lightning voltage went down the cable from one building to the next, and so the entire voltage of the strike appeared between the cable and the safety ground in the adjacent buildings. Poof go the interfaces - and anyone who happens to be touching the cable - in an adjacent building.

Additional hazards:

In addition to faults on high voltage distribution lines, the same kind of hazard can exist in other places due to leakage currents or local short circuits in buildings. Here is a list of places where such hazards are likely to exist:

1. A common location not readily noticed is the farm outbuilding serviced by a feeder from the house. If there is no ground stake at the outbuilding, a substantial voltage can develop between the safety ground and the earth floor of the outbuilding.
2. Where interconnected equipment, such as sound equipment at a concert, is powered from multiple breaker-box circuits. The hazard is larger if the pieces of equipment are powered by different subpanels.
3. Likewise, stage lighting equipment can cause the same hazards. All lighting battens and pipes must be bonded to the electrical system safety ground at the service panel.
4. In industrial settings, the power for a large motor and for its control circuits might be on different grounds.
5. Where power is being borrowed from one building to supply construction work in another building.
6. In food preparations areas where much of the equipment is on different circuits due to high electrical loads.

What to do

Remedies:

• In each case, the existence of different grounds constitutes a hazard. Methods must be developed for dealing with these different grounds in a way to render them safe without creating ground loops that conduct high currents.
• In new construction, the answers are quite obvious. Each outlet in such an area must have an isolated safety ground going all the way back to the main service entrance, with no other outlets connected to the same ground wire.
• All utility services, including cable TV, water, gas, telephone, and sewer must be bonded to the house electrical ground.
• All metal in the construction, including rebar, support columns, fences, swimming pool plumbing and rebar, and anything else metal or concrete that is in contact with earth, must be bonded to the house electrical ground.
• For sound systems, a subpanel should be provided to supply all outlets being used for interconnected sound equipment. It should supply no other outlets, and all sound equipment related outlets should be on the same phase.
• Ground fault circuit interrupters (GFCI) must be connected to all circuits where ground faults might occur on the protected circuit, or on circuits with outlets adjacent to connected equipment that might constitute a hazard. If both surge suppression and GFCI are to be used, the surge suppression must be on the service panel side and the GFCI on the load side.
• Placing a surge suppressor in the electric meter socket is an excellent idea. Most electric companies now offer such equipment.
• For lightning and surge suppression, MOVs (metal-oxide varistor) are a good idea from phase to neutral. Do not connect MOVs from phase to safety ground, from neutral to safety ground, or from phase to phase. Why not? If a surge comes, one MOV fires. The surge then finds it easier to go THROUGH the protected equipment to the fired MOV than it does to fire another MOV. Fried equipment and operators result.
• If surge protection is necessary between all conductors (phase, neutral, and safety ground), a three terminal gas discharge device is better than a cluster of MOVs. When the surge fires the first gap, the other two gaps become ionized, preventing the surge voltage from rising above the gas conduction voltage in the other conductors, instead of the firing voltage. The result is that no voltage difference rises above either the gas conduction voltage of the device, or the rated insulation of the wiring. The surge does not pass through the protected device. If the entire grounding system is properly bonded, the operator should not feel the surge either.
• For additional protection, inductors that have very little reactance at ordinary power line frequencies, but have high reactance to surge frequencies, can be placed upstream of the MOVs or gas discharge devices.
• Three terminal gas discharge devices were combined with inductors to solve the lightning damaged interfaces on signal cables between buildings.
• To reduce both surges and noise, .1 uF 2 KV capacitors can be placed from the phase to the neutral.
• If existing wiring must be used with interconnected equipment powered by different breaker-box circuits, the following precautions must be observed for safety:
  • NEVER disconnect the safety ground on any piece of equipment.
  • Use a meter to see if the grounds are at different potentials. If they are, demand another power source, or repairs on that one.
  • Never run a piece of wire to connect the grounds of the two different circuits together. Ground loop currents of tens or even hundreds of amps could flow. Disconnecting such a current could cause electrocution.
  • Remember that extension cords are not allowed to be run through doors or windows.
  • Get rid of old two-wire amplifiers with ground switches, or convert them to three-wire operation.
  • Use GFCI equipment on each power distribution strip. This will shut down power faster if a ground fault current develops.
  • An isolation power transformer or a motor-generator can be used to power the equipment served by one circuit, provided that the neutral and safety grounding terminal on the load side is connected to the safety ground of the other circuit used with heavy gage wire.
  • Use wireless signal links where possible.
  • Ground loop isolation transformers can be used in signal lines to prevent high currents from flowing from one ground to the other. If a potential difference appears between the two grounds during normal use, use TWO ground loop isolation transformers, one at each end of the cable run.
  • NEVER interconnect equipment connected to two different electrical services
  • Mike the guitar amp.
  • Never let a signal cable carry ground loop or ground equalizing currents arising from the use of two different breaker-box circuits. The person who disconnects that cable could be electrocuted. The cable could also be damaged by heating.
• And if you are in doubt of the safety or bonding of a system, DON'T USE IT!

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