Equipotentiality is one means of preventing electric shock. Grounding is a specific kind of equipotentiality.

For some kinds of power distribution systems, the resistance of the grounding circuit in an electrical product is critical to establishing equipotentiality.

This paper explains equipotentiality and how it provides protection against electric shock. This paper also describes the grounding circuit for the various power distribution systems. Finally, this paper shows the effect of the product grounding resistance on the voltage of accessible parts, and validates the value required by various safety standards.

Probably the most well-known protective scheme is that of protective

Grounding.

Grounding is a supplementary protection scheme. It is equivalent to

Supplementary insulation in a double-insulation protective scheme.

For both protective grounding and double-insulation schemes, the principal protection is Basic Insulation.Grounding and supplementary insulation are supplementary protection

Schemes because they are only called into operation in the event of a failure of the principal safeguard, Basic Insulation.

The operating concept for product safety is that safeguards be interposed

between the body and the hazardous electric energy.

Furthermore, protection is provided by both a principal safeguard (for normal operating conditions) and a supplementary safeguard (for the event of failure of the principal safeguard).

For electric shock, the principle safeguard is Basic Insulation.

In the event of failure of Basic Insulation, there are two supplementary

safeguards, (1) Supplementary Insulation and (2) Protective Grounding.

Grounding provides protection against electric shock by creating an

Environment that approaches an equipotential environment.

An equipotential environment is an environment where there is no potential difference between any two conductors. If there is no potential difference, then there can be no current between those conductors

Faraday Cage is the name given to a device that shields its inside from electric fields generated by static electricity. Usually a complete conductive shell, it collects stray charges and, because like charges repel, stores them on the outside surface (where they can be further apart than on the inside). The electric fields generated by these charges then cancel each other out on the inside of the cage.

Reference: Museum of Science, Boston.

(http://www.mos.org/sln/toe/glossary.html#faradaycage)

The Faraday Cage also creates an equipotential environment (or Equipotential Cage).Because all of the conductors are connected together, there is no potential

Difference between any two conductors. No matter how much current or how high the voltage, there is no potential difference in the cage. If there is no potential difference between any two conductors, then there is no current in the body.

(This model assumes the resistance of the conductors and connections that comprise the cage is negligible.)

The Equipotential Cage can be grounded or it can be isolated from ground. An equipotential environment is established regardless whether the Cage is grounded or not. For the ungrounded Cage, there is no current return path. Regardless of the Cage voltage, there is no potential difference within the Cage, and there is no current in the body.

For the grounded Cage, the current path is that of least resistance (in

Accordance with Kirchhoffs Laws). Since there is no resistance in the

Conductors that make up the Cage, there is no voltage across the body, and there is no current in the body.

In these Equipotential Cage equivalent circuits, there is no current through the body because the body resistance is shorted out by the Cage conductors.

If the Equipotential Cage is grounded, then the resistance of the cage

Conductors and connections is critical to maintaining the equipotential

Environment.

The concept of the equipotential environment is quite simple: Connect all

Conductive parts of the environment together.

Notice that the parts need not be connected to ground to create the

equipotential environment.

If the conductive parts are connected only to each other, then in the event of a fault of Basic Insulation, there is no current as there is no return circuit path.

If there is no current, then the resistance of the Equipotential Cage conductors and connections is inconsequential. This concept is important because in some power distribution schemes the grounding path resistance is relatively high.

These schemes are discussed elsewhere in this paper.

Normally, there is no cage. To accomplish the equipotential environment, the accessible conductive parts of electrical equipment must be connected to other conductive parts in the local environment. A wire connects the equipment to the local conductive parts.

In principle, the conductive parts need not be connected to ground.

In practice, most conductive parts such as water pipes, heating ducts, and

Similar parts are connected to ground. So, for convenience, all conductive

Parts are connected to ground. The ground provides the common connection for all conductive parts, and no special connections need be made.

We would not be concerned with the ground circuit characteristics except that most power systems are grounded. This means the grounded equipotential cage provides a current path back to the electric energy source. The design of the cage must account for fault currents through the cage conductors.

The world has several different power system grounding configurations.

In IEC standards, the various power system configurations have designations based on the connection of the Neutral wire to earth, and on the connection of the Protective Conductor connection to earth. The IEC designation is comprised of two letters, the first for the Neutral conductor, and the second for the Protective Conductor.

The letters designate the means by which the respective conductor is

Connected to earth.

The TT system has two, separate ground rods.

The neutral is connected to its ground rod at the service entrance.

The protective conductor is connected to its own ground rod, remote from the neutral ground rod. In some cases, the ground rod may be the steel frame of the building. In any case, there is no direct copper connection between the enclosure and the supply system.

The IT system also has two, separate ground rods.

The neutral is connected through an impedance to its ground rod at the service entrance.

The protective conductor is connected to its own ground rod, remote from the neutral ground rod. In some cases, the ground rod may be the steel frame of the building. In any case, there is no direct copper connection between the enclosure and the supply system.

One characteristic of the IT system is that the system is tolerant of a fault to ground. That is, a fault to ground does not operate the circuit breaker, so the system remains operational. (An alarm identifies the fault to ground, but the system continues to operate.) As in the TT system, there is no direct copper connection between the enclosure and the supply system.

The TN-S system has a single ground rod.

At the service entrance, the neutral conductor is connected to the ground rod.The protective earth conductor is connected to the neutral at the service

Entrance. The S in the designation means that the protective earth conductor is a separate system conductor.

Unlike the TT and IT systems, in the TN system the equipment and man are grounded through different paths. If the current through the different paths is different, then a potential difference will occur between the equipment and the man, and current will pass through the man. To minimize the potential difference due to the difference between the equipment and the man, it is imperative to keep the equipment ground circuit resistance as low as practicable.

This paper analyzes the TN-S grounding resistance and voltage differences between the equipment and the man.

The TN-C system has a single ground rod.

The neutral conductor is connected to the ground rod located at the service entrance.

The protective earth conductor is connected to the neutral in the equipment. There is no separate protective conductor.

The C in the designation means that the protective earth conductor is

Combined with the neutral conductor. The TN-C system is used for electric dryers, electric ranges, and electric water heaters in the United States.

To analyze the circuit, the various circuit parameters must be known. The

Parameters needed are: source resistance, maximum load, and protective

Conductor resistance.

Electrical engineers design power distribution systems for a specific system voltage drop. The voltage drop cant be zero because sources and conductors have a small but finite resistance. Most systems are designed for 3% typical and 6% maximum voltage drop. Using Ohms Law, the circuit resistances can be calculated from the system voltage drop, Maximum load is the rated load of the over current device (usually a circuit breaker).

The protective conductor is the same size as the phase and neutral conductors.

Therefore, for the purposes of this analysis, the protective conductor has the same resistance as the phase and neutral conductors. (Note that the percent system voltage drop is comprised of both the generator resistance and the conductor resistances; for this analysis, we have assumed that all of the resistance is in the wires.)

This circuit is used for the analysis.

Maximum load current is taken as the circuit breaker rating.

The load voltage is the nominal system voltage minus the voltage drop across the source resistance due to the load current.

The supply source resistance is the system voltage drop divided by the

Maximum load current. It is attributed to the wires, each wire having half of the source resistance.

The protective conductor resistance is the same value as the resistance of the other wires (because it is constructed of the same size wire and is in parallel with the other wires). The protective conductor normally has zero current.

There are three analyses.

The first analysis is: What is the equipment resistance that will maintain no more that 30 volts on accessible parts? The voltage on accessible parts is a function of both the system voltage drop and the fault current.

The second analysis is: What is the voltage on accessible parts when the

Equipment resistance is 0.1 ohm? The voltage is a function of both the fault current and the source resistance (system voltage drop).

The third analysis is: What is the voltage on accessible parts when the

Equipment resistance is 0.1 ohm and when the fault is a short circuit, i.e., when the fault current is limited only by the resistance of the source and the 0.1 ohm equipment grounding resistance?

The power system resistances are calculated from this circuit.

At maximum current, the load voltage is 120 V minus 6 percent of the supply voltage, 112.8 volts. This means 7.2 volts (120 - 112.8) is dropped in the source resistance. One-half of 7.2 volts, 3.6 volts, is dropped in each wire.

The resistance of each wire is 3.6 volts divided by 15 amperes, or 0.24 ohms.

These calculations can be repeated for various system voltage drops, 1% up to6%.

The same calculations can be repeated for the different supply systems, 120 V, 20 A, and 230 V, 16 A.

This table shows some representative source resistances for the different power sources and for different system voltage drops. Source resistance approaches 0.5 ohm maximum, and 0.1 ohm minimum. As previously mentioned, engineers design for 3% system voltage drop, or about 0.2 ohm source resistance.

Note that as system voltage drop goes down, the source resistance goes down. Logical!

Note also that source resistance is a function of the maximum current of the system, not the voltage of the system.

This schematic shows the normal-current (blue) and fault-current paths in the TN-S system.

The fault current is created by an insulation fault. This fault can be any

Resistance from megohms to zero ohms (i.e., short-circuit). For this analysis, only those resistances that cause fault currents exceeding the circuit-breaker rating are considered.

The fault current divides, most of it through the protective conductor, and

some through the body to earth. This analysis does not address the current, but instead examines the voltage at an accessible (grounded) part of the equipment.

The question is: If the insulation fault allows a current of twice the maximum load current (i.e., twice the rated current of the circuit breaker), what is the maximum value of equipment resistance that will limit the voltage of an accessible part to 30 volts?. (At twice rated current, the circuit-breaker will take up to 2 minutes to operate; so, for those 2 minutes, safety must be assured by the grounding circuit.)

The system voltage drop in this example is 3%. Therefore, the source

Resistance is 0.12 ohms (see #20). The voltage drop across the 0.12 ohm

Resistance of the protective conductor is 3.6 V.

For 30 V at accessible grounded parts of the equipment, 30 V is the sum of the voltage drop across the protective conductor resistance, 3.6 V, and the voltage drop across the equipment resistance, 30 minus 3.6, or 26.4 volts.

The equipment resistance is 26.4 volts divided by the current, 30 amperes, or 0.88 ohm. This is much greater than the 0.1 ohm required by various safety standards. These calculations can be repeated for various system voltages drops, 1% up to 6%.

This is a display of equipment resistances that give 30 volts at accessible

Grounded parts for various fault currents as a function of system voltage drops.

The resistance calculated on the previous slide (#22), is 0.88 ohm at 30

Amperes and 3% system voltage drop.

The resistance for 30 volts for 30-ampere fault current was re-calculated for system voltage drops from 1% to 6%.

The same calculations are shown for 20- to 60-ampere fault currents. Note that the maximum resistance for all fault currents up to about 80 amperes is greater than the 0.1 ohm required by safety standards!

If the equipment resistance is 0.1 ohm as required by the various standards, what is the voltage at accessible grounded parts for various fault currents? Is the voltage always less than 30 volts?

Once again, the analysis is for a 120-V, 15-A, 3% system voltage drop circuit.

The current is an arbitrary 150 amperes (10 times the circuit-breaker rating).

This current will clearly operate the circuit-breaker in a relatively short time.

Using similar calculations as for the equipment resistance, the accessible part voltage is 33 volts.These calculations can be repeated for various system voltage drops, 1% up to 6%.

This is a display of voltages at accessible grounded parts for various system voltage drops as a function of fault currents.

The voltage calculated on the previous slide (#24), is 33 volts at 150 amperes and 3% system voltage drop.

The voltages for 0.1 ohm resistance for 3% system voltage drop was recalculated as a function of fault current.

The same calculations are shown for 1- to 6-percent system voltage drops.

Note that the maximum voltage for all fault currents from 80 amperes to 220 amperes is less than the 30 volts required by safety standards. So, the 0.1-ohm resistance does a reasonable job of limiting the voltage. Note that circuit breaker operating time decreases rapidly with increasing current. So, the duration of voltage is limited by the current-time operating curves of the circuit-breaker.

Note that as the system voltage drop decreases, the current for 30 volts

Increases. The lower the resistance of the source, the more effective the 0.1-ohm equipment resistance!

If the equipment resistance is 0.1 ohm as required by the various standards,

What is the voltage at accessible grounded parts in the event of a short-circuit?

Clearly, the voltage will not be less than 30 volts? How high is the voltage?

Once again, the analysis is for a 120-V, 15-A, 3% system voltage drop circuit.

The current is limited only by the source resistances and the 0.1-ohm

Resistance of the equipment. (This current will clearly operate the circuit breaker in a relatively short time.) Using similar calculations as for the 150-ampere fault current, the accessible part voltage is 77.7 volts volts.These calculations can be repeated for the three major supply systems, 120 V/ 15 A, 120 V/20 A, and 230 V/16 A as a function of system voltage drop.

This is a display of voltages at accessible grounded parts for short-circuit currents as a function of system voltage drops.

The voltage calculated on the previous slide (#26), is 77.7 volts at 3% system voltage drop.

The voltages for 0.1 ohm resistance and short-circuit current was re-calculated as a function of system voltage drop.

The same calculations are shown for 120 V/15 A, 120 V/20 A, and 230 V/16A supply circuits. Note that the maximum voltage always succeeds the 30 volts required by safety standards. Note that circuit-breaker operating time is minimum under short-circuit conditions. So, the duration of hazardous voltage is limited by the current-time operating curves of the circuit-breaker.

For a TN-S system, grounding does not provide an equipotential environment due to the finite resistances of the equipment grounding circuit. However, equipment grounding through its protective conductor does serve to limit the voltage for low fault currents.

For higher fault currents, another scheme provides protection against electric shock: limited duration of the current through the body by means of automatic disconnection of the supply (operation of the circuit-breaker).

By limiting equipment grounding resistance to no more than 0.1 ohms,

Protection against electric shock (by limitation of voltage) is achieved for fault currents up to about 80 amperes.

The 0.1-ohm limit is easily achievable.

Ohms Law allows prediction of the maximum current at which the voltage on accessible parts will exceed the usual 30-volt electric shock limit. For a nominal 120-V, 15-A, 3% system voltage drop, the maximum current is about 125 amperes.

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