ETMS2.comBasic Electrical Safety
Electrical Safety Concerns
Electrical Codes, Standards and Guidelines are filled with specific requirements concerning how electrical energy should be safely handled. These requirements usually come out of hard experience based on safety incidents that have occurred and are focused on preventing them from happening again in the future. Any sound safety program must take into account these specific requirements.
Electrical Energy Hazards
However, it is useful to explore the common aspects of electrical safety that underlie these specific requirements. Electrical energy has three general types of hazards associated with it:
Traditional electrical equipment safety has been concerned with risk of electric shock and risk of fire. A new emerging concern is the hazards associated with arc flash, as the evidence has accumulated indicating this is a serious hazard that needs to be taken into account.
Electric Shock or Electrocution
Electric shock occurs when electrical current flows through a person’s body due to exposure to a voltage potential. Electric shock is distinct from electrocution. Not only does the heart’s normal cycle stop during electrocution but the body’s internal organs are also heated in a destructive manner.
Factors in Electric Shock
Many factors impact the severity of injury resulting from electric shock. These include:
- Timing relative to the heart’s cycle;
- Ground path;
- Contact area and pressure;
- Body mass;
- Environment (Wet Locations).
The protections provided by Circuit Breakers and Electric Shock, and Safe Voltage Levels are also discussed below.
Timing
A fatal electric shock can be caused by a current of short duration that flows through the body and interrupts the normal heart function. The heart is highly susceptible to this interruption during a particular period in its normal cycle.
Figure #1 shows an electrocardiogram of the normal heart cycle and the period of time (Ventricular Relaxation) during which it is most vulnerable to current pulse interfering with its function.
When the body is subjected to a pulse of current at this point (T) in the cycle, Ventricular Fibrillation occurs, as shown the electrocardiogram in Figure #2. As a result the heart stops performing its blood pumping function.
When considering the fact that the heart is only highly susceptible to electric shock during a portion of its cycle, it becomes clear that there is an element of chance involved in surviving an electric shock. The severity of the injury is simply a matter of luck when one is subjected to the momentary pulse of current. If one person is subject to a current pulse at the right time in the heart cycle he may walk away unharmed. Another person, subject to the same current pulse, may be hit with it at the wrong time in the heart’s cycle and he will experience a total stoppage of normal heart function. This is the random nature of momentary electric shock.
Ground Path
There are a number of factors that will impact the amount of current the body will draw at any given voltage. Furthermore, the amount of current passing through the heart that could result in a lethal electric shock is even more difficult to determine. The ground path, which the current passing takes through the body, is one major factor. Was the path the current took to ground: hand to hand; hand to foot; foot to foot; or worse hand to chest?
Contact Area and Pressure
The surface area of contact will significantly impact the current draw through the body at any given voltage – greater contact area means more current through the body. Equally significant is pressure of contact with the potential fatal voltage source.
Body Mass
Another major determining factor in the current passing through an individual’s body is their body mass. A simple circuit model of a 1,500 ohm resistor, shunted by a 0.15μF capacitor, is often used by safety professionals. This model is necessarily overly simplified however, its simplicity makes it more useful for practical application. Figure #3 show how this model is used, voltage is measured across the resistor and the “leakage current”, or current through the body, is calculated by V/R = I.
Circuit Breakers and Electric Shock
This model can be used to help illustrate a point about electric shock and circuit breakers. Figure #4 shows the circuit diagram of an unfortunate individual whose body has provided a ground path for a 220VAC source. In this case, the XXX current that this individual is drawing will not even be enough to cause the circuit breaker to trip, as a result the current will be continuous. This will be a case of electrocution where not only the heart normal cycle will stop, but internal organs will be heated in a destructive manner.
There is another reason why a circuit breaker tripping will not protect the human body from electric shock. Even if the current was enough to trip the circuit breaker – circuit breakers normal take a minimum of 1-1/2 cycles to open. That means that the full voltage would contact the body and resulting full current would still pass through the body, causing a potentially lethal electric shock. Although, circuit breakers are important safety devices, they do not provide humans protection from electric shock.
Safe Voltage Levels
The fundamental approach to protect against electric shock is to prevent contact with voltages that can cause a hazardous current through the body. Voltages above 30 volts rms or 42.2 volts peak or 60 volts dc are considered great enough to potentially cause a lethal electrical shock to humans. Much of electric safety is concerned with avoiding human contact with voltages above these levels under both normal operating and fault conditions.
Electric Shock and Wet Locations
The discussion so far has been about dry locations, but what if conductive fluids are involved in the contact surface that delivers the current to the human body? As the resistance is reduced, conductive fluids will greatly increase the amount of current through the human body that will be produced by a given voltage. In these situations the focus is on limiting the amount of current that can be supplied by a circuit under fault conditions.
Normally this is done with a Ground Fault Circuit Interruption (GFCI) device – also known as an Earth Leakage Circuit Breaker (ELCB). There is disagreement among international standard setting committees on the issue of how much current must pass through the body for it to be an electric shock concern. However, within the semiconductor industry, 5mA has been accepted as a maximum limit. This is consistent with the number given in the US National Electric Code. This means that if electric shock is a concern in a wet location, the current available to a ground fault should be limited to 5mA.
Fire or Burn
The risk of fire in electrical systems primarily results from two causes:
- Excessive heat generation usually due to a fault; and,
- Inadequate heat dissipation usually due to a fault.
In both cases sufficient electrical energy must be present for a fault condition to manifest itself in fire risk. Power levels of 240 volt-amps (VA), or greater, are generally considered to have enough energy to present a fire risk if not adequately protected against. One important thing to observe is that risk of fire is driven by power levels, unlike risk of electric shock that is strictly a function of voltage levels. If power is available to drive a fault a risk can start, even if voltage levels are low enough to protect against electric shock.
Excessive Heat Generation
Risk of fire due to excessive heat generation can occur as a result of numerous different types of faults. However, all of these different faults have one thing in common, that is a component or subsystem generates more thermal energy than it was designed for and as a result the system capacity for heat dissipation is overwhelmed causing a fire to start.
Several common faults which result in excessive heat generation are discussed below with possible mitigating designs or controls:
- Poor connections
- Circuit Breakers and Fire Protection
- Short Circuits
- Overload
Fire Due to Poor Connections
One of the most common causes of industrial fires are poor electrical connections. This is also one of the most difficult to protect against. If electrical connections are well made they will have very low resistance. In fact electrical design engineers will usually consider their resistance to effectively be 0. This assumption is valid when connections are properly made. However, if a wire connection is loose and the electrical contact surfaces between the two conductors are not tightly made the resistance in a connection point can be significant. An examination of the simple circuit in Figure #5 and Figure #6 illustrates this point.
Figure #5 shows the circuit functioning as it was designed, the circuit draws 20A and all power is dissipated in the heater. Figure #6 shows the same circuit when a poor connection is made at the terminal block.
The terminal block now has 12 ohms of resistance as a result of the fault, and it is, in effect, functioning as an unanticipated heating element. The terminal block and its installation in the system was not designed to handle the 1200 watts of thermal energy that is being generated in it. With the extreme elevated temperatures generated by the poor connection, the terminal block it self can become a source of fuel.
Circuit Breakers and Fire Protection
This example also illustrates that fact that this type of fault is not protected by the common protection of properly sized over current protection – a circuit breaker or fuse. Before the fault (Figure #5), the circuit was drawing 20A. After the fault (Figure #6), it was drawing 10A – a properly sized circuit breaker will not detect this type of fault.
Proper torque applied to the connection is one common way to avoid this problem in new equipment. Other types of electrical connections have similar solutions such as ensuring proper crimping for crimp connector assembly. A more difficult problem is that this type of problem can manifest itself over time. Normal operation and vibration of equipment can loosen connections over a period of years, causing this type of problem. Companies often enact a policy of annual thermal scans of power distributions panel to detect elevated temperatures indicative of problem connections before they can cause problems.
A preventative measure such as this can help reduced the risk of a major facility fire.
Fire Due to Short Circuits
A short circuit most commonly occurs when there is a line to ground fault in a system, although a line to line fault is also another form of a short circuit. Under either of these conditions, resistance will in effect go to 0 and as a result current will attempt to spike towards infinity. When current surges in such a drastic manner even protective circuit breakers with grossly over sized trip values will open up and cut off the current source. Features that protect against short circuit conditions are relatively easy to design. However, even with these protections in place, there are other system features that may result in a fire if a short circuit occurs.
Fuel Sources
A short circuit will most commonly result in a fire when a ready source of fuel is present, such as a flammable atmosphere or combustible plastics. In these cases even a fault that is quickly cut off by a circuit breaker can result in a fire. The solution to this problem is to eliminate the source of fuel.
When dealing with a flammable atmosphere the previously discussed 240VA lower limit, generally considered to be sufficient to cause a fire, does not apply. The reason for this is that even a very small spark can ignite a flammable atmosphere. The common solution is to ensure that electrical energy and flammable atmospheres can never come together, even under fault conditions, in a design.
Fire Due to Overload
Another common fault condition that causes an electric fire is an overload condition. Overload is distinct from a short circuit. An overload condition occurs when there is an electric fault in a system that results in abnormally high amounts of current but far less than a short circuit. In other words, an overload fault results in significantly reduced resistance, but resistance that also remains significantly above 0 – unlike a short circuit fault. Complex industrial systems present many opportunities for such faults. In Figure #7 there is a simple power circuit that can be used to illustrate an overload fault.
This circuit consists of 3 identical heaters that are 4 ohms each and generate 1600 watts of energy each. They are in series so the total resistance of this circuit is 12 ohms. Now if a fault occurs across the base of two of the heaters as shown in figure #8, these two heaters are effectively taken out of the circuit reducing the total resistance of the circuit to 4 ohms.
The voltage drop across the remaining heater will increase by a factor of 3 and because of the reduced resistance the current will increase by a factor of 3. The power generated by the heater will increase form 1600 watts to 14400 watts. The physical design of this system will probably not be able to safely dissipate such a major increase in the thermal energy in this location. More over a circuit breaker with an over sized trip value will not eliminated this fault. The circuit breaker must be appropriately sized for the load to ensure that it will detect this type of fault and disconnect power to this circuit.
Figure #9 shows how this system can be modified to protect against the risk of fire that results from this single fault. This example demonstrates is why sizing circuit breakers appropriate to the load is an essential part of electrical safety and the correct way to protect against over load faults.
Fire Due to Inadequate Heat Dissipation
Inadequate heat dissipation by design may result in a heat build up and a fire but a more common hazard once a system is installed is the loss of a system’s ability to remove heat under fault conditions. One classic problem that has occurred in the semiconductor industry is associated with heated chemical baths. Figure #10 shows a simple design of a heated chemical bath. As intended by the designer, this system was engineered so that the fluid is heated by the 4800 watt heater.
However, there is also a latent function associated with this design. The fluid provides a heat sink for the heater. It removes thermal energy away from the heater. Loss of the fluid means that the heater will lose its heat sink, and the design of the system will probably not be able to safely dissipate the heat generated by the heater without the fluid.
Figure #11 shows this system when this fault condition (loss of liquid) has occurred. This problem was often exacerbated in the semiconductor industry by the fact that these baths were usually constructed out of plastics that can be a good source of fuel for a fire. The solution to this type of problem is often level sensors and over temperature that is not fluid dependant for its function.
Arc Flash
Whether of not an arc occurs through air is a simple function of distance and voltage. The hazards associated with arc flash are not however, a simple function of the fact that an arc occurs. The safety concerns associated with arc flash result from the energy released by an arc. When there is significant energy in an arc, the arc will produce a shock wave of both powerful percussion and intense heat. The effects of these factors on a technician working on an electrical panel that experiences arc flash can be devastating.
The Effects and Mitigation of Arc Flash, and the steps to Identifying an Arc Flash Hazard are discussed below.
Effects and Mitigation of Arc Flash
The heat wave can be in excess of 2000 °C and the shock wave can blow him back off his feet. Increasing numbers of arc flash incidents have been documented and the 2002 edition of the NEC requires all panels with potential arc flash hazards to be labeled accordingly. When a technician works on a live electrical panel, which has a potential arc flash hazard, NFPA 70E requires that he be provided with a flash suit to protect him against this hazard. This is an emerging requirement that will significantly impact the practice of electrical maintenance of industrial electrical panels.
The challenge of providing adequate arc flash protection is to know which electrical panels present an arc flash hazard and therefore will require technicians to wear “flash suits” when they work on them. A common misconception is that arc flash hazards are only present on electrical panels with voltages in excess of 480V or 600V. Simply stated, this is not true. Arc flash hazards are a function of the energy in the arc. Figure #13 shows the anatomy of an arc.
The size of the air gap and the magnitude of the voltage determine if the arc gets started, but the energy in the arc is a function of the voltage causing the arc and the current in the arc. The current in the arc is not what is limited by up stream over current protection, because most circuit breakers take a number of cycles before they trip. It is a function of the maximum momentary fault current that is capable of being supplied by the facilities power grid. While voltage varies from 120V to 480V, the energy in the arc will increase by a factor of 4, current may vary for 100A to 20,000A or more, which in turn increase the energy in the arc by a factor of 200. Therefore, the current available to an arc is the single most important determinate of an arc flash hazard.
Identifying an Arc Flash Hazard
Figure #13 shows the essential steps in determining if an arc flash hazard is present.
While each of these steps requires a set of complex engineering calculations, a brief overview will serve to better understand the process. An arc flash study necessarily looks at an electrical panel as it is installed in a given facility. Therefore, it makes no sense to attempt to do any arc flash calculations on a uninstalled piece of equipment, because the outcome of such a study will not only be greatly impacted by the facility in which the equipment is installed, but also by where it is installed in that particular facility.
The first step in conducting an arc flash study is determining the bolt current. This is largely accomplished by conducting a classic short circuit study. The arc current is next calculated, and this is directly a function of the bolt current. The incident energy is next calculated as a function of the arc current and the arc duration. The incident energy then is used to determine the flash protection boundary or the area over which hazardous heat and force will be propagated as a result of the arc. This in turn will determine whether or not a flash protection suit is needed for a technician working on this electrical panel in accordance with NFPA 70E.
