812Common Places Where Shorting Occurs

The following is a list of items or locations where electrical shorting leading to fires often occurs. The list is not intended to be exhaustive, but simply represents items that seem to occur regularly.

Staples — In residences and light commercial buildings it is common practice in some areas to simply staple Romax type conductors to wood

Plate 8.4 Aluminum electrical cable. The cable heated from shorting and pulled apart forming "points" on the cable strands. Note white color of oxidized aluminum.

members. If the staple is applied too tightly, the insulation around the conductors can be crushed, cracked, or even torn. In some cases, the staple itself may bite or cut into the insulation. Over time, the damaged insulation may further degrade because of tears and penetrations in the protective sheathing. Current may leak from one conductor to another due to the damaged insulation and precipitate shorting.

Corners — When nominally straight conductors are bent at a sharp angle, the conductor cross-sectional area may be deformed. When the bend is sufficiently sharp, the conductors may develop a crease, or otherwise become distorted and lose some of their effective current-carrying cross-sectional area.

The resistance of a conductor with uniform cross-sectional area is given by the following:

where r = the specific resistance of the conductor usually given in ohms per cm3, l = length of the conductor, and A = cross-sectional area of conductor.

From inspection of Equation (xxiii) it is apparent that if the cross-sectional area is halved, the resistance is doubled. And if the resistance is doubled, then the amount of heat generated by the resistance at that point is doubled, as per the power equation given below.

In addition to deformation of the conductor itself, sharp bending of a conductor may cause the insulation on the outside radius of the bend to become stretched. This will reduce its thickness, and may even cause small microholes or microtears to develop. When such microholes develop in plastic materials, it is often the case that the color of the plastic will "whiten." Clear plastic will often become milk white or opaque where the micro-holes are concentrated.

When the insulation wrap around the conductor stretches and becomes thin, or develops microholes due to excessive strain, the insulation will lose some of its dielectric properties. Because multiple conductors are often in common casings, the insulation between the hot and cold conductors may be sufficiently damaged for a current leak to develop between conductors.

Skinning — When wiring is installed, it is often pulled through conduit, weather heads, holes, or box openings. Sometimes as the conductors pass over edges, around corners, or over rough spots, the exterior insulation around the conductor will be "skinned," that is, some of the exterior insulation wrap will be abraded away. This causes the insulation to be thinner at that location. As such, the "skinned" area is an insulation "weak spot" in the conductor. If the thinning is sufficient, it is possible that current can leak across the weak spot.

Conductors not only have to handle the normal voltage supplied by the utility, but also the common voltage spikes and transients generated by switching activities within the utility system. If insulation is skinned, it may be able to withstand the nominal voltages, but may not be able to stand up to large spikes and transients. Breakdown of the skinned insulation may occur after repeated spikes, or after a single spike if it has sufficiently high voltage.

Edges — Wires and conductors are not immobile although they may appear to be wholly static. First of all, they expand and contract with temperature change. Also, they often jerk slightly in response to large current rushes, such as occurs when a motor is turned on. (The amount of jerk can be calculated by the application of Maxwell's equations.) Because the conductors are often fastened to various portions of a structure, portions of the conductor will move as the structure moves in response to wind, temperature, and loads. Thus, conductors in contact with a sharp edge may have their insulation cut after a period of time due to small relative motions between the conductors and the sharp edge.

Whenever conductors or other types of wiring are run through boxes, walls, or other items that have sharp edges, the NEC requires that smooth bushings be provided. (See NEC 410-30; 410-27(b); 370-22; 345-15; and 300-16.)

Flexible cords — Flexible electric cords typically have stranded conductors. The conductor is composed of many individual hair-like strands of conductor bundled together similar to a rope. Over time, the cord may be bent back and forth at a particular point in the cord causing fatigue damage to the individual strands.

As the strands break apart, the ability of the conductor to carry current diminishes. If enough of the strands break apart, the cord will develop a hot spot at that point for the same reasons as discussed under "corners" in the previous paragraphs: loss of current-carrying cross-sectional area.

From experience, the two most common points in a flexible cord where fatigue damage to the strands occurs is where the cord exits the appliance, and where the cord connects into the male plug. These are the two points where the cord will often be bent the most.

Some people have the habit of disengaging a plug from a wall outlet by pulling on the cord. When this is done, the force needed to disengage the male plug from the outlet is carried by the conductor, since the conductor is usually more rigid than the exterior insulation wrap. Not only can this cause breakage of the electrical connection between the conductor and the male plug, but if the cord were pulled while it was at an angle with the plug, individual stands may be broken by combined bending and tensile stresses in the strands. This is why it is recommended that a male plug be disengaged from a wall outlet by grasping the plug itself.

In addition to fatigue breakage, flexible cords often sustain physical damage to the cord due to mashing and folding. This may occur, for example,

Plate 8.5 Beading at ends of individual strands of wire. ©2001 CRC Press LLC

when an extension cord is run through a door jamb and the door is closed on it, when a knot in the cord is pulled tight, or when the cord is walked on often. I have observed where extension cords have been laid across the burners of cook stoves, laid across the rotating shaft of a bench grinder (take a guess on this one as to what happened), laid under rugs in busy hallways, and laid across garage floors where the car was regularly driven. I have also seen them nailed to walls with the nails driven between the conductors, stapled to walls and ceilings, tied around nails and hooks, and embedded into wall plaster to avoid having an unsightly cord hanging down the wall.

One of my favorites combines several types of abuses at once. A breeze box fan was suspended from the ceiling by its own cord, which had been tied with a square knot to a hot steam pipe. The fan had been suspended over the top of a commercial steam cooker, which was regularly opened and closed. This all goes to show, of course, that there is simply no end to the creativity a person can apply to the abuse of flexible cords.

Lastly, besides being physically damaged as noted above, flexible cords are often just electrically overloaded. Many light application extension cords are rated for 15 amperes. If a 20 or 25 ampere appliance is plugged into the cord, the appliance may draw more current than the extension cord can safely handle. The cord will then heat up due to "I2R" heating effects. There will be some point along the cord where it will heat up a little more than elsewhere, and if left unchecked the cord may, in time, fail and short at that point.

It is not uncommon for a 15 ampere-rated extension cord to be equipped with three or more outlets. Thus, if three appliances, each drawing 10 amperes are plugged into the extension cord and are operated, the cord would then be carrying twice its rated load. Unfortunately, few homeowners are cognizant of the current ratings of extension cords. Most people simply use whatever extension cord is handy, has the appropriate length, or is the cheapest.

Lugs and terminals — Lugs and terminals are used to connect conductors together. The ability of the lug or terminal to carry current is a function of the contact area between the two materials, and the quality of the metal-to-metal contact between them. In most cases, the conductors, lugs, and terminals are cleaned prior to installation, and pressure is applied to the connection to ensure good contact.

If the contact area between the two conductors is reduced or the contact quality is degraded, the connection will lose its ability to carry current. The connection point can then become a hot spot due to "I2R" heating effects. Two of the more common reasons for a lug or terminal to lose its ability to safely carry current are: looseness and corrosion.

Looseness of the connection can occur because of vibrations, impacts to the panel box, temperature effects, material creep, chemical attack, and a host of other less obvious causes. Looseness can cause a loss of current-carrying area because in most cases, the amount of contact area is partially a function of the compression between the two materials. When the compression is firm, the two materials are in intimate contact. When the compression is not firm, there may be an air space between the materials that acts as an insulator. A firm connection also tends to push through the light layer of oxide that usually forms on the conductors during storage and shipment.

It is not uncommon for some manufacturers of screw-type lugs to recommend that the lugs be periodically checked for tightness. Some manufacturers of industrial type bus bars have employed "break away" lug connections to ensure that screw-type connections do not back off and become loose. Other manufacturers have employed crushable threaded lugs to prevent lug back off. Most have specific tightening specifications to ensure a tight connection that will not back off.

Corrosion at a lug or terminal usually causes problems in two ways. First, the products of corrosion are often not good conductors of electrical current. While copper and aluminum are excellent conductors, copper oxide and aluminum oxide are not. A layer of corrosion products between the conductors typically increases the resistance of the connection, which then results in heating of the connection.

Secondly, corrosion may cause material damage to the connection. It may result in material loss, weakening of the material, or even dimensional distortion. In the latter case, the distortion may occur due to a change in material properties while being subject to the same loading or stress.

Corrosion of lugs and terminals is often caused by exposure to water, constant high humidity, or chemicals. A water drip may occur directly over an electrical box, or the humidity in the area may be constantly saturated. Chemical vapors from processes or materials storage may also contribute to and accelerate corrosion. It is important to note whether the electrical box and connections have been rated for the specific environment in which they are used. There are many types of electrical boxes that are variously rated for outside use, use in high humidity, use in explosive environments, etc.

The corrosion of lugs and terminals can also be caused by the electricity itself flowing through certain combinations of dissimilar conductor materials. This is called galvanic corrosion, and is basically the same phenomenon used to produce gold or silver plating in jewelry.

In galvanic corrosion, the passage of electrical current literally causes one of the materials to plate out onto the other. This results in material loss to the donor material, which can be substantial depending upon the circumstances. The material receiving the transferred material does not benefit by this plating action either. Typically, the transferred material quickly oxidizes and becomes a nonconducting crust of hoary fuzz or "crud" around the lug or terminal.

The formation of this crud further accelerates the galvanic process because the crud itself acts as a surface for gathering water moisture to the connection. The presence of moisture around the connection helps to promote galvanic corrosion because the moisture provides a medium, or a chemical solution if you will, in which the reaction can take place.

Thus, the primary factors influencing the rate of galvanic corrosion include.

• The type of materials in physical contact with one another.

• The amount of current flow.

• The voltage across the terminal connection itself (a relatively highresistance connection will help promote the process).

• The humidity or moisture that may come into contact with the lugs or terminals.

For these reasons, it is not proper to directly connect aluminum conductors to copper conductors. Copper and iron is also a bad connection combination due to its tendency to quickly corrode. It is also not proper to connect aluminum conductors to lugs and terminals that are rated only for copper conductors, and vice versa.

If a mixed connection between a copper conductor and an aluminum conductor must be made, a dual-rated connection box should be used. Also, connections between copper and aluminum conductors can be safely done when an intermediary material is used that is compatible to both. Tin is often used for this purpose. Thus, copper conductors and aluminum conductors that will be connected together are often tin coated or "tinned" at the point where direct contact will be made. Other types of solder coatings or "tins" have been developed to be used as intermediaries.

However, these coatings must be used with great care. Over time the coating may crack, become abraded, corrosively degrade, etc. Breaching of the coating may then allow the two dissimilar metals to come into direct contact with another and set up a galvanic corrosion cell.

With respect to dissimilar conductors, the National Electric Code, Section 110-14, states in part that:

Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors (such as copper and aluminum, copper and copper clad aluminum, or aluminum and copper clad aluminum) unless the device is suitable for the purpose and conditions of use.

When a lug or terminal connection is loose or corroded, often a "chattering" or buzzing noise can be heard emanating from the problem connec tion. This noise is generated by the 60 Hz alternating current arcing across or within the connection. Because there is not enough current-carrying contact area, current is literally arcing across air gaps in and around the connection.

When such arcing occurs, it typically causes pitting of the connection, and scatters tiny blobs of conductor material in the vicinity of the arcing. The area around the chattering is often blackened due to the formation of carbon residues from the air by the arcing. The local temperatures of the arcs themselves will range from 2500°F to as high as 10,000°F, which is the same range of temperatures found in arc welding or lightning.

If the chattering is allowed to continue, it usually results in overheating of the connection, electrical shorting and failure, and possibly fire. Sometimes after a fire, an occupant of the building may recall having heard a chattering or buzzing sound coming from the electrical box.

Motor burn out — When electric motors become worn out, or the rotating shaft becomes locked, perhaps due to a seized bearing, the windings in the motor can overheat and short out. This is especially true of motors that are not equipped with internal thermal switches. Such switches, also known as high-heat-limit switches, shut off the motor should its windings overheat. Some older motors have flammable insulation shellac around the windings that can ignite and then sustain fire.

CB radio coaxial cables — Because citizen band radios do not require licenses, the people who use them are often not trained very well in the basic principles of radio and radio transmission. Some CB hobbyists inappropriately (and illegally) use powerful linear amplifiers on low power rated equipment to boost their signal output. They may also mix and match various types of antennas with their transmitters without matching antenna impedance to transmitter impedance. Thus, one of the common points where CB radios overheat and cause fires is the antenna coaxial cable, leading from the transmitter or linear amplifier to the antenna. Usually the whole line will heat up, especially after the transmitter has been in use a long time during an extended gab session. When sufficiently hot, the antenna line can melt away its insulation sheath and ignite materials that come in contact with it.

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