Posts Tagged ‘forensic inspection’

Wire Size and Electric Current

Sunday, March 13th, 2011
     Whether or not you live or work in a city, you are probably aware of rush hour traffic and how frustrating it can be.  As a matter of fact, this traffic is the number one reason many choose to live within cities providing public transportation.  Instead of watching the cars pile up in front of you, you can be checking your email or reading the paper.  And no matter where you live, you’ve probably encountered a narrow one-lane road at some time.  If this road were to be spotted with traffic lights and double parked cars, the resulting frustration would reach a new high, one which has you craving the freedom of a crowded three-lane expressway.  At least there’s the possibility of movement there.

      Generally, the wider the road and the fewer the impediments, the better traffic will flow.  The problems presented by vehicular traffic are analogous to those present in electrical wires.  For both, obstructions are impediments to flow.  You see, the thicker the metal is in a wire, the more electrical current it can carry.  But before we explore why, let’s see how electric wires are classified.

     If you’ve ever spent any time hanging around a hardware store looking at the goodies, you’ve probably come across wire gauge numbers, used to categorize wire diameter.  American Wire Gauge (AWG) is a standardized wire gauge system, used in North American industry since the latter half of the 19th Century.  Handy as it is, the AWG gauge numbering system seems to go against logic, because as a wire’s diameter increases, its gauge number decreases.  For example, a wire gauge number of 8 AWG has a diameter of 0.125 inches, while a gauge number of 12 AWG has a diameter of 0.081 inches.  To make things easier on those who need to know this type of information, wire diameter is tabulated for each AWG gauge number and readily available in engineering reference books.

      So what does this have to do with electric current?  To begin with, the larger the AWG number, the less current it can safely carry.  If we turn to an engineering reference book, and look up information relating to an 8 AWG insulated copper wire, we find that it can safely carry an electrical current of 50 amperes, while a 12 AWG insulated copper wire can safely carry only 25 amperes.  This information allows us to make important and relevant design decisions regarding a myriad of things, from electrical wiring in electronic devices, to appliances, automobiles, and buildings. 

      So, why are bigger wires able to carry more current?  Well, as you’ve heard me say before, no wire is a perfect conductor of electricity, but some metals, take copper for instance, are better conductors than others, say steel.  But even the best conductors are inherently full of impurities and imperfections that resist the flow of electricity.  This electrical resistance acts much like traffic lights and double parked cars that impede the flow of traffic.  The larger the diameter of the wire, the less electrical resistance is present.  The logic here is simple.  Wire that is larger allows more paths for electrical current to flow around impurities and imperfections.

      The congestion present in rush hour traffic results in travel delays and hot tempers, and heat is also present in electric wires that face resistance to electricity flow.  If the resistance to electric current flow is high enough, it can cause overheating.  Road rage within the wires is a possibility, and if the wires get hot enough, electrical insulation can melt and burn, creating a fire.  Known as the “Joule heating” effect, this phenomenon is responsible for its share of building fires.

      We’ll learn more about Joule heating and how wires are sized to keep electrical current flow within safe limits next week.  Until then, try to keep out of traffic.



Machine Safety, Operator Safety, And Keeping Those Fingers

Sunday, September 27th, 2009

     Crushed fingers, amputations, burns, blindness, these are all too common undesirable occurrences involving moving machinery.  Eliminating the risk of such accidents is an integral part of the engineering design process, where risk assessment goes hand and hand with industry standards in order to provide adequate machine safeguards and protection to operators as well as bystanders.

     Machine safeguards fall into three basic categories: Guards, Devices, and Distance.

     Guards are physical barriers that are added to machines with the goal of keeping body parts, clothing, etc., separated from potentially hazardous areas.  An example would be a metal cage surrounding drive belts and pulleys.  Guards can also serve to keep material fragments and debris from flying out of machines while in operation, such as when an enclosure is built around the grinding wheel of a bench grinder.

     Devices can consist of automatic controllers, often connected to sensors on machine componets.  These controllers use a form of “safety interlock logic” to monitor the operating state of machinery.  They must act quickly and automatically to stop the normal operation of a machine if they sense that an undesirable object, say a person’s forearm, is in danger of entering a hazardous area.

     Controllers can be in the form of hard-wired electromechanical relays, embedded microprocessors, or programmable logic controllers (PLCs).  Their sensors can include electrical switches embedded in floor mats or mounted on movable guards, incorporated into control handle grips, or linked to an access door latch.  Still other sensors are more elaborate, using more sophisticated methods to maintain safety, such as photoelectric devices known as laser curtains.  These act by spreading beams of light across an opening which may be a gateway to a dangerous area.  If the beam is broken by an object, the controller takes appropriate action and renders the machinery inoperable.

     Distance safeguards operate as you would infer them to, by designing machinery so that hazardous areas are kept a great enough distance from body parts, etc., so as to eliminate any danger of them being drawn into an unsafe area.  An example of this factor at work would be when machinery is developed so that moving gears and other potential hazards are kept far out of the reach of someone by virtue of their overall design. 

      Sometimes even the best machine safeguard designs can be rendered ineffective after a piece of machinery is put into actual operation.  The reasons for this are varied, from poor maintenance of equipment, to lack of training for operating personnel, to inadequate supervision of workers, or perhaps the machine has been modified to operate outside the parameters of its design capacity.  Whatever the reason, people can be put at risk for serious injury and even death if machine safeguards are bypassed, eliminated, and defeated.