## Posts Tagged ‘electrical safety’

### Wire Size and Electric Current – Joule Heating

Sunday, March 20th, 2011
 Ever take a peek inside the toaster while you’re waiting for the toast to pop up?  If so, you would have noticed a bright orange glow.  That glow is produced when the toasting wires heat up, which in turn creates a nice crusty surface on your bread or waffle.  It’s the same phenomenon as when the filament inside an incandescent bulb glows.  The light and heat produced in both these cases are the result of the Joule, pronounced “jewel,” effect at work.      To understand Joule heating, let’s first refresh our memories as to electrical current resistance.  We learned previously that wire is not a perfect conductor, and as such resistance to flow is encountered.  This resistance causes power to be lost along the length of wire, in accordance with this equation: Power Loss = I2 × R Where I is the electric current flowing through a wire, and R is the total electrical resistance of the wire.  The power loss is measured in units of Joules per second, otherwise known as watts, “watt” denoting a metric unit of power.  It is named after the famed Scottish mechanical engineer, James Watt, who is responsible for inventing the modern steam engine.  A Joule is a metric unit of heat energy, named after the English scientist James Prescott Joule.  He was a pioneer in the field of thermodynamics, a branch of physics concerned with the relationships between different forms of energy.      Anyway, to see how the equation works, let’s look at an example.  Suppose we have 12 feet of 12 AWG copper wire.  We are using it to feed power to an appliance that draws 10 amperes of electric current.  Going to our handy engineering reference book, we find that the 12 AWG wire has an electrical resistance of 0.001588 ohms per foot, “ohm” being a unit of electrical resistance.  Plugging in the numbers, our equation for total electrical resistance becomes: R = (0.001588 ohms per foot) × 12 feet = 0.01905 ohms And we can now calculate power loss as follows: Power = I2 × R = (10 amperes)2 × (0.01905 ohms) = 1.905 watts      Instead of using a 12 AWG wire, let’s use a smaller diameter wire, say, 26 AWG.  Our engineering reference book says that 26 AWG wire has an electrical resistance of 0.0418 ohms per foot.  So let’s see how this changes the power loss: R = (0.0418 ohms per foot) × 12 feet = 0.5016 ohms Power = I2 × R = (10 amperes)2 × (0.5016 ohms) = 50.16 watts      This explains why appliances like space heaters and window unit air conditioners have short, thick power cords.  They draw a lot of current when they operate, and a short power cord, precisely because it is short, poses less electrical resistance than a long cord.  A thicker cord also helps reduce resistance to power flow.  The result is a large amount of current flowing through a superhighway of wire, the wide berth reducing both the amount of power loss and the probability of dangerous Joule heating effect from taking place.       Our example shows that the electric current flowing through the 12 AWG wire loses 1.905 watts of power due to the inconsistencies within the wire, and this in turn causes the wire to heat up.  This is Joule heating at work.  Joule heating of 50.16 watts in the thinner 26 AWG wire can lead to serious trouble.      When using a power cord, heat moves from the copper wire within it, whose job it is to conduct electricity, and beyond, on to the electrical insulation that surrounds it.  There the heat is not trapped, but escapes into the environment surrounding the cord.  If the wire has low internal resistance and the amount of current flowing through it is within limits which are deemed to be acceptable, then Joule heating can be safely dissipated and the wire remains cool.  But if the current goes beyond the safe limit, as specified in the American Wire Gauge (AWG) table for that type of wire, then overheating can be the result.  The electrical insulation may start to melt and burn, and the local fire department may then become involved.          That’s it for wire sizing and electric current.  Next time we’ll slip back into the mechanical world and explore a new topic: the principles of ventilation. _____________________________________________

### Wire Size and Electric Current

Sunday, March 13th, 2011

### Transformers – Electric Utility Power Savers

Sunday, January 2nd, 2011

### Arc Flash Dangers

Sunday, September 13th, 2009

Imagine being hit by a bolt of lightning.  Like lightning, an arc flash can unexpectedly release tremendous amounts of energy, resulting in serious injuries and even death.

An arc flash is the result of a short circuit or electrical fault in energized equipment.  Current flows through the air and creates an electrical arc, very much like the phenomenon of lightning.  But unlike lightning, arc flash dangers are present in a myriad of circumstances that do not require storm conditions to manifest.

Over 80% of electrically related injuries involve some type of arc flash.  They can be caused by a wide variety of factors, including:  equipment malfunctions, inadequate safety procedures, carelessness, lack of training, dropped tools, etc.   The amount of energy released by the electrical arc depends on the amount of electrical current flowing through the arc and how long the current will flow before it is interrupted by a circuit breaker or fuse.

The radiation released in an arc flash can be so intense and so rapid that it can instantly burn skin and ignite clothing.  Temperatures at the electrical arc can rapidly climb to tens of thousands of degrees.  At temps this extreme metal becomes liquid, then vaporizes, and the air surrounding the arc becomes superheated to approximately 30,000°F.  The superheated air and metal vapor together expand with explosive force.  This creates a dangerous and potentially lethal pressure wave of hot gas, molten metal droplets, and solid metal shards that can create burns and shrapnel wounds.

 Temperatures at the electrical arc can rapidly climb to tens of thousands of degrees, creating a dangerous and potentially lethal pressure wave of hot gas, molten metal droplets, and solid metal shards that can create burns and shrapnel wounds.

The Occupational Safety and Health Administration (OSHA) requires employers to assess the workplace for arc flash hazards that are present or that are likely to be present.  Assessment is done using standards developed by the National Fire Protection Association (NFPA) and the Institute of Electrical and Electronics Engineers (IEEE) to specifically address arc flash hazards.

If hazards are identified, employers must mitigate risks by adhering to a six point compliance plan such as the following:

1.  Implement a worker safety program with defined responsibilities.

2.  Perform engineering studies that include calculations to determine the degree of arc flash hazards.  These studies also must be updated when any changes to the electrical system are made by the employer or the electric utility.

3.  Provide workers with the correct personal protective equipment (PPE) based on the study results.  These PPE must then be maintained on site to protect workers.

4.  Provide worker training on the hazards of arc flash. This training must be documented and workers must demonstrate proficiency through testing.  Worker training must also be updated whenever any changes occur to the electrical system.

5.  Provide appropriate tools for safe working.

6.  Place conspicuous warning labels on equipment to warn workers about potential arc flash hazards.

OSHA, like other government agencies, expects employers to keep up with regulations and take the necessary steps to remain compliant.  For example, OSHA won’t send notices out to employers to inform them that they must implement an arc flash program in their plant.  It’s up to the employers to know that and institute the necessary precautions on their own.

It must also be noted that OSHA doesn’t walk employers through the steps of setting up an effective worker safety program.  This means that workers can be exposed to arc flash hazards simply because their employer is ignorant of regulatory requirements and is operating based on misconceptions.

Some workers are unfortunate enough to work for employers that don’t take potential dangers like arc flash seriously enough to implement an effective safety program.  When their wakeup call comes, it’s often too late.

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Do you want to know more about implementing an effective Arc Flash Program in your facility? Phil O’Keefe developed and presents a 90-minute webinar entitled: “Arc Flash Program Fundamentals.”  Contact him for more information about conducting this webinar for your organization.