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Doug Baney
Last edit Jan 1 2013

Electrical Safety

Part 1: Electrical basics

Part 2: Electrical Hazards

Part 3: Safe operating around electricity

Part 4: Student Test

Part 1: Electrical Basics:

Volts, V, is the force that pushes electrons through space or material, measurement units are volts. Batteries supply voltage, AC outlets supply alternating voltage at typically 60 times per second. The higher the voltage the more force there is to get electrons across barriers (such as human skin), at high enough voltages, electrons will travel through air creating sparks. Car batteries are about 12 volts, while house voltages is 10X higher alternating between positive and negative voltages. (1 mV i.e. 0.001volts, 1KV i.e. 1000 volts)

Current, I, is the amount of electrons flowing (like a river), measurement units are amperes, or just amps. Batteries can readily supply hundreds of amperes, say to start a car engine starter, an LED flashlight may draw just 0.1 amperes, or 100 mA.

Resistance, R, in units of Ohms, refers to the ability to resist current flow, like boulders in a river.  Measurement units are ohms. Resistance is simply a proportionality constant relating the voltage lost across a device for a certain current passing through the device.

In electrical circuits, there is a simple relation between V, I and R, voltage is current multiplied by resistance:

V = IR

Example: Given a 3 ohm resistor connected across a 12 volt battery, what is the current flow?

Answer: I = V/R so I = 12/3 = 4 Amps

Example: Given a 12 volt battery and

Part 2: Electrical Hazards

Current passing through your body can cause electric shock, resulting in 3 types of potential injuries:

1. Burns (arcs burn with heat & radiation)

2.     Physical injuries (broken bones, falls, & muscle damage)

·         At 10 mA, the muscles clamp on to whatever the person is holding.

3.     Nervous system effects (stop breathing at 30 to 75 mA alternating current at 60Hz, fibrillation at 75 to 100 mA)

  Fibrillation = heart is "twitching" and there is no blood flow to the body.

·         Just 1 mA passing directly through the heart can be deadly

·         200 mA current through the body can be deadly

 

The heart can be damaged because it is in the path of the most common routes electricity will take through the body:

Hand-tohand

Hand-to-foot

Quiz:

Assume  Buffy cuts his fingers on each hand and accidentally touches the terminals of a 24 volt battery, assume that the human blood resistance is 120 ohms for this example. What would be the electricity flow through Buffy, is it potentially dangerous?

 Answer: I = 24/120= 0.2 amperes. Yes

 

Ground Loops: Problematic and sometimes hazardous.

Ground loops are caused when there is some resistance between an intended ground point, and the actual ground. Current flow along that path then causes the intended ground point to float to a non-zero voltage.  A supposed ground connection, for example from a co-axial cable made to the elevated voltage point and run to an actual ground in a different place will cause current flow in the ground conductor of the cable. Ground loops can cause electrical equipment malfunction, hazardous shocks are also possible from ground loops, as well as causing annoying hum in audio systems. The use of star ground methods and very low resistance connection to ground will reduce ground loops.  It is a wise practice to meaure ground quality (resistance) with a multimeter.

 

 Part 3: Safe operating around electricity

 Please follow these general guidelines pertaining to electrical safety. 

Never work alone when working with electricity

Turn off power and unplug from the wall before working on electric or electronic circuits, except when energized testing is required.

Do not work on electrical equipment in a wet area or when touching an object that may provide a hazardous earth ground path.

Replace defective cords and plugs. Inspect cabling for defects such as frayed wiring, loose connections, or cracked insulation

Remove metal jewelry, watches, rings, etc., before working on electrical circuits.

Work with just one hand where possible, keep the second hand away from anything that conducts electricity.

Never place containers of liquid on electrical systems.

Never defeat the purpose of a fuse or circuit breaker. Never install a fuse of higher amperage rating than that specifically listed for your circuit.

Make sure equipment chassis or cabinets are grounded. Never cut off or defeat the ground connection on a plug.

Safely discharge capacitors in equipment before working on the circuits. 

Wear shoes with rubber soles. The rubber will help prevent shocks by keeping electricity from flowing through you into the ground.

Do not attempt to solder electronic devices that are still connected to a power source

Read more: Safety Rules for Working on Electronics | eHow.com http://www.ehow.com/list_7442608_safety-rules-working-electronics.html#ixzz2EE3sG17e 

Part 4: Student Test

1. Working with 12 volt circuits or 48 volt circuits presents equal hazards: True/False
2. It is OK to work barefoot with electrical circuits since human skin is non-conductive: True/False
3. It is OK to wear jewelry when working on electrical circuits as long as it does not have a battery: True/False
4. When working with electricity, safety glasses are needed to protect from flying parts: True/False
5. Ground loops are caused by overhead power lines creating eddy currents in the earths crust: True/False