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ECE DEPARTMENT LAB SAFETY
Compiled by Mark Bailey
We are worried about two kinds of safety: safety of you and safety of the equipment.
The flow of electricity can adversely affect your body in at least three ways: it can cause muscle spasms and paralysis; it can electrolyze water in your blood and produce gas bubbles; and it can burn the skin. Very little current can do all of these things, although how much real damage there is depends upon what part of your body is affected. A temporarily paralyzed arm isnÕ too serious, but a paralyzed heart is fatal. As little as 50 milliamps (0.05 amp) can do this if it passes, say, from one arm to another, a path which includes the heart. One tenth of this amount of current will give you an unpleasant shock. Burns are produced on the skin if there is sparking.
First aid for someone who is unconscious from electrical shock starts by removing cause of the shock if it is still present. Disconnect the circuit, if possible. Try to separate the victim from the electrical circuit but do not take hold of him otherwise you may become part of the problem. If he is not breathing, give him artificial respiration. (Lung paralysis is common. Other organs may also be affected, but there is little you can do about them except to get a doctor as soon a as possible.)
When the victim is conscious, keep him comfortable, let him rest lying down, make sure a doctor has been called.
Prevention, though, is better. Avoid becoming a victim. Don't allow your body to come across any voltage. Don't touch any conductor unless you can be sure the circuit is disconnected. Even then, treat everything as if it were live. You can also reduce the chance of shock by working with only one hand, keeping the other hand behind your back. Even that may not always work. Almost all power circuits are connected to ground at some point. In the case of 120 volt AC as in wall plugs, one of the two conductors is grounded. With higher voltages, usually a center tap is grounded. With three-phase circuits, the neutral point is grounded. What this means is that you only need one conductor and a ground to complete the circuit. Standing on a concrete floor, with wet shoes, you only need to touch one wire to have a current path through your body.
Another way to get a surprise shock is from a charged capacitor. Capacitors (also called condensers) can store electrical energy for as much as a month in some cases. How much energy depends on the value of the capacitor and the voltage it was charged to. Treat capacitors as if they were charged even if nothing is connected to them.
If you get a shock, the amount of current that will flow through your body depends on the voltage you have come in contact with and your body resistance. Your body resistance depends mostly on how you have come in contact with the conductor. Most of your body resistance is skin resistance at the point of contact. If the skin is dry and the contact area is small (light pressure), your skin resistance will be high and the shock will be small. If your hands are sweaty (sweat is saltwater and saltwater is a good conductor) and the contact area is large, your resistance will be low, and the shock could be severe or deadly. Ordinary house voltage (120 volts AC) can kill. Remember, one side of it is connected to ground, so half the connection to you may already exist. Higher voltages are more likely to kill and also more likely to cause skin burns.
There are non-electrical hazards around electrical and electronic equipment. Storage batteries contain corrosive sulfuric acid in a package which can leak if you drop it. Also, these batteries give off explosive hydrogen gas when charging or discharging.
Cathode ray tubes ("picture tubes") have a high vacuum inside and are made of glass. This means that there is a pressure of near 15 pounds per square inch on the glass. There is over two tons of force on the face of a 24-inch television tube. If it breaks, broken glass will shoot out at high speeds. Oscilloscope tubes, although smaller, are no less dangerous. (There is always high voltage around cathode ray tubes, ranging from one or two thousand to thirty thousand volts. This voltage is almost always present because they store charge like a capacitor even when the equipment is turned off).
Low voltages are not "safe", particularly if much power is available. It may be true that you can't get a dangerous shock from a 12-volt battery, but a short circuit across such a battery will result in currents of several thousand amps, which is enough to melt, almost instantaneously, all but the heaviest copper wire, tools, finger rings, etc.
Almost all electrical power is used on a "constant voltage" basis. That is, a constant potential difference is supplied, and the amount of current which flows is determined by the resistance of the load that is connected to the supply. Since power is proportional to the current (amps) multiplied by the potential difference (volts), the resistance of the load determines the amount of power taken from the supply.
Things are damaged when too much power is applied to a circuit for too long a time. Fuses and circuit breakers disconnect the power when current flow is too large, but don't expect them to protect much else. For example, a circuit protected by a 20-amp fuse will still deliver several hundred amps if it short-circuited before the fuse blows. The fuse will open in a fraction of a second, but a lot of damage may have already been done.
Use conductors and connectors appropriate for the amount of current flowing through them and make sure all the connections are tight. Power resistors usually get hot; some of them very hot. Do not touch them when the circuit is or has recently been energized. Before turning on the power supply, calculate how much power the resistor will have to dissipate and make sure that it is within the rating of the resistor.
Meters are expensive, delicate, and vulnerable. Voltmeters and ammeters usually look alike and work on the same basic principle, but they are completely different in their electrical characteristics and applications. A voltmeter has a high resistance so that a small current will flow through it when itÕ connected to the voltage to be measured. An ammeter, however, has a very low resistance so that there will be a small voltage across it when the current is flow through it. If an ammeter is connected across a voltage supply, very large currents will flow and can destroy the meter and/or harm the operator.
You can make fewer mistakes by remembering: Voltage across, current through. DO NOT connect an ammeter across the line. To measure current, you have to interrupt a current path by disconnecting a conductor. Do this when the circuit is DE-ENERGIZED. The ammeter is then connected in series with the circuit across the two open ends of the disconnected conductor.
Ammeter shunt switches give additional protection for ammeters. They act as short circuits in parallel (across) with the ammeter, and are opened when a reading is taken. When the ammeter shunt switch is properly used and a wiring error has been made, dangerous current will flow through the shunt and bypass the meter. Be sure the meter is connected so that itÕ full-scale reading is more than the voltage or current youÕe measuring.
Finally, the universal safety rule: If you're not certain, ask the instructor.
Miscellaneous Additional Items Related to Personal and Equipment Safety
- Meters, if adjustable, should be placed on the highest possible range before energizing the circuit.
- It is important that meters be hooked with the correct polarity.
- All resistors should have the power ratings at least as much as the maximum expected power dissipation.
- Should it be necessary to alter the circuit, de-energize it first.
- For minor cuts and burns, there is a first aid kit available, locations are posted in all labs. Should the injuries be more severe, apply first aid , contact the health center immediately and, if possible take the victim there. If the victim cannot be moved, call 911. The instructor should be notified of the accident, its cause and any action taken.
- Often fuses in equipment blow at inconvenient times. In any case, always obtain a replacement of the correct value.
- When using a resistor, capacitor or inductor substitution box, be sure that you do not exceed its power and current ratings.
- It is best to turn an ohmmeter off after you have completed your measurements. This will insure against the battery being run down if the leads are accidentally shorted together. On most VOM's and DMM's, the ohmmeter section can be de-energized by switching to a "volts" position. It is best to leave the range switch on the highest position if there is no ÒffÓposition.
- In the event that you think an instrument is broken, notify the instructor and put a yellow tag on it (available in the front on the lab). Do not attempt to fix it. If upon entering the lab you find any of the equipment on your bench damaged, notify the instructor before attempting to use it.
- Severe burns and fires result from careless use of soldering irons.
- Poor insulation can do more than blow fuses; it can give someone a shock or start fires.
- As a general rule, one should not replace components while circuits are energized.
- To prevent dangerously high voltage, be careful never to reverse primary and secondary connections on transformers or disconnect inductive components while they are energized.