Test Equipment and Safety
Electricians and electronics technicians rarely experience a serious electrical shock because they understand the nature of electricity and take the necessary steps to prevent this often unpleasant sensation. Taking the basic steps to avoid electrical shock should become habit, not only to prevent injury, but to also prevent damage to equipment.
The Danger Of Current Over Voltage
Electrical shock occurs when current flows through the body. Though a high voltage can overcome the resistance of the skin, it is the amount of current flowing that causes injury. The old saying is, "Current Kills". The worst case scenario is a shock between the hands. The current path from one hand to the other is across the heart, and as little as 100 mA (milliamps or 1/thousandths of an amp) can prevent the heart from beating properly. This can cause unconsciousness and death if sustained.
Here are some pointers that may help you get a feel for how to avoid getting shocked when working on electrical or electronic components.
Know Your Equipment
Being familiar with the device being worked on, and with the test instrument being used, is a must. It is very easy to make a malfunctioning piece of electronic equipment into a permanently damaged piece of electronic equipment. Expensive meters and other test instruments can be damaged or destroyed with improper use as well.
Start Off Small
Your first repair projects should not include things like TV sets or tube-type amplifiers. Both of these devices, though greatly different in design and complexity, have very high and potentially lethal operating voltages present! TV sets require isolation to prevent damage to some test instruments. Inadvertently touching the test lead of a voltmeter between the plate connector of a vacuum tube and a source of ground (which may only be 1/4" away) can have serious and permanent consequences!
Locate And Isolate Yourself From Ground Sources
Use carpeting or a rubber mat under your feet to insure that you are not grounded through contact with a concrete floor when working around voltage. Being "grounded" means that a (low resistance) path for current flow has been created. If your body is not in contact with any object that is grounded, it is not possible to experience an electrical shock. You can touch a high voltage source with no shock or sensation if there is no path through your body for current to flow to ground.
The metal chassis of an amplifier is usually grounded through the round pin on a "three-prong" plug. Ground or earth represents an infinite supply of electrons! Resting one hand on the chassis of an amp while reaching into the device with a test probe is inviting a nasty shock!
Voltage measurements will usually be taken with respect to this ground. An alligator clip attached to the negative test lead and clipped to the chassis will free one hand while you use the other test lead to take measurements. Even if you should touch a source of voltage and current while reaching with a test lead, you will not experience a shock if you are not grounded.
Electrical or electronic equipment should be serviced only after the device has been turned off and unplugged. On the other hand, it is often necessary to work on a piece of equipment with it powered up, depending on the problem you are looking for. Knowing this, you will want to make sure that you are not in contact with a source of ground (a place where electrons can come from or go to) while your hands are in close proximity to voltages. Never work on anything electrical while standing on a concrete floor! Use carpeting or a rubber mat under your feet to insure that you are not grounded in this manner.
Try To Test "Single-Handedly"
Checking high voltage in a power panel or device that uses high voltage must be done with extreme care and forethought. Most electricians will place one hand in their pocket when reaching into a power panel where high voltage is present with the other hand. This habit precludes the possibility of placing the other hand on a grounded object and receiving a high voltage shock between the hands.
It is still possible to get your hand or fingers between a voltage source and ground and receive a shock through your hand. The minor shock and surprise will cause many to "snatch" back their hand as a reflex, resulting in a scrape or cut .
Impatience and carelessness causes most electrical shocks, and if you work with electricity and test equipment, sooner or later you will get shocked. The negative reinforcement leads most to adhere to basic rules of "do's" and "don'ts" when working around voltage of any kind. This also includes knowing when to admit that servicing or repair of a given device be left to someone with more knowledge or experience.