BK Precision 4011A Provozní pokyny stáhnout pdf (Strana 45)

Appendix A: Safety

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f) Do not exceed the voltage or current ratings of circuit elements or instruments. This par-

ticularly applies to wattmeters, since the current or voltage rating may be exceeded with

the needle still reading on the scale.

g) Be sure any fuses and circuit breakers are of suitable value.

When connecting electrical elements to make up a network in the laboratory, it easy to lose track

of various points in the network and accidentally connect a wire to the wrong place. One proce-

dure to help avoid this problem is to connect first the main series loop of the circuit, then go back

and add the elements in parallel.

Types of Equipment Damage

Excessive currents and voltages can damage instruments and other circuit elements. A large

over-current for a short time or a smaller over-current for a longer time will cause overheating,

resulting in insulation scorching and equipment failure.

Blown fuses are the most common equipment failure mode in this laboratory. The principal

causes for these failures include:

 incorrectly wired circuits;

 accidental shorts;

 switching resistance settings while power is applied to the circuit;

 changing the circuit while power is applied;

 using the wrong scale on a meter;

 connecting an ammeter across a voltage source;

 using a low-power resistor box (limit ½ amp) when high power is required;

 turning on an autotransformer at too high a setting.

All of these causes are the result of carelessness by the experimenter.

Some type of insulating material, such as paper, cloth, plastic, or ceramic, separates conductors

that are at different potentials in electrical devices. The voltage difference that this material can

withstand is determined by design (type, thickness, moisture content, temperature, etc.). Exceed-

ing the voltage rating of a device by an appreciable amount can cause arcing or corona, resulting

insulation breakdown, and failure.

Some electrical devices can also be damaged mechanically by excessive currents. An example is

the D’Arsonval meter, the indicator in most analog metering instruments. A large pulse of over-

current will provide mechanical torque that can cause the needle to wrap around the pin at the

top of the scale, thereby causing permanent damage even though the current may not have been

on long enough to cause failure due to overheating.

After-Accident Action

Since accidents do happen despite efforts to prevent them, preparation for an accident can save

valuable time and lives. This preparation should include immediate availability of first aid mate-

rial suitable for minor injuries or for injuries that are likely because of the nature of the work.

Knowledge of how to obtain trained assistance such as Emergency Medical Service (EMS)

should be readily available for everyone.

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BK Precision 4011A Provozní pokyny Strana 45