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Also referred to as electric shock. When a person touches a motor that has a "leak", a path can be created from the motor frame to the hand, body and feet of the person to the floor he is standing on to allow a current to flow through it, sometimes resulting in a fatal accident.
The seriousness of a shock hazard widely varies depending on the amount and duration of the current that flows through the person' s body. His constitution, age and medical condition are also variation factors, but in general, at a frequency of 50 or 60Hz, stimulus to the skin is felt at 1mA, considerable pain occurs at 5mA, pain is unbearable at 10mA, there is difficulty in releasing the "leaking" object because of intense muscle contraction at 20mA, it is considerably dangerous at 50mA and fatality is likely at 100mA. For the safety limit for a fatal current, which causes ventricular fibrillation, Professor Dalziel proposed the following equation from numbers of experiments on animals.
I = 165√t
Where, I = current (mA) and t = time (sec).
From this theory, the maximum duration for a current of 165mA is 1 second.
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This is a test based on the principle that an ideal insulation will produce identical readings at all voltages, while an insulation which is being over stressed, will show lower insulation values at higher voltages. During the test, the applied voltage incrementally steps by a certain voltage taking successive 5-time measurement. Degradation of insulation may be doubted when insulation resistances become lower at higher applied voltages.
Ex: SV display of KEW 3128
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A device that uses the voltage developed by the junction of two dissimilar metals to measure temperature. One junction, called the measuring junction, is placed at the point where temperature is to be measured. The other junction, called the reference junction, is maintained at a reference temperature. The voltage developed between the two junctions varies depending on the difference between the temperatures of the two junctions and the type of thermocouple.
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Most alternating currents and voltages are expressed in effective values, which are also referred to as RMS(Root-Mean-Square)values. The effective value is the square root of the average of the square of alternating current or voltage values.
Many clamp meters with rectifier type circuits have scales that are calibrated in RMS values for AC measurements. But, they actually measure the average value of input voltage or current, assuming the voltage or current to be a sine wave.
The conversion factor for a sine wave, which is obtained by dividing the effective value by the average value, is 1.1. These instruments are in error if the input voltage or current has some other shape than a sine wave.
CF : Crest Factor = Peak value/RMS value
DC = 1
Sine wave = 1.414 |
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