Instrument Transformer and Power Management (P1) Course
Chapter (2) ; Safety Basic
2.7. Step and Touch Voltage
Other hazards exist in a substation due to current flowing. This can be as a result of unbalanced loads, induction due to conductors that run parallel, electrostatic charges or fault currents.
One of the most dangerous situations is when fault current flows, as these very large currents give rise to very large voltages.
In the case where a feeder contacts the ground in this instance the substation will supply large currents along the feeder to the fault.
This current must return to the source and does so through the ground, causing voltage drops as it does.
The current that enters the ground will distribute it self over a very large area, so there will be little voltage drop at locations away from the fault or substation.
At the fault, and again at the station, the current will be concentrated and will cause large voltages to be developed. These voltages will normally be distributed exponentially shown in the following figure.
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| Fig. 4 |
This means that any one walking near the fault or near the substation will be exposed to high voltages unless precautions are taken, putting some numbers on the above figure we can obtain the following.
2.7.1 Step voltage :
From Fig. you can see that, a person with their feet about 0.5 m apart can experience a potential difference of more than 1000 volts. This known as 'step potential"
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| Fig. 5 |
2.7.3. Touch potential ;
This figure shows that a person can be exposed to several thousand voltages in such an instance.
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| Fig. 6 |
source : http://www.electricnet.com/article.mvc/Ground-Rules-by-John-Cadick-Power-System-Stud-0004
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