7. A landscape crew is in the process of trimming a tall hedge the day after a h
ID: 2292761 • Letter: 7
Question
7. A landscape crew is in the process of trimming a tall hedge the day after a heavy rain. Four workers, using various types of equipment, are trimming the hedge as described below. List the severity of the possible electrical hazards that each of the four workers may experience, in order from 1 (least hazardous) to 4 (most hazardous). Worker A is standing on an aluminum ladder and using an electric hedge trimmer with a metal housing. The earth ground of the trimmer has been eliminated, as it is powered using an old two-pronged extension cord without a ground wire Worker B is standing on a wooden ladder and using an electric hedge trimmer with a plastic housing. The trimmer is powered using a new extension cord with an earth ground, plugged into a GFCI outlet. Worker C is standing on a wooden ladder and using a manual hedge trimmer, and wearing a rubber suit. Worker D is standing on an aluminum ladder and using an electric hedge trimmer with a metal housing. The trimmer is powered using a new three-prong extension cord, plugged into a grounded three-prong outlet. A. B. C. D. For the most hazardous situation described above, determine the maximum current that could flow through the worker if the resistance of the worker's body, between her hand holding the trimmer and her feet standing on the ladder, is 1000 ?.Explanation / Answer
Severity of hazards:
Worker A: order of severity (4)
Since the hedge trimmer housing, ladder are conductors and the ground wire is eliminated and there is no mention of any protective suit worn by the worker, this case will be of highest severity.
Worker B: order of severity (2)
The hedge trimmer has a plastic housing and it has Earth ground provided. If the wooden ladder is dry, it will provide sufficient resistance. Hence the severity is reduced.
Worker C: order of severity (1)
Assumptions: 1. Ladder is dry 2. Manual hedge trimmer has a wooden or plastic handle ( as is the usual case)
Since the worker has a rubber suit( electrical grade rubber used with correct voltage rating) risk is greatly reduced.
Worker D: order of severity (3)
The only protection the worker has is that the trimmer cord has grounground wire. Even though the ground wire diverts current in case of a fault, some amount of fault current will flow in the path created by trimmer, worker and aluminium ladder.
Most hazardous situation is of worker A.
The path taken by fault current is: hedge trimmer- worker- ladder- ground.As the resistance offered by metal parts is negligible, Fault current will be opposed only by the body resistance of the worker- 1000ohms.
Fault current= phase Voltage of the electrical system/ 1000 ohms.
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