Fig. 1 shows the electrical arrangement beside a garden pond after rain. A submersible pump has a metal case and is connected to an outdoor socket through a...
Question 1 Report
Fig. 1 shows the electrical arrangement beside a garden pond after rain. A submersible pump has a metal case and is connected to an outdoor socket through an RCD. The flex has a split in its outer insulation close to the pump. Water has collected on the paving stones. The gardener notices that the pump sometimes stops and the RCD must be reset. The supply is 230 V and the pump takes a current of 0.80 A when it is working normally. A warning label on the control box states that the RCD trips when the current in the live wire is not equal to the current in the neutral wire.
(a) What is the power of the pump when it operates normally? Show your calculation. [3]
(b) Which part of the arrangement provides a low-resistance path to Earth if the metal case becomes live? [1]
(c) What is meant by the RCD detecting a difference between the live current and neutral current? [2]
(d) When a fault causes current to flow from the live wire through water or a person to Earth, explain how the RCD reduces the danger. [4]
(e) Which two actions should the gardener take before anyone touches the damaged flex? [2]
(f) What is the main reason that rainwater makes this fault more hazardous? [2]
Answer Details
(a) \[P=VI=230\ \text{V}\times0.80\ \text{A}=184\ \text{W}.\] [3]
(b) The earth wire, or earth conductor, provides the low-resistance path to Earth. [1]
(c) A difference means that some current has left the intended circuit by another path, as leakage current. Therefore less current returns in the neutral wire than flows in the live wire. [2]
(d) A fault current can flow to Earth through water or a person. This makes the live and neutral currents unequal. The RCD detects the difference and disconnects the supply rapidly, limiting the duration of an electric shock and reducing risk of serious injury. [4]
(e) Two suitable actions are to isolate or switch off the supply and unplug the pump. Keeping people away from the water and arranging competent repair or replacement are also acceptable. [2]
(f) Rainwater contains dissolved ions, so it can conduct electricity. It gives current an easier path through a person or to Earth, making the fault more hazardous. [2]
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