Discussion
The steps of removing a wattmeter from a current transformer circuit in service are listed below. Put them in order.
1. Disconnect the instrument leads 2. Close the shorting link across the secondary terminals 3. Remove the instrument 4. Check that the shorting link is making good contact
Answer: A
Correct answer: 2, 4, 1, 3
The shorting link goes on first and is then checked, because an open-circuited current transformer secondary is dangerous: the primary current becomes pure magnetising current, the core saturates hard and very high voltages appear across the open terminals. Only once the short is proved may the instrument leads be disconnected and the instrument removed. This ordering is a safety rule, not a preference.
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