402 High voltage control transformers— In high voltage systems having high resistance earthed neutral, voltage transformers in switchgear and control gear may be installed without primary side protection when installed between phase and earth. The thermal withstand capacity shall be adequate to prospective fault current and the transformers shall not be based on a single core (i.e. phase to phase short circuit is impossible).— In three phase systems with neutral insulated, voltage transformer primary side protection will not be required providing that transformer is not formed based on single common core.
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