Ground Detector Circuit

Hello, everyone (especially current or former USN Electrician's Mates). As some of you probably know the U.S. Navy uses ungrounded shipboard
A.C. power distribution in order to ensure "battle short" availability of on-board electronic equipment. In Fig. 3-13 of USN training manual 14344 (available as a downloadable pdf at http://ebookbrowse.com/navedtra-14344-electrician-s-mate-pdf-d60422134 ) is a simple circuit for determining whether a fault condition (short to ship hull) exists on any phase of a 3-phase system. My question is what is the purpose of hull grounding of the midpoints of the transformer secondaries in Fig 3-13? Thanks for your time and comment. Sincerely,
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J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

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