the situation when a submain distribution board is supplied from a main distribution board.
is it necessary that the smdb has a main cb (or fuse) at its incoming.
i have seen some designs with identical cbs at the outgoing of the mdb and the incoming of the smdb.in this case ,what is the purpose of the cb at the incoming of the smdb as the cable will be protected by the cb at the mdb.
in other designs ,there is a main switch not cb at the incoming of the
smdb , this arrangement seems logical for more economy,
the question is ,is it nessary that every distribution board has a main cb , or a main switch will be enough,
please excuse my poor english.

There is no requirement for an additional circuit breaker at the sub db

thanks for replying, another question please, when a 3 pole cb rated 150 amp ,does it mean 50 amp per pole or the 150 amp is the sum of them all.

A 3 phase cb is rated per pole-ie a 32A triple is 32A per phase.
Your post above should say 50A per pole or 150A per pole.

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Those who make no mistakes do very little work!!......

yes, you're right,that was a mistake.
and thanks a lot for replying.

Merry Christmas, you're welcome.

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Those who make no mistakes do very little work!!......