DIYbanter - View Single Post - Does a CU on a sub-main need an integral isolator switch?

Roger Hayter[_2_]

John Rumm wrote:

On 13/02/2019 12:18, wrote:

A situation that I can't found an answer to ...
There is a DP isolator before the main CU (which has an isolator). This
CU will be fitted with a 50A MCB feeding a 20m sub-main to a second CU
elsewhere in the house. There are therefore 3 ways to isolate the sub-main.
Is there a requirement for the second CU (at the end of the sub-main) to
have an integral isolator switch?

If its a CU and not a distribution board, then you will need a main
switch. There are two main reasons that spring to mind:

1) Expectation - from a user's PoV they expect there to be a main switch
present. A guest / future owner etc will not have the background
knowledge to know its a sub CU fed from another, and might be looking
for a way of killing the power in an emergency.

2) Practical, you need some way to terminate the live of your incoming
sub main on the the live bus bar(s) of the CU. The live bus bar has no
terminals of its own, and relies on those of the main switch to connect
the incoming feed to the bus bar.

I had been wondering the same question. And, now that you mention it,
your point 2) is pretty conclusive. It would be a shame for an amateur
such as myself to discover one had insufficient ways to fit a main
switch and and no other way of putting the thing together *after*
ordering the parts!

--

Roger Hayter