Discuss Forgive me, it's late... (stupid question alert) in the Electrical Forum area at ElectriciansForums.net

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IAmSparkytus!

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Good evening,

As the title suggests this could be and probably is a stupid question.

I've been flicking through posts and noticed a post regarding a fused switch isolator being installed before a run of SWA feeding a DB.

Why would a fused isolator be necessary if feeding the isolator is the main incoming fuse which itself obviously had a fuse in similar to the fused isolator? Why wouldnt it be possible to just fit a 2 pole 100A switch for example?

Don't see why this is any different to the main fuses feeding a board located directly adjacent to the incomer if the SWA is adequately sized.
 
It harks back to an old DNO regulation about 3M max tails to be connected to their service cut-out

you could in theory just use a DP switch to extend the tails beyond 3M but the DNO might not connect you
 
As stated above, the dno only allows up to 3m from their fuse without additional protection such as a fuse, it's a bit silly really as they may fit a 60A fuse, and you fit a 100A downstream - which complies with the dno rule, but of course is actually irrelevant as the dno fuse will likely go first in this example.
 
Not a silly question.
Beyond the DNO regs - it is of course more economical in terms of cable costs to fuse down and size the SWA for the demand, as opposed to sizing the SWA to carry the full rating of the suppliers fuse.

( As a point of interest we also have a reg (433.2.2) that says the overcurrent protection can be along the length of a conductor if it's length doesn't exceed 3m. It recently threw me why in a 3 phase chamber everything was 35mm except for 4 x 6mm feeding a fused switch, so I had to look up and remind myself why that was ok. )
 
The BS regs do allow just an isolator switch if you have the DNO's permission to use their fuse for protection. See 434.3 (iv) for the text.

I suspect getting that is going to cost more in time & effort than just putting in your own fused-switch
 

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