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Obvious question more than likely.

Discuss Obvious question more than likely. in the UK Electrical Forum area at ElectriciansForums.net

J

jwc

Second question of mine on here and it's one I've never got round to getting an answer for.


Mains comes into a building through a 60A main fuse for instance.
Why then are main switches rated at 100A or 80A?
Does this mean any load above 60A would blow the main fuse before it blew the main breaker?


Don't be harsh people. I'm fully aware the answer's probably an obvious one.
 
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a lot of domestic propeties have a 100a main fuse so why do many domestic boards have 100a main switch then 80/63a rcds surely there is potential for overload under fault conditons or am I missing something silly?
 
I understand what your saying but think of a ring it is wrong but you come across a 20 amp double pole switch which feeds a single socket for appliance the appliance is fused but you would have to change that double pole switch a its only rated to 20amp but under fault conditions could pull 32amp so if this is wrong whats the difference with a 100a fuse supplying downstream an 80 amp rcd with no downrating? I understand what your saying about balancing the load but why are they not 100a rcds inline with modern cut out fuses? also light switches do far more than 1 light in commercial and rooms with downlights etc a lot of old switches are five amp if your doing eicrs too
 
@Bigshow84
Explain your fault conditions you refer to.... a short will see a possible current over 1000amps this will operate the protective device in a time that damage isn't caused to the cables, accessories etc

You can spur a single socket direct off the ring you wouldn't fit a 20DP switch, the fuse within the plug top will limit current ... see appendix 15
You keep saying fault conditions but your are not specific by what you mean....

Following the regs you would not design a circuit capable of giving a small overload for a long duration, please explain this fault that is getting you confused.
 
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im scared to the fact this question comes up numerous times from 'Qualified' Electricians now im starting to think you in yourself are not to blame here but either your tutor or the course itself

Oit! remove those apostrophies from the word 'Qualified' :D



Not neccessarily mate


How could such knowledge be missed or not taught if you have completed all the above, and gained the qualifications!
I was refraining from saying anything, but it seems like the rest of you haven't held back! I agree with jamesBoater. how can the OP not know this basic knowledge after having gained said quals?!

With the standard of training available today, it isn't surprising that this simple bit of knowledge is often missed. Believe me, I know!
 
Not neccessarily mate

With the standard of training available today, it isn't surprising that this simple bit of knowledge is often missed. Believe me, I know!



i hope you're wrong D. if a time served spark is installing main switches based on their overload capabilities, then the trade's in a worse state than we all imagined.




EDIT: and to avoid confusion, by "overload capabilities", i mean ability to interrupt overloads.
 
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Whats even more worrying is the large schemes out there be it Nappit, Elecsa or Niceic etc all welcome them with a golden ticket and fuel this dumbing down of the industry ...its all ok expressing they have stringent strict guidelines to gain entry but means nothing when you ask the wrong questions and allow the member to pick and choose the jobs you can check..... if they ever were done by you in the first place.
 

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