Obvious question more than likely.

[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.

 

[telectrix]

light switches are invariably 10A and only feed 1 light.

 

[Guitarist]

As tel says, the load should be fairly evenly distributed. Up to you as the designer to make sure the RCD's don't get overloaded.
Pretty basic stuff this.

 

[brightspark2010]

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

 

[Darkwood]

@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.

 

[D Skelton]

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'

rubbish.

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!

 

[Guitarist]

Oit! remove those apostrophies from the word 'Qualified'




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!

Are you saying that it's hard to cover even the basics in 18 days?

 

[ayjay]

imagine it had a 5 amp fuse fitted-------

 

[D Skelton]

I'm saying that on occasions, even the basics aren't covered in four years!

 

[shagbite]

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.

 

[Darkwood]

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.

 

[D Skelton]

Based on overload capacities then yes, I agree, but with the thought in the back of their mind that it has an overload capacity? Well, up until a year ago that was me. I'm time served and was taught this both at college and as an apprentice despite my reservations! I still designed my circuits based on the overload capacities of the OCPDs but always thought that stand alone RCDs had overload protection too. So I fully understand where someone is coming from who says they're qualified but don't know this.

 

[shagbite]

Based on overload capacities then yes, I agree, but with the thought in the back of their mind that it has an overload capacity? Well, up until a year ago that was me. I'm time served and was taught this both at college and as an apprentice despite my reservations! I still designed my circuits based on the overload capacities of the OCPDs but always

thought that stand alone RCDs had overload protection too.

So I fully understand where someone is coming from who says they're qualified but don't know this.

one of my strongest memories of college is one of the tutors shouting to the class..."AN RCD PROVIDES EARTH FAULT PROTECTION ONLY.....IT OFFERS NO OVERLOAD OR SHORT CIRCUIT PROTECTION WHATSOEVER!!"

he must have drilled this into us about a dozen times.

 

[Darkwood]

I recall many years ago i did have my grey area's but they weren't what i would call the Basics so its this that worries me and as i said earlier i don't blame the student i blame the system thats been chopped down and simplified ...we had written exams as well as a small multi-choice and there was a high failure rate as well as a high thresh-hold before you were given a pass... nowadays i bet the majority would fail the same level of exam i took and out of the class of 22 only 5 passed ..... its amazing what a mess the goverment can do when ensuring more kids are in jobs hence we are where we are.

 

[Guitarist]

When I was training, it was quite common for many apprentices to fail either their practical training, college, or both. Nowadays with training centres pretty much guaranteeing a "pass", it's no wonder there are so many out there with little basic knowledge. No more sorting the wheat from the chaff.

 

[D Skelton]

The problem I found when I was at college is that we were being taught about different types of motors and their control before we'd even been taught ohms law! We were doing complex equations in year one and learning about bonding in year two! My education was back to front until year three so I definitely had my grey ares, but weirdly, had strong areas too. I learned basic ladder programming before I learned that an RCD had no overcurrent protection! I wouldn't say my education was weak, it just had weak elements. One or two tutors that had never been on a site in their life but then I had tutors who were absolutely excellent!My point is, it is very easy to know your stuff but at the same time have gaps in your knowledge. I did bonding to death at college, so I consider that basic knowledge, but I'm always stunned by the amount of qualified electricians who still get confused by it and think things like "it's there to stop the pipes becoming live if a cable touches them". Again, they have just had weak teaching in that area. I'm sure there are many people out there confused by bonding that DO know that a main switch is just a switch.The fact remains though that any spark worth his weight, if unsure will ask! It's the ones who think they know it all after 18 days but plough on regardless that worry me.

The best sparks are the ones that KNOW they have grey areas!