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]

you are confusing the main switch with a fuse. the rating of the switch is the max. load it will carry. it is not an overcurrent protective device

 

[xylophone]

Hi. The answer is in the terminology. When you write "Why then are main fuses rated at 100A or 80A?" you probably mean "Why then are main switches/RCDs rated at 100A or 80A?"
The switch rating is not related to your supply rating (but needs to be same or more)

 

[jwc]

But why have a main switch rated at 80A if the main fuse is only 60A?

 

[telectrix]

60A fuse will happily run @ 80A for about 1 hour. that's why you have a switch rated above the fuse rating.

 

[jwc]

Just received an inbox stating that the main switch doesn't trip on overload? :/
Really? Thought the main switch was essentially an MCB to carry the load of the other MCB's if that makes sense?

 

[dan86]

at the moment you're making no sense man

 

[jwc]

How? haha.
I just wanna know what's the point in having a main switch with a higher rating than the main fuse which supplies it?
Surely you want your main switch to disconnect before the main fuse blows?

If your main fuse is 60A, main switch is 100A, and your house is pulling 80A, what use is the main switch?

 

[telectrix]

Just received an inbox stating that the main switch doesn't trip on overload? :/
Really? Thought the main switch was essentially an MCB to carry the load of the other MCB's if that makes sense?

totally wrong. the switch is a 2 pole switch, nothing more. each circuit is protected by it's own MCB. the installation as a whole is protected by the DNO fuse. think of the switch as your car. it will do 110 mph, but you run it at no more than 70mph.

 

[jwc]

Thanks for explaining, Telectrix.

 

[Darkwood]

JWC - Its hard to respond to a question sometimes without been harsh but 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 .... may i ask what kind of course you have taken... now you realise that the main switch is just a mechanical switch and nothing more its brings to question are other areas of your learning lacking in basics ..... this could be dangerous in design in some situations if you thought the main switch on a standard dist-board afforded overload protection.

Hope you dont take as a dig at you...

To note hear you can get main switches although usually in larger commercial and industrial boards that do afford overload protection 'MCCB's and or a combination of earth leakage protection.... maybe this is where you may have confused yourself, but your bog standard 17th edition board DP switch is exactly that - a switch without the perks.

 

[jwc]

Thanks for the explanation, Darkwood. I've completed a full apprenticeship, AM2 and NVQ Level 3. Everything's much clearer now knowing that the main switch is exactly that... A switch.

 

[shagbite]

Thanks for the explanation, Darkwood.

I've completed a full apprenticeship, AM2 and NVQ Level 3.

Everything's much clearer now knowing that the main switch is exactly that... A switch.

rubbish.

 

[trev]

Also not having a go but- How on earth can you think something called a main SWITCH is anything other than what it claims to be?

 

[jamesBoater]

How could such knowledge be missed or not taught if you have completed all the above, and gained the qualifications!

 

[ian.settle1]

On a similar vane I had an electrician with 2391 who carried out a PIR and recommended that a 40A 30mA RCD should be changed to a 32A one, asked him why he replied because the fuse protecting the circuit is only 32A thus it would go before the RCD if the was an earth fault and not the RCD as it was rate at 40A, shook my head and walked away in disbelieve

 

[Guitarist]

There is a simple reason for switches being 100A.... They can be used on a system with a 60A fuse, an 80A fuse, or a 100A fuse. In other words, one size fits all.
As already said, a switch is just that, the same way as an RCD is just that...they do not "trip" on overcurrent. While I agree that nobody knows everything, this is pretty basic stuff.

 

[SirKit Breaker]

But why have a main switch rated at 80A if the main fuse is only 60A?

Because thats what the manufacturer makes, and it will do for any supply rated at 80a or less.

 

[dan86]

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?!

 

[richy3333]

Thanks for the explanation, Darkwood. I've completed a full apprenticeship, AM2 and NVQ Level 3. Everything's much clearer now knowing that the main switch is exactly that... A switch.

FFS and people on this forum lambast Electrical Trainee!

Stick a 63A Main switch in and everyones happy?!

 

[SPARTYKUS]

FFS and people on this forum lambast Electrical Trainee!

Stick a 63A Main switch in and everyones happy?!

fair comment. Och aye.

 

[richy3333]

fair comment. Och aye.

Don't bring racism into it too

 

[trev]

Sore point Richy, some knacker accused me of that in a roundabout waylast night on here. Least said soonest mended bonny lad

 

[richy3333]

trev your'e the last person I'd think of a racist. Tell em to **** in their hat and punch it! (not a very scottish term, but its what came to mind)

 

[SPARTYKUS]

Don't bring racism into it too

Ahh dinna ken wha yer sayin, laddie ;-)

 

[ipf]

trev your'e the last person I'd think of a racist. Tell em to **** in their hat and punch it! (not a very scottish term, but its what came to mind)

Could use their sporran, then

 

[trev]

There's a French saying, Je te pis au cour
Something like that anyway, I should have said that to him

 

[brightspark2010]

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?

 

[telectrix]

because there are 2 80/63A RCDs, hopefully the load is reasonably equally shared between them.

 

[brightspark2010]

ok good point but isnt that similar to say a 10 amp lighting circuit with 5amp switches in different rooms surely the light switch is underated and could be overloaded under fault conditions?

 

[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!

 

[brightspark2010]

I understand what everyone is saying here but i give you 2 different scenarios totally then the kitchen ring with a 20a double pole switch connecting a socket for an appliance low level (thats wire theres a switch) years ago when touring some sidn off sites with my old niceic supervisor pointed out to me this was wrong and it needed to be a fused connection as the 20a dp switch was not rated to 32a is this correct I have seen this over the years a few times and noted it as a fault.

also say a 32a mcb then 6mm cable to an isolator 20amp rated iisolator 10amp load air con unit surely doing aan eicr on this the ciruit would need work to repace the isolator? or not potentially something could happen that lets more thn 20amps pull through the isolator surely the isolator has to be higher or equal to the protective device and not just rely on the fact the design current of the load is below the isolators rating or am i missing something

 

[Des 56]

quote
my old niceic supervisor pointed out to me this was wrong and it needed to be a fused connection as the 20a dp switch was not rated to 32a



http://www.mkelectric.com/Documents/English/EN%20MK%20Technical%20Specifications/T02%20LOGIC%20PLUS%20Tech%20355-389.pdf

Use the link above and scroll down to page 13


MK Fused connection units and 20 amp double pole switches have exactly the same supply terminal capacity
terminal capacity suitable for 3 x 2.5mm conductors
. 2x4mm
. 2x6mm

A double pole swicth rating of 20 amp is the maximum rating of the switch contacts

Whether the device incorporates a 13 amp fuse or not,it will make no difference to the supply terminal capacity,they are exactly the same

If you are happy to accept the one, then the other should not be an issue

 

[shagbite]

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.

i started writing a post highlighting this problem late last night, but i stopped halfway through because i was knackered.

at my college, failure wasn't an option. the college's funding was based on the success of the students.

i think i've mentioned this before, but i always found it strange that the really weak students (who really shouldn't have been accepted on the course) could fail some of the exams one week, and then get a distinction in the re-sit a week later.

in one of the online multiple-guess exams, one of the poorer students was sat at my '5 o'clock' - just over my right shoulder. throughout the entire exam, one of the tutors (who was supposed to be acting as an invigilator) was sat at the computer with him. when we'd finished i asked the lad what the tutor had been doing. he freely admitted that, whilst not giving him the answers outright, he'd been reading through the questions with him and had been pushing and prompting him towards the right answers.

i don't blame the tutors though. they were constantly under pressure (from the college hierarchy) to produce results. the same hierarchy that accepted these sub-standard students onto their courses.

it might sound harsh, but they should use a more stringent vetting process when they interview prospective students. you don't have to be einstein to be an electrician but you should be able to read & write and do a bit of maths.

 

[D Skelton]

but you should be able to read & write and do a bit of maths.

I disagree, proper sparks should be mathematicians long before they get their hands on any cables!

 

[Engineer54]

I'm just totally gobsmacked at the total lack of common knowledge displayed on this thread by supposedly time served electricians. In fact i can't and don't believe that they are, not too sure if they are even electrical trainee's! God Help this Industry!!

My hat off to Darkwood & Telectrix for having the patience to go back to bare basics and spoon feed these guy's the why's and wherefores of the operation of a CU main DP isolator/switch!! I know i haven't that kind patience!! lol!!