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!

 

[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!!

 

[D Skelton]

By that definition Eng that makes me a Electrical Trainee. I find that rather offensive.

There was a time when you didn't know it all.

 

[Engineer54]

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.

In my day, we had to sit a series of aptitude and math tests to be accepted by my company as an apprentice!! There was no such thing as multiple choice C&G exam questions either. Each question if answered, needed to be answered in full, so if you didn't know, then you failed, it was as simple as that!!

 

[Engineer54]

By that definition Eng that makes me a Electrical Trainee. I find that rather offensive.

There was a time when you didn't know it all.

I can't help how you define what i've stated, or if you want to take offence or not!!

Yes, .....it was a very long time ago, but nothing as basic as knowing what a CU DP main switch is!! lol!!

 

[D Skelton]

So you knew that from birth did you? Wow!

You can help how I define what you've stated, and that is by not stating it. What you define as basic knowledge is completely subjective and even still, how can someone who was taught this most 'basic' of knowledge from the word go by more than one 'qualified' tutor and by his site supervisor be at fault for accepting what he was being told? I don't know the OPs situation, whether or not he is a Electrical Trainee or a time served spark, but what I do know mainly through personal experience is that questions like this can be answered without jumping to conclusions.

What I can't abide by is supposed 'qualified' sparks asking stupid questions like: "Why can't I use one neutral conductor for two different circuits?", "does the water need bonding if it comes in in plastic?" and "can you put a socket in a bathroom?" when the answer is there in black and white in the BGB. Unfortunately, there are things that aren't in the BGB that need to be taught, sometimes they just happen to be taught wrongly or not at all. There was a recent post on here from a valued member asking questions about adiabatic equations and how 't' is applied in different circumstances. This is basic stuff for me, calculations, formulae, thermal effects, inductance, capacitance, trigonometry, transposition blah blah... but clearly it isn't basic for him. Still, I know through some of the advice he has given others, stuff that is basic for him is good advice and that he knows what he is doing but most importantly, knows his limitations at this point in time. Should I be slating him for his lack of 'basic' (in my opinion) knowledge?

Like I said, I was learning ladder logic before I even knew about bonding! At the time I still thought that an RCD had overload protection (not that it affected my work at all) I was wiring and programming complex automation systems, fault finding motors and designing installations, but by your definition I would be a $hit electrician because I lacked 'basic' knowledge. Well sorry mate but we all lack a little basic knowledge here and there.

I refuse to accept that someone who can realise that they need to ask questions, however basic at times, must be crap at what they do!

I bet you couldn't tell me off by heart the minimum distance control gear can be placed from a swimming pool or basin? I know it, it's basic knowledge Eng!?!?!. Does that make you a crap spark? Of course not!

 

[Guitarist]

In my day, we had to sit a series of aptitude and math tests to be accepted by my company as an apprentice!! There was no such thing as multiple choice C&G exam questions either. Each question if answered, needed to be answered in full, so if you didn't know, then you failed, it was as simple as that!!

I think you've hit the nail on the head there Eng. Back in the 80's when I started my apprenticeship, you weren't even accepted as a trainee without GCE maths and physics, along with a day of aptitude tests just to see whether you could apply logic and common sense. Only once you got through that (about 25% managed it the year I started), then the 4 years of "fun" began.
Now you can go from being someone who can't tie their shoelaces to becoming "qualified" in 18 days!
Heaven help us....

 

[Engineer54]

No-ones saying we don't have grey areas, or grey moments, but when you need to be led by the nose step by step on the function of a DP switch, then i'm sorry that's very worrying and doesn't instill confidence in that person!!
And that's all i going to say on the matter, it's not worth the hassle...

Yes i do know!! Most of my projects involve provision of swimming pools etc where residential accommodation is included and/or indoor pools for medical purposes. Also know the recommendations that are applied by other international standards too, ...Yes they do differ.

 

[D Skelton]

when you need to be led by the nose step by step on the function of a DP switch

Well that is a little different to either thinking one thing or the other. If someone needs it explaining to them again and again then there are obvious problems there, some of which aren't electrically related if you know what I mean... But simply believing one thing, asking a question and then being put right isn't the same is it.

 

[Guitarist]

Every day is a school day, no mistaking. Thing is, during a long apprenticeship you would cover most of the basics and have a few grey areas to iron out as the years progressed. Nowadays, you can't possibly cover even the basics in a few weeks before being let-loose on peoples' homes. I bet that at least once a week I am still shocked that someone had the absolute bare-faced cheek to charge for what is frankly appalling.
I have no problem with what people charge, as long as the customer is aware up-front, but I find it utterly disgraceful that people who can't install a backbox level are being certified as "qualified". Surely, these training courses should at least cover the basic building skills which are essential for domestic work???

 

[brightspark2010]

Hi all

Well done D Skelton

firstly I am a time served electriian i started in September 2001 and finnished in May 2005 with 2391 2006
I have a Agrade gcse math year 2000 and double AA Science
2001 Ihave physics as level grade d and mathematics grade e

I never asked about the function of main switch or wether a an rcd as overload protection or not I always knew they do not

However what I did believe is that any electrical componet not fused down in anyway making up the supply to aload had to be of equal value or greater (ampage) than its protective device and not merely designed so that you wouldnt exceed it current rating nobody answered is a 32a mcb using 6mm cable to a 20amp isolator feeding a 10/20amp whatever below 20amp load acceptable as the load is designed not to be above the 20amp isolator .

A

 

[D Skelton]

Simple answer mate: Yes!