A quick mcb calc for the newbies on here.

[i=p/u]

240 tutored hours a year overs 3 years = 720 plus nvq on 4th years which is two hours a fortnight and plenty to do at home

now if you half that as thats what you done as practical = 360hrs is roughly only 45 days actual hands on expierience... so you really need to be out with employer as moving along.... which i think is the case..


my tudor told us its based on domestic now as they only talk about everything uoto 100A cut out/meters...

but we did cover trunking , tray, steel pipe , swa, pyro and most practical scinarios in small detail even to just get the feel of it and name..

as for theory he told us he covering more than prosepectus allows him and when you in he talked for for 8hrs solid..... he said loads to get through even tho h didnt have to cover some of it..

tudors background , one of head electrical engineers in du pont factory and i take my hat off to him even tho i dont wear hats cause they make me itchy

 

[i=p/u]

if you use a contactor does this not activate lights at full voltage taking away inrush???

 

[i=p/u]

and as you say , i have expierienced this going into comercial and floods tripping but contactor cured...ive also seen recognised companys putting 20 x 50W halogens on 5A fuse which customer had 3 x10pks fuse sitting beside cu. and yes i cured from help from youselves by adding type c mcb

Thanks for asking for the solution well il attept to answer... When using inductive loads especially multiple loads that share the same MCB and are subject to striking together then the first port of call is manufactures guidelines (tables or by direct enquiry)... subject to their advice on the mcb rating required to eliminate all chances of nuisance tripping will have a great bearing on the design of the install especially csa of conductors.

The 400w high pressure sodium has no specific answer for mcb rating but choosing the mcb to protect it then i can give you a sample answer;

Merlin Gerin C60 range of 60898 recommends a 6amp(c) which is within the normal thinking channels but if you have 2 x 400w then its a 10amp.

Square D KQ range of 60898 recommends a 10amp(c) for one 400w fitting and a 16amp for 2 x400w

Hopefully you can see that not knowing the manufacturers mcb's characteristics with a particular inductive load whether it be a motor, transformer or lamp(s) etc can make your standard teachings for designing a circuit and sizing a cable wrong, the colleges are in the habit of teaching you a rule of thumb method and when you progress from a domestic or small commercial situe you can quickly find your installs having issues of tripping and the only to rectify it would be a increase in cable size to allow for the correct mcb according to manufacturers.

This is my beef with the dumbed down system its been stripped back so much (not the students fault) that they are ill prepared and are still rubber stamped with the term electrician although they could not effectively work out what correctly what would seem a simple circuit, going by college a 400w hp sodiums = 3.13amps- a 6amp (c) mcb but Square D KQ range specifies a 10amp whereas Merlin agrees with the 6amp rating.

Ironically both Merlin and Square D are made by the same manufacturer 'Schneider' ...now the examples i have shown are to emphasise the issue but as the inductive trait of the load increases i.e. a large motor then the standard teaching become more problematic... ive seen so many installs designed as taught where the mcb trips from a standard inrush and upgrading the mcb to suit the job would require the cable to be upgraded.

 

[Darkwood]

if you use a contactor does this not activate lights at full voltage taking away inrush???

A contactor is just a switch it has no bearing on the inrush, what inrush occurs without a contactor still occurs with one, contactors are used so a small control circuit can operate a larger rated circuit so you could have a 4amp mcb control to a bank of switches which pull in 16amp lighting runs over many phases

 

[i=p/u]

yep just a switch, whiuch to be honest took ages to grasp bt once i wanted to know , 2 days running through my head in how and where could be used.. i know now or do i really know lol

 

[Chr!s]

We have 5 230 V high pressurer sodium light's 400W each. The Displacement Power Factor is 0.9, Capiatance of each Fitting 22.5 uH, PFC(Isc) at fitting is 338 AmpsWhats the ;Total Active Power?The Apparent Power?Total RMS Current?Peak Current?Total Capitance?The Line Inductance?The Peak Inrush Current?What Sizeand Type Breaker Is Required?All the Info is there

The solution and the main factors that decide which MCB is contained in the above, it's a good exercise and gived a better understanding.

 

[SPARTYKUS]

College can't teach everything. Like passing a driving test, start really learning afterwards....

 

[Darkwood]

@chris ... i see your angle but the manufacturers have already done the maths and each brand differs in its tolerences so you would still have to consult with manufacturers...
@Spartykus.... yes your right but they fitted it all in in my day and its not just that its the illusion that the college leads you to believe that you can use the same teachings throughout the industry .... if its not part of the course then they should make the tutors the limitations of the calcs they have been learning, at least this way they would not walk blindly into commercial or industrial and be aware further education is required.

 

[Chr!s]

@chris ... i see your angle but the manufacturers have already done the maths and each brand differs in its tolerences so you would still have to consult with manufacturers...
@Spartykus.... yes your right but they fitted it all in in my day and its not just that its the illusion that the college leads you to believe that you can use the same teachings throughout the industry .... if its not part of the course then they should make the tutors the limitations of the calcs they have been learning, at least this way they would not walk blindly into commercial or industrial and be aware further education is required.

The manufacture's only play one part, they have no control over the system to which they are connected, the system can be the major factor, otherwise you may as well go down the route of 1.8 and a type C.

 

[Darkwood]

Well i already pointed out the 1.8 factor isn't reliable outside domestic and yes agree with you if the install is unusually large or unusual but i use tables that have a balanced scope that have already taking into account standard install practices, i.e. if i recall one set is a guide for lighting based on a 20m to first fitting then 7m between with a lighting trunking scheme, its a quick reference and its always done me good, but yes if say your feeding a high Inertia motor with long start-up times then its back to the calcs rather than the tabled format..... what i was trying to show other than the missing education of new sparkies was that a simple thing such as brand of protective device can make a big difference to the whole install as some tabled results are not the same for all brands.

Im a one man band so my own workload limits the scale of the install down to sizes where the tables are relevant and a good guide and give all kinds of lighting and how many per mcb are allowed for said type, anyway ive decided i dont mind the stripdown of the college as its keeps me in work when they venture into my field and mess it up :greedy:

 

[sbelec]

t's interesting you said the manufacturer is only one part, given the current level of legislation and blame culture installing anything to a design which does not either comply with or improve on the manufacturers instruction manual if fraught with risk. You would need to be iso9002 registered with all the back up documentation to prove that your design was safe and fully tested and proven, otherwise ......
Mind you it doesn't help when the manufacturers instructions are worse than useless and contradictory or just down right wrong!
I install electric boilers and the boiler recommended cable sizes are at least one size up than required but the small print says the guarantee is only valid if the installation complies with the installation instructions.
Whilst I accept that current day qualifications lead a lot to be desired, I got 100% on my 17th edition test when it came out but there were guys in the room with more hands on experience than I have who failed! I have installed transformers, motors up to 175kW, PLC's and MCC's, and I have never installed a sodium lamp. In fact other than domestic and small commercial never done any lighting either design or installation. The definition of an electrician can be very misleading, I know my limitations of experience and with constantly upgraded kit I would sometimes prefer to check it out than rely on my memory of a 35 year old calculation method.

In the case of your example I would have simply priced a 1.5mm cable and 6amp C type mcb, which would have been more than a match for your lamp but had somebody asked me for a bay of 10 then I would have been checking my design guides.

 

[Chr!s]

t's interesting you said the manufacturer is only one part, given the current level of legislation and blame culture installing anything to a design which does not either comply with or improve on the manufacturers instruction manual if fraught with risk. You would need to be iso9002 registered with all the back up documentation to prove that your design was safe and fully tested and proven, otherwise ......
Mind you it doesn't help when the manufacturers instructions are worse than useless and contradictory or just down right wrong!
I install electric boilers and the boiler recommended cable sizes are at least one size up than required but the small print says the guarantee is only valid if the installation complies with the installation instructions.
Whilst I accept that current day qualifications lead a lot to be desired, I got 100% on my 17th edition test when it came out but there were guys in the room with more hands on experience than I have who failed! I have installed transformers, motors up to 175kW, PLC's and MCC's, and I have never installed a sodium lamp. In fact other than domestic and small commercial never done any lighting either design or installation. The definition of an electrician can be very misleading, I know my limitations of experience and with constantly upgraded kit I would sometimes prefer to check it out than rely on my memory of a 35 year old calculation method.

In the case of your example I would have simply priced a 1.5mm cable and 6amp C type mcb, which would have been more than a match for your lamp but had somebody asked me for a bay of 10 then I would have been checking my design guides.

I think your misunderstanding, ISO registered, what on earth for? what do the manufactures state with regard to inrush currents?

 

[sbelec]

The manufacture's only play one part, they have no control over the system to which they are connected, the system can be the major factor, otherwise you may as well go down the route of 1.8 and a type C.

The manufacturer may have stipulated the type of system. I install electric heating boilers and the manufacturer stipulates the cable size which I believe is too big by at least one size but failure to comply can be deemed to contravene BS7671 (can't remember the section) and in this case actually invalidates the warranty.
So if you do not heed manufacturers instructions you could have to show evidence that the design you have used is better, following iso9002 document methodologies would prove this.

In the event of an incident you could be asked for documentation proving that you had designed the system correctly prior to installation. Houses are great for this as BS7671 has standard circuits! I remember the amount of documentation that came out of design team for the factories I used to commission.

 

[Chr!s]

The manufacturer may have stipulated the type of system. I install electric heating boilers and the manufacturer stipulates the cable size which I believe is too big by at least one size but failure to comply can be deemed to contravene BS7671 (can't remember the section) and in this case actually invalidates the warranty.
So if you do not heed manufacturers instructions you could have to show evidence that the design you have used is better, following iso9002 document methodologies would prove this.

In the event of an incident you could be asked for documentation proving that you had designed the system correctly prior to installation. Houses are great for this as BS7671 has standard circuits! I remember the amount of documentation that came out of design team for the factories I used to commission.

I'm sorry, but that's a load of nonsense.

 

[LX-1]

I was considering going to college to try and make me more employable, if what your saying is that the standard that colleges are teaching is not up to scratch is college worth paying for, i am domestic only and would not dare take on work that i was not 100% confident i was safe to carry out. not that it seems to stop alot of people! what would you recommend?