Electrical system design 615 level 3 help!!!

[Garry0256]

Hello everyone I am doing my level 3 electrical system design project 615 and I need some help with design current etc. at the moment all the colleges are closed so i am unable to find any help from teachers. i would really appreciate if someone show me the right path how to calculate it.

 

[pc1966]

Have you got specific questions you want advice on? Many folk here will try to help out.

Also I think there are closed trainee forums that might be more useful, but I don't know. Others on here will know a lot more about this.

 

[Garry0256]

Have you got specific questions you want advice on? Many folk here will try to help out.

Also I think there are closed trainee forums that might be more useful, but I don't know. Others on here will know a lot more about this.

I need help with question 3 and 4.

 

[pc1966]

Start with Q3. There are two separate aspects requested, and before you select the devices you need to determine what the requirements are. So how would you go about:

• Establishing the PFC at the DB where the protective devices will be fitted?
• Examining the circuits (and their wiring routes) to decide what protection is needed?

Folk here won't answer them for you, but they will help you find the route to answering yourself!

 

[Garry0256]

Start with Q3. There are two separate aspects requested, and before you select the devices you need to determine what the requirements are. So how would you go about:

• Establishing the PFC at the DB where the protective devices will be fitted?
• Examining the circuits (and their wiring routes) to decide what protection is needed?

Folk here won't answer them for you, but they will help you find the route to answering yourself!
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Start with Q3. There are two separate aspects requested, and before you select the devices you need to determine what the requirements are. So how would you go about:

• Establishing the PFC at the DB where the protective devices will be fitted?
• Examining the circuits (and their wiring routes) to decide what protection is needed?

Folk here won't answer them for you, but they will help you find the route to answering yourself!

For lighting circuit I am thinking to use 6A type b rcbo and do you think that you be fine for pool lighting ?

 

[Strima]

For lighting circuit I am thinking to use 6A type b rcbo and do you think that you be fine for pool lighting ?

All depends on what luminaires are to be fitted. Some LED units require C type OCPD due to large inrush currents, some circuits might need to be 10 amp due to the number and type of luminaires.

Select your luminaires then you have some numbers to work from.

 

[pc1966]

For lighting circuit I am thinking to use 6A type b rcbo and do you think that you be fine for pool lighting ?

It is not what I think that matters, it is the examiner's view! But you are missing the point, an exam is not about giving the right answer, it is about demonstrating understanding to the required level of skill.

So back to the point, you have been asked for a suitable protective device and answered with a 6A RCBO. Why? What are your reasons for selecting that as the target device and, presumably, going from there to a specific part to match?

As a professional electrician you need to have many different skills and this sort of exam is to cover your understanding of basic electrical principles, knowledge of the UK wiring regulations, and an ability to translate them in to a practical solution fitting the client's budget and functional goals.

This is about circuit protection and you may, or may not, have the following:

• Short circuit protection
• Overload protection
• Leakage protection
• Arc fault protection

For this circuit what are needed and why? You need to show the examiner the reasoning for each point, if it is needed and possibly according to what part of the regulations, and finally what sort of device, or devices, you would use to provide that.

 

[pc1966]

Divide and conquer. With any complex problem it can be split in to more manageable section and each of them becomes easier to solve. So start with the first aspect, do you need short circuit protection? If so what are the characteristics?

Now it might seem obvious to you that it needs this protection, but the IET regs have some specific exceptions. Does your circuit fall in to any of them? If not, eliminating those options is you clear argument that it is needed.

So if you are protecting against a short, what do you need to achieve? What happens if the short is at the DB? What if it is at the furthest device on the circuit?

 

[Mark Wright]

Divide and conquer. With any complex problem it can be split in to more manageable section and each of them becomes easier to solve. So start with the first aspect, do you need short circuit protection? If so what are the characteristics?

Now it might seem obvious to you that it needs this protection, but the IET regs have some specific exceptions. Does your circuit fall in to any of them? If not, eliminating those options is you clear argument that it is needed.

So if you are protecting against a short, what do you need to achieve? What happens if the short is at the DB? What if it is at the furthest device on the circuit?

Prospective Short circuit can't increase further away from the board so the value at the board is the only relevant value - not the correct terminology I'm sure but in a way the further away the better, more resistance smaller fault current.

the only reason earth Fault current is so important in the U.K away from the board is because we cheap on CPC and use smaller conductors to our lines and might result in the protective device not operating but with MCB and RCD,RCBO its alot better(not perfect keep those Zs values down) than fuses witch is what the design project was probably based on.

 

[pc1966]

the only reason earth Fault current is so important in the U.K away from the board is because we cheap on CPC

Only? There is more to Zs than R2 alone!

But generally correct. Some would argue that relying on RCD to overcome a high Zs, while perfectly within the regs, is not ideal practice given the significantly higher chance of RCD electronics failing compared to the thermal/magnetic trip side. Now if only they were regularly tested...
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Bonus qestion, why might you choose a BS88 fuse over a MCB?

 

[Mark Wright]

Only? There is more to Zs than R2 alone!

But generally correct. Some would argue that relying on RCD to overcome a high Zs, while perfectly within the regs, is not ideal practice given the significantly higher chance of RCD electronics failing compared to the thermal/magnetic trip side. Now if only they were regularly tested...
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Bonus qestion, why might you choose a BS88 fuse over a MCB?

Obviously Ze Z=Impedance E=External but Ze is one place you shouldn't expect a high resistance unless TT. RCDs might have higher chances of falling but with good Zs you should be getting instantaneous just with MCB,

RCD's are still 100% more reliable than fuses if you want to put Supplementary bonding as a protective method you need to be very careful you are not introducing a fault current or different potential of course what needs bonding etc is often subject to discussion LOL but on the design project you have no idea of test result of what is an extraneous conductive part and making an assumption about what is extraneous in real life is not good.

Obviously aim to meet the requirements for no Supplementary bonding.


Explaining why you choose your protective device is a bit out dated as RCBO are the standard your not going to use anything less are you.

 

[pc1966]

Obviously Ze Z=Impedance E=External but Ze is one place you shouldn't expect a high resistance unless TT. RCDs might have higher chances of falling but with good Zs you should be getting instantaneous just with MCB,

Max Ze for a TN-S system is 0.8 ohms so can I always use a 32A C-curve MCB?

RCD's are still 100% more reliable than fuses if you want to put Supplementary bonding

For electric shock protection yes!

But I'm willing to bet that a fuse will fail-safe leaving a circuit disconnected but a MCB with stuck contacts wont. Also generally fuses limit the peak fault current but MCB won't. Finally most MCB have an max PFC of 6-15kA but many BS88 fuses are good to 50kA or more, to do that with a breaker you need to look at MCCB and adjust your budget up a bit...

Explaining why you choose your protective device is a bit out dated as RCBO are the standard your not going to use anything less are you.

In every case? What if your boss is a tightwad and looks for the cheapest solution for 200 new homes, is an all-RCBO CU cheaper then a split RCD+MCB combination? What if this is an office and you are feeding a UPS with a dozen servers on it?

Now you and I might both agree that in this case an RCBO is the best solution to this question's requirements if we were installing this, but how do you convince the examiner that your reasons are sound?

 

[Mark Wright]

Max Ze for a TN-S system is 0.8 ohms so can I always use a 32A C-curve MCB?


For electric shock protection yes!

But I'm willing to bet that a fuse will fail-safe leaving a circuit disconnected but a MCB with stuck contacts wont. Also generally fuses limit the peak fault current but MCB won't. Finally most MCB have an max PFC of 6-15kA but many BS88 fuses are good to 50kA or more, to do that with a breaker you need to look at MCCB and adjust your budget up a bit...


In every case? What if your boss is a tightwad and looks for the cheapest solution for 200 new homes, is an all-RCBO CU cheaper then a split RCD+MCB combination? What if this is an office and you are feeding a UPS with a dozen servers on it?

Now you and I might both agree that in this case an RCBO is the best solution to this question's requirements if we were installing this, but how do you convince the examiner that your reasons are sound?

Always seemed a good idea to Size plug Fuses accordingly not just for rings but I guess that is a bit out dated as well to be honest I never really look a fuse Trip Curves(at DB types)but I wouldn't mind betting there are a few old school guys who prefer them also I would be interested in any MCB that tripped with less fault current than x5 but going well off-topic or Overload I2 below 1.45 like trip staright away on 32.5 amps.

Take care.

 

[pc1966]

Some of this is slightly off-topic but it is intended to make you think though your choice, not to say it is wrong in any way.

Now you would never see fuses in any new CU for all sorts of reasons, but if you were feeding a sub-main to an out building CU you probably would use a fused-switch split from the meter tails for that. In an exam situation you may well be expected to justify it both in terms of what regulations requires this, and in terms of your choice of technology (e.g. fuse versus MCB on the sub-main source).

So trying to keep on the subject your choice of a RCBO is a good one, but you should be able to give a couple of reasons why it is the best choice in your view. Not just technical, as cost is always a factor, but if you can justify why an RCBO is better to the end user then they will often be happy with it.

 

[Edtwozeronine]

In question 3 it asks about requirements for additional protection (fused spurs powering radiators or hand driers) and special locations (zones 0,1&2 where water is present and you need RCD protection at minimum for circuits or you might need extra low voltage circuits)

It's the sort of thing that they want you to demonstrate that you know; for example if you read manufacturers enclosed documents included with an extractor fan for a bathroom, they tell you that you need to power it from a switchable fused spur.