City & Guilds 2396 Exam

[Spottydogs]

Hi all

Do we have anyone on here doing the C&G 2396 at the moment?

 

[Zdb]

Yes. What would you like to know?

 

[Spottydogs]

Yes. What would you like to know?

Hi Zdb

I was just wondering how many on here had done or are currently doing it? Any advice on areas to concentrate revision on would be helpful, although I appreciate it's a big book with a lot of topics they can question on!

Currently trying to brush up on my cable calculations. I notice that the Examiners Report for the last exam talks about candidates 'must be aware of the two forms of adiabatic equation'.

S>SqRootI2t/k (sorry I haven't a clue how to do symbols!!)

For the 2nd are they referring to Iz>In>Ib?

Any other helpful tips from anyone who has done it would be greatly appreciated!

 

[Spottydogs]

Ok, I think I was having a bit of a brain fart this morning,

The equation I'm looking for is just a re-arrangement of the adiabatic equation for Prot Conductors above isn't it?

t=k2xs2/I2

 

[Zdb]

Yep you got it. The second one is all about using the steel amouring as the CPC.

The design question is broken up into 10 sub-questions.

1. Working out the design current (Ib).

2. Selecting the nominal protective device (In).

3. Installation method based on the information given to you in the question.

4. Rating factors that apply. E.g ambient temperature (Ca), Grouping factors (Cg) etc.

5. The table number and column used to select current carrying capacity within BS7671.

6. Minimum size of live conductors after you've applied rating factors and volt drop.

7. Actual voltage drop.

8. Zs. This must be calculated after selecting your cable size. Ze and length of circuit will be given to you in the question.

9. Is the Zs acceptable?

10. Is the size of the CPC acceptable? (adiabatic equation).

 

[Zdb]

Apologies for the poor quality scan, I did it using my phone.

 

[Strima]

I did last years exam and have just submitted my project.

A lot of people were let down on their basic understanding and interpretation of the regulations. Also on basic testing principles and how to interpret results.

The written exam isn't about blindly quoting the book parrot fashion.

One example is why was Cmin brought in. Do you understand the reason behind Cmin and how varying loads on the network can effect impedances on circuits?

Also when designing you need to understand how thermal constraints can effect the circuit.

We all know about ambient temperature but what about conductor operating temperature. 70c may feel hot to the touch but a well designed circuit may well operate at 70c with no adverse effects to the cable or its surroundings.

You've also mentioned the adiabatic. Do you know why we work this out, and not just to get the correct CPC size? A conductor working at 70c has a potential to raise another 90c under fault conditions but why is this?

You may very well know all the maths but the written exam is aimed more at why and how much you understand the basic principles.

Also forget Zs=Ze+(R1+R2) and remember Zs=Ze+1.2(R1+R2).

More to come when I can remember what else in the three hours that I can't remember...

 

[Spottydogs]

Thank you for your replies, much appreciated. I've got a couple more days before exam time but clearly I've still a bit of revision to get through!

Regarding Cmin values, from what I recall from when it was covered in the early evening classes, it was a factor to allow for variations in the supply? For example to allow for the variations in supply voltage to the minimum -6% of 230V. So I assume you're referring to large loads causing a drop in voltage on the network which would in turn see fault currents reduce which would affect disconnection times?

The revised Zs formula I assume is taking into account multipliers from table 13 OSG? Do they nearly always use 70c Thermoplastic for the exam examples?

One thing I was getting mixed up with was when to use In or Ib at the beginning of cable calculations. In for circuits which might overload and Ib for when overload protection isn't required, I assume this isn't very often? Motor circuits where overload is provided on the motor protection/fixed loads?

I ordered Brian Scaddan Design and Verification on amazon last night and it dropped through my letterbox about an hour ago. From the bits Ive flicked through, I wish I'd bought it earlier as it seems a bit more user friendly in terms of language used.

 

[Strima]

When designing to a load you need to know if its fixed or variable, resistive or inductive.

If designing to a variable load it is safe to assume the OCPD rating is that of the circuit, say 20 amp for a radial socket circuit.

Fixed load then you can play around a little more. For example 6mm T&E enclosed in conduit on a wall is rated at 38 amps. If you have a shower rated at 8.5kw which is 37 amps and the installation method is favourable then you could chuck it in on 6mm cable. However you can't get 38 amp MCBs but you could use a 40 amp as there's no risk of over current.

Also remember to apply power factor if given as this can have a big impact.

If you stick with load/cable/fuse then you can't go far wrong but have a play around with the numbers, you'd be surprised what you can get away with.

Always use BS7671 unless otherwise stated, and they can use any cable from the same book on the exam, just read the question twice!!!

 

[Spottydogs]

Apologies, another quick one.

At College when calculating the If we have calculated:

Cmin x Uo / Zs

And then used this value for checking Thermal constraints. However, if the value of fault current is more than the minimum required for 0.1s (less than 0.1 in time), do we then use the value given in the table rather than the calculated amount?

And for time we either use the 0.1 second or plot using the graph and use the calculated If we have had to plot?

Hope that makes sense!

 

[Spottydogs]

Which makes sense I guess as the fault current will never reach the amount calculated in the above scenario?

 

[Mark Culpin]

View attachment 39605

View attachment 39606

Apologies for the poor quality scan, I did it using my phone.

Hi Thanks for the post. Have you noticed that the question seems to confuse the Final Circuit with the Distribution Circuit. There seem to be no details to indicate size of load on the final circuit.
Regards
Mark

 

[Deleted account]

My advice is keep the answers simple. If they ask for 10 characteristics of .....
List the 10 characteristics as listed in the blue book or yellow.
Don't start going into reams of pointless explanations if the question doesn't request it. You get 10 points per question.
These are easy 10 points. One point per characteristic listed.

I got a question what should you take into account on designing an installation in a working harbour?
Read the question. It's not asking for regs or Ingress, impact,seismic protection codes.

I didn't even look in my book for that one.
Answers I gave where- potential for impact from waves, and stones and other materials in the waves in stormy weather, access to members of the public, salty corrosive conditions, exposure to cold, exposure to direct sunlight etc...
It didn't ask what I would do to combat these conditions, so I didn't right pointless thinspgs like use barriers etc.. Again a list of 10 things to take into account all numbered 1to 10. Easy 10 points.


Hope that helps.