*** Trainee Comp 2 ***

[Resu]

Lol, bit of a random one to put you in the mood but each to their own!

 

[pennychew]

And the winner is.......

Simonslimline!

I'll get it in the post on tuesday for you mate, and big thanks to Paul and especially Richard for their help in getting this done.

Do you want to post the answer and explanation up Richard?

 

[jimmyray]

Well done mate!

 

[Richard Burns]

At last!

Well done Simonslimline.

I do not suppose anyone needs to know the answers, they will have already worked it all out!
Let me know if you want to know!

Oh I now have to find the answers!

 

[Resu]

Well done Simon! Can honestly say I enjoyed this one. Looking forward to the next!

 

[Simonslimline]

And the winner is.......

Simonslimline!

I'll get it in the post on tuesday for you mate, and big thanks to Paul and especially Richard for their help in getting this done.

Do you want to post the answer and explanation up Richard?

Excellent, i am well chuffed with that.

Suppose it's my turn to do a comp next. Ill have a think on that one.

 

[Richard Burns]

For those who got correct answers and want to compare with my thoughts and for those who want to know the answer this is the short and sweet version of my approach to answer this question:

The question asks:
If the new circuit addition complies with the AMD2 version of BS7671.
(It is not asking for good practice or compliance with any other documents.)
If it is not compliant in what way can it be simply redesigned to be compliant.

To design a circuit you need to know the type of circuit, the maximum load and the planned installation method, then you can select appropriate cables and protective device(s) and calculate the earth fault loop impedance and volt drop.

From the question:
The whole existing installation is already compliant so the Ze, Zdb and bonding are already OK. So we only need to consider the new circuit.

The circuit is on agricultural premises (Section 705 special location)
However most of the requirements of section 705 do not apply in a standard office location and any that do apply are already covered.

The BS1363 socket circuit will be used to capacity so the load is 32A.
The installation method is one circuit run in conduit and trunking using 4.0mm² single core insulated cables (method B).
The protective device is a B32 RCBO, so the sockets rated <20A are protected by an RCD in accordance with 411.3.3 and 415.1.1.
EFLI and VD can be calculated from the values given in the diagram.

Current carrying capacity
The current carrying capacity tabulated in table 4D1A for 4.0mm² singles in conduit is 32A so this just complies as there are no other rating factors to apply.

Earth fault loop impedance
Because you have 30mA RCD protection BS7671 permits a maximum Zs of 1667Ω so for the purpose of the question we can say Zs will be nominally complaint with the minimum requirements of BS7671 even on the most cursory look at the circuit resistances.
[I should mention that it would be very good practice normally to comply with the overcurrent device (32A type B) Zs which is 1.44Ω at maximum temperature or 1.16Ω at 20°C for AMD2. (Lowered by 5% in AMD3.) This circuit does also comply with these values.

Volt drop
Volt drop is the limiting factor on most long circuits. We are given the values in the diagram
Calculating this using:
Volt drop (V) = mV/A/m * A *m / 1000

11 * 32 * 45 / 1000 = 15.84 V

Which is greater than the maximum permitted in appendix 4 of 5% of 230V = 11.5 V
Therefore the circuit is not compliant.

So solutions to this problem are (in order of ease):
Close the gap to create a ring final circuit instead of a radial.

Reduce the circuit length by rearranging the cable connections, say to two 30m branches rather than 1 15m branch and 1 45m branch.

Split up the circuit into two circuits to reduce the current demand on each circuit.

Increase the cross sectional area of the cable to reduce the volt drop rating.

I can give a more detailed breakdown if you want covering all the approaches.

 

[Loki]

Congratulations Simon well done mate.

loki

 

[pennychew]

For those who got correct answers and want to compare with my thoughts and for those who want to know the answer this is the short and sweet version of my approach to answer this question:

The question asks:
If the new circuit addition complies with the AMD2 version of BS7671.
(It is not asking for good practice or compliance with any other documents.)
If it is not compliant in what way can it be simply redesigned to be compliant.

To design a circuit you need to know the type of circuit, the maximum load and the planned installation method, then you can select appropriate cables and protective device(s) and calculate the earth fault loop impedance and volt drop.

From the question:
The whole existing installation is already compliant so the Ze, Zdb and bonding are already OK. So we only need to consider the new circuit.

The circuit is on agricultural premises (Section 705 special location)
However most of the requirements of section 705 do not apply in a standard office location and any that do apply are already covered.

The BS1363 socket circuit will be used to capacity so the load is 32A.
The installation method is one circuit run in conduit and trunking using 4.0mm² single core insulated cables (method B).
The protective device is a B32 RCBO, so the sockets rated <20A are protected by an RCD in accordance with 411.3.3 and 415.1.1.
EFLI and VD can be calculated from the values given in the diagram.

Current carrying capacity
The current carrying capacity tabulated in table 4D1A for 4.0mm² singles in conduit is 32A so this just complies as there are no other rating factors to apply.

Earth fault loop impedance
Because you have 30mA RCD protection BS7671 permits a maximum Zs of 1667Ω so for the purpose of the question we can say Zs will be nominally complaint with the minimum requirements of BS7671 even on the most cursory look at the circuit resistances.
[I should mention that it would be very good practice normally to comply with the overcurrent device (32A type B) Zs which is 1.44Ω at maximum temperature or 1.16Ω at 20°C for AMD2. (Lowered by 5% in AMD3.) This circuit does also comply with these values.

Volt drop
Volt drop is the limiting factor on most long circuits. We are given the values in the diagram
Calculating this using:
Volt drop (V) = mV/A/m * A *m / 1000

11 * 32 * 45 / 1000 = 15.84 V

Which is greater than the maximum permitted in appendix 4 of 5% of 230V = 11.5 V
Therefore the circuit is not compliant.

So solutions to this problem are (in order of ease):
Close the gap to create a ring final circuit instead of a radial.

Reduce the circuit length by rearranging the cable connections, say to two 30m branches rather than 1 15m branch and 1 45m branch.

Split up the circuit into two circuits to reduce the current demand on each circuit.

Increase the cross sectional area of the cable to reduce the volt drop rating.
View attachment 29192
I can give a more detailed breakdown if you want covering all the approaches.

Great explanation [emoji106]

 

[Paul.M]

....and big thanks to Paul and especially Richard for their help in getting this done.

All the thanks goes to Richard and Pennychew. I was sat on the subs bench for this one, you wanted to run one, not as easy as you think is it?

Well done Simon and if you fancy setting up the next comp give me a shout. I'm sorting out a prize with one of the forum sponsors later in the week.

 

[Paul.M]

Lol, bit of a random one to put you in the mood but each to their own!

Lol, it was more the case of having a bit of time together with feet up in front of the telly. Was not expecting text messages from Pennychew but I did offer to help out with the draw.

 

[Simonslimline]

All the thanks goes to Richard and Pennychew. I was sat on the subs bench for this one, you wanted to run one, not as easy as you think is it?

Well done Simon and if you fancy setting up the next comp give me a shout. I'm sorting out a prize with one of the forum sponsors later in the week.

Will do mate, i will give a question some thought and get back to you. Sure i have a worthy prize i can donate also.

 

[Richard Burns]

I have now posted a more detailed answer in this thread that contains both the question and the breakdown of the answer including calculations for later reference.

 

[pennychew]

As a little update Richard has very kindly asked me if he can donate a socket tester and r2 plug to each of the 5 guys who also got the question right but didn't win.. I think its a great idea and a great gesture from a top man!

If the following posters could PM Richard with your address you will receive these in the post!



1 - Resu
2 – Loki
3 – Leo chaos
4 – Jimmyray
5 – Newtothis

Get your address's sent in chaps!

 

[Leo_chaos]

As a little update Richard has very kindly asked me if he can donate a socket tester and r2 plug to each of the 5 guys who also got the question right but didn't win.. I think its a great idea and a great gesture from a top man!

PM sent.
Thanks to Richard for the generosity !