Pop Quiz - Trainees Cooker Question

[Nigel]

So I am going to ponder a design question as a potential real-life situation for you all to consider.

Rules:

• The winner may not necessarily be just a correct answer. Backing up you thoughts with detail and Reg numbers will get you above your competition.
• TRUSTED ADVISORS will decide the WINNER. Likes are encouraged but the answer with the most 'WINNER' votes will be the winner. Trusted Advisors can vote more than once but TRAINEES cannot use the WINNER button.
• Bouncing ideas off of one another is encouraged as is probing questions from Trusted Advisors.
• Winner will be decided Sunday night. The prize is this. Prize will be made available to pick up from your local Rexel branch.

Question:

A customer wants to change their gas oven and hob standalone unit to an all electric one. The oven they are installing is an Electrolux EKC6461AOW Ceramic Electric Cooker.

You attend the house and survey the run. The CU is a BS3036 type. The installation will be chased into the wall which is all brick and the floor boards are able to be lifted to wire through the ceilings. There is really old insulation in the ceiling and is about 50mm in depth. The circuit will be a 25 meter run. Bonding to the gas and water is 2.5mm.

The customer has asked to not have an isolator installed and wants the circuit connected directly to the CU. If she absolutely has to have one she wants one with a socket on. She wants your advice on if she has to have an isolator backed up by a Regulation, if possible.

The customer would like to understand fully how you are going to do this job and wants to know how what size cable is being installed and what type of cable. The customer also wants to know if there are any other issues that you can see with the existing installation that may need to be addressed and the reason.

There are no silly answers or ideas. All ideas are welcome in this section of the forum so do not be afraid to give it a shot with your best answer.

Good luck and enjoy.

 

[MDJ]

Too difficult to try to answer , a good puzzle for the trainees definately.

 

[gazdkw82]

Earthing arrangement?
Size of the tails?

 

[Nigel]

Earthing arrangement?
Size of the tails?

25mm tails.

TN-C-S (PME)

 

[littlespark]

Does the customer make a good cup of tea and does her husband work away a lot?

 

[Resu]

Sounds like a nightmare customer! Good luck guys!

 

[gazdkw82]

Right, I've had a go. Best I could do with the kids screaming around me (the other half is working late)

I know I've made mistakes but it's been over a year since I touched any design scenarios.

Best I could do for now

 

[MFS Electrical]

Liking the regs @gazdkw82

 

[Nigel]

Right, I've had a go. Best I could do with the kids screaming around me (the other half is working late)

I know I've made mistakes but it's been over a year since I touched any design scenarios.

Best I could do for now

View attachment 44784

View attachment 44785

Excellent start.

 

[telectrix]

so my idea of a few yards of red/black 7/029 salvaged from a recent rewire and piggy backed off the ring with a nail in the 30A 3036 is a non-starter?.

 

[Jay 1987]

firstly i want to say i hope the staff will give a full explanation of this because i am absolutely baffled hahaha

anyway shouts out to gasdkw82 nice work mate

my contribution is more research based then working out figures.

its not much and it maybe wrong

however here we go......

firstly we rock up to the house and id let gaz do the maths and dimensions and this would be my bit hahaha

regarding circuit protection


BS 3036 semi-enclosed fuses cannot be relied upon to operate within 4 hours at 1.45 times the nominal current of the fuse element.

not exactly sure what this means...... however

Correct protection can be obtained by modifying the normal condition In≤ Iz such that the fuse rating does not exceed 1.45/2 = 0.725 times the rating of the circuit conductor. For this reason, larger cables may need to be selected where overload protection is provided by semi-enclosed fuses than when it is provided by a cartridge fuse or fuses or circuit-breaker (Appendix 4 of BS 7671 and Guidance Note 6 give further guidance).

So this has a bearing on the type of cable and size used its in relation to circuit protection. (i think)

If she does not want a isolator but wants it connected to the consumer unit then firstly not sure if thats legal or not however what i found was according to reg 533 01-04 it requires fuse elements in semi enclosed fuses to be in accordance with manufacturer instructions. where this is missing table 53 a of bs 7671 gives some guidelines and for the cooker she is having which is 240v, the nesscessary fuse is 30A and the the fuse element required for a 30 A fuse is 0.85 mm of tinned copper wire.


this is all i managed to source . can't wait for the real explanation lol

 

[Nigel]

So I havr not forgotton but I have been really busy.

We are seriously lacking answers here and have yet to declare a winner.

So another few weeks will be added.

 

[Jay 1987]

Can we have a small clue or guidance as to what we are supposed to be looking for ?

 

[Richard Burns]

Can we have a small clue or guidance as to what we are supposed to be looking for ?

You want to define the size and type of cable required to safely connect the oven to the CU. Taking into account the description of the cable run, installation methods, protection type and connected load.

You want to identify any other issues that may need to be addressed. Refer to the description of the installation and identify any problems.

You need to assess if a local isolator switch is required and if so why.
Reference the regulations on isolation and switching.

From your post you have stated that rewireable fuses take more current (than 1.45*In as is used for MCBs) to ensure disconnection on overload so the cable will need to be bigger to not overheat before the fuse goes. This is a good start on defining the cable size.
gazdkw82 has made significant inroads into the
calculation and information that show the approach expected.

Keep on thinking about it and checking the regulations for the items mentioned in the description.

 

[Jay 1987]

You want to define the size and type of cable required to safely connect the oven to the CU. Taking into account the description of the cable run, installation methods, protection type and connected load.

You want to identify any other issues that may need to be addressed. Refer to the description of the installation and identify any problems.

You need to assess if a local isolator switch is required and if so why.
Reference the regulations on isolation and switching.

From your post you have stated that rewireable fuses take more current (than 1.45*In as is used for MCBs) to ensure disconnection on overload so the cable will need to be bigger to not overheat before the fuse goes. This is a good start on defining the cable size.
gazdkw82 has made significant inroads into the
calculation and information that show the approach expected.

Keep on thinking about it and checking the regulations for the items mentioned in the description.

Ok thanks

 

[Nigel]

Ok thanks

Burnsy had detailed exactly what we are looking for. It is very easy to get caught up in just saying things without really knowing where to back it up in the Regs book. That is why I have specifically mentioned Regs.

 

[Jay 1987]

yes i understand, thank you, will try again !

 

[Nigel]

Ok thanks

Burnsy had detailed exactly what we are looking for. It is very easy to get caught up in just saying things without really knowing where to back it up in the Regs book. That is why I have specifically mentioned Regs.

yes i understand, thank you, will try again !

My advice would be to go deeper into calculations on cable size. Think correction factors and how they will influence your design.

 

[Jay 1987]

Ok

 

[Nigel]

Ok. Forum Mentors. Can we vote on a winner please???

Use the ‘Winner’ to vote.

 

[Jay 1987]

Ok. Forum Mentors. Can we vote on a winner please???

Use the ‘Winner’ to vote.

Is there no more probationary period ?????

Don’t want to leave it till Xmas day lol

 

[gazdkw82]

It would be nice to get an outcome with answers

 

[Nigel]

It would be nice to get an outcome with answers

I will rustle something up over the weekend and then await to be picked to pieces by others. Haha

 

[telectrix]

I will rustle something up over the weekend and then await to be picked to pieces by others. Haha

hopefully there won't be the wrong kind of snow on the tracks.

 

[MDJ]

and then await to be picked to pieces by others. Haha

I cannot wait

 

[gazdkw82]

Still waiting ;-)

 

[Nigel]

@gazdkw82 - can you explain where you got the Ci rating factor of 0.88 from please?

PS - I know the answer ;-)

 

[Nigel]

Ok so here goes:

Cable Calculations:
Rating factors we need to consider are as follows:
Ca = ambient temperature
Cf = OCPD
Ci = Insulation

So, we won't realistically exceed 25 degrees in a dwelling so the rating factor here is 1.03 (Table 4B1 70 deg thermoplastic).

As the OCPD is a BS 3036 we need to apply a RF to this also as the breaking accuracy is rubbish. For this we apply 0.725 (App 4 Section 4).

Then we have insulation which is already covered when we decide a cable and installation method. So, for T&E cable, Table 4D5 has already applied the RFs and then printed the max CCC of the different size cables. Our reference method will be 100 for T&E above plasterboard in less than 100mm thermal insulation (Table 4A2).

So now we need to look at our design current (lb). The cooker’s maximum demand is 49.6 amps. Now, we are allowed to apply diversity to this load as it is unlikely that the cooker will every run at full load (311.1). The ONG gives us some calculations here:

Remove 10 of 49.6 (leaves 39.6)
Take 30% of that figure away (leaves 11.88)
Put back the 10 we took away earlier = 21.9A

So; lb = 21.9A

If we then looked at Table 4D5, which is what we have already agreed is our table for CCC for T&E cable and reference method 100 then for a cable to safely hold 21.9 amps we would need a cable size of 4mm.

However, we have not applied our rating factors yet; so:
lb
(Ca x Cf)
-----------------------------------
21.9
(1.03 x 0.725)
-----------------------------------
21.9 / 0.75
New lb = 29.2A

So, we now look again at Table 4D5 and we can now see we need a 6mm to take 29.2A and an OCPD of 30A.

We check the voltage drop which we are told is not to exceed 5% on a non-lighting circuit (Table 4Ab) so our maximum permitted VD is 6.9v (we use Nominal Voltage for VD not measured voltage. Nominal is always 230v)

So, the calculation for VD is:

(mV/A/m) x lb x Length
1000

We get our mV/A/m figure from Table 4D5 which for a 6mm T&E is 7.3.

7.3 x 29.2 x 25
1000
----------------------------
5329 / 1000 = 5.33

VD = 5.33v (which is less than our maximum permitted of 6.9v) so within limits.

So, we would need to install 6mm T&E for this cooker based on what we know so far.

In terms of no isolating switch Regulation 463.1.3 & 464.1 clearly stipulate isolation is required. Also, it could be argued that Regulation 465 also applies.

Now we know the customer has to have an isolator we have two choices. One with a socket or one without. Diversity states we need to add 5 amps if the isolator has a socket. If we add 5A to our lb, that becomes 34.2A. Checking Table 4D5 again and now we need to install a 10mm cable and an OCPD of 45A. This changes the job completely and an option with costs would need to be presented to the customer to make an informed decision.

Now to look at the CU. There is currently no RCD protection and Regulation 522.6.202 clearly states that we must have RCD protection for our new circuit. If we were to install a cooker isolator with a socket Regulation 411.3.3 will also stipulate RCD protection for the socket outlet regardless of the installation method.

As BS 3036 do not do RCDs we would need to offer the customer either a CU change or install a standalone RCD and enclosure to protect our circuit. The preference being the CU change.

Bonding – Table 54.8 states that in a PME system the minimum cross-sectional area of the main bonding conductor where the tails are 35mm or less is 10mm. So, the customer would need to upgrade the bonding conductors to 10mm also.

In conclusion:

The customer needs a CU upgrade to include RCD protection, main bonding increased to 10mm and either a 6mm or 10mm T&E depending on if they want a socket with the isolator.

 

[gazdkw82]

Brilliant breakdown.

I'm going to go through it in more detail later when I can get some books out.

I messed up on a few things so id like to review.

Didn't realise about the dersting factor for BS3036.

 

[Nigel]

Brilliant breakdown.

I'm going to go through it in more detail later when I can get some books out.

I messed up on a few things so id like to review.

Didn't realise about the dersting factor for BS3036.

You referenced 0.725 in your calcs????

I know you got your Ci Rating Factor from Table 52.2 but you need to read 523.9 carefully to understand why this is incorrect.

 

[gazdkw82]

You referenced 0.725 in your calcs????

I know you got your Ci Rating Factor from Table 52.2 but you need to read 523.9 carefully to understand why this is incorrect.

Well that clearly shows I'm doing something and not actually digesting the information.

523.9 - iv had a good read. Key bits of information. For thermosetting cables, no correction factor for insulation is required.

If a cable is totally surrounded by thermally insualted material for more than 0.5m then the CCC should be x0.5 of that cable (ref C)

 

[Nigel]

Well that clearly shows I'm doing something and not actually digesting the information.

523.9 - iv had a good read. Key bits of information. For thermosetting cables, no correction factor for insulation is required.

If a cable is totally surrounded by thermally insualted material for more than 0.5m then the CCC should be x0.5 of that cable (ref C)

You are correct but in our example the key part of 523.9 is the second paragraph which directs us to App 4.

523.9 is only for cables installed to Ref C and partially going through insulation.

App 4 Tables already take in to account thermal insulation and publish CCC with that taken into account.

 

[gazdkw82]

Ah, your refefencing from 7671?

I was looking in OSG.

Looked in 7671, I see what you mean but I need to read it further to understand it better.

 

[Nigel]

Ah, your refefencing from 7671?

I was looking in OSG.

Looked in 7671, I see what you mean but I need to read it further to understand it better.

7671 needs to be your main reference. The OSG is just a guide.

 

[Richard Burns]

A pictorial reference to demonstrate the differences in applying rating factors or differing installation methods to a cable installed with or passing through insulation.