Help with Maximum Demand needed

[Gigsy]

Hi, I am studying to become an electrician. I got a question wrong on a practice test.

Can anyone help me understand maximum demand.

this is the question I got wrong.....



Question 27: With reference to the nature of the supply, which one of the following can be determined by calculation, enquiry or measurement? The:
A maximum demand of the installation
B rating of the circuit protective device
C prospective short-circuit current at the origin of the installation
D c.s.a. of the tails

You said prospective short-circuit current at the origin of the installation (C), which is the wrong answer. It is clear from your answer you have a good understanding in the subject of supplies. Note that answers A), B) and C) are correct for this question and either of these would have been acceptable. The correct answer is maximum demand of the installation (A).



I thought the maximum demand was the maximum current your installation will draw. If your maximum demand is more than the suppliers main fuse you can use diversity to bring your maximum demand down.

If I am correct you cannot obtain your maximum demand by enquiry, because nobody else would know the details of the installation you just designed and installed and they do not know what appliances you have on your circuits, therefore you have to calculate the maximum demand of your installation.

But in BS7671, it does say you can obtain your maximum demand by enquiry. Please refer to attached document.



You can however obtain the prospective short circuit current at the origin by enquiry, you can phone your supplier and ask the value of Ze, then you can use the equation 230 / Ze to get the current. I = V/R

So my question is, do I have all the correct or is the maximum demand the maximum current I am allowed to draw i.e. the value of the main fuse?

 

[buzzlightyear]

Question 27: With reference to the nature of the supply, which one of the following can be determined by calculation, enquiry or measurement? The:

C prospective short-circuit current at the origin of the installation

is the answer

 

[Gigsy]

is the answer

Thank you, so my tutor is wrong then

Am I right about maximum demand? Why does it say in the regs that you can obtain the maximum demand by enquiry?

 

[buzzlightyear]

is he asking about max demand by reading the question 27 you put on post 1.
or are you confused. max demand is loading of the installation .

 

[Gigsy]

is he asking about max demand by reading the question 27 you put on post 1.
or are you confused. max demand is loading of the installation .

I know he is answering question 27. I agree with him, my tutor is wrong with his response.

I was asking if my explanation about maximum demand is correct. By what I typed do you think I understand what maximum demand is.

 

[Gigsy]

is he asking about max demand by reading the question 27 you put on post 1.
or are you confused. max demand is loading of the installation .

How can you obtain the maximum demand by enquiry if maximum demand is the total load of the installation.

 

[buzzlightyear]

IET Forums - Maximum Demand - http://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=24451
may be this will explain .

 

[The Ghost]

Tbh I would have gone with C as well but seeing your whole post and thinking about it I would agree with your tutor. Because clearly it is not the OCPD or tails so there are only the two left. PFC is by measurement or calculation but not enquiry. Although saying that the DNO or energy will provide a notional max demand based on Kwh hours used in Kva. Given that you have been getting bills. This would not apply in a new build for instance. But then they do work on expected and usual demands at the design and supply level for new builds. So yes you could enquire, calculate or possibly measure when the installation is in use I suppose. Difficult one and it underlines the strange grammar and syntax of C&G question which are a notorious trap in exams.

 

[buzzlightyear]

https://www.amazon.co.uk/Electricia...F8&qid=1506373732&sr=8-8&keywords=wiring+regs
this book might help you .

 

[Gigsy]

https://www.amazon.co.uk/Electricia...F8&qid=1506373732&sr=8-8&keywords=wiring+regs
this book might help you .

Do you think i need help tho, don't you think what i typed is correct? What have I said that makes you think I do not understand something?

 

[Gigsy]

Tbh I would have gone with C as well but seeing your whole post and thinking about it I would agree with your tutor. Because clearly it is not the OCPD or tails so there are only the two left. PFC is by measurement or calculation but not enquiry. Although saying that the DNO or energy will provide a notional max demand based on Kwh hours used in Kva. Given that you have been getting bills. This would not apply in a new build for instance. But then they do work on expected and usual demands at the design and supply level for new builds. So yes you could enquire, calculate or possibly measure when the installation is in use I suppose. Difficult one and it underlines the strange grammar and syntax of C&G question which are a notorious trap in exams.

BS7671 says you can obtain prospective short circuit current. You mentioned prospective fault current, that is not an option, only prospective short circuit current is an option. You could not obtain prospective fault current by enquiry because the supplier does not know the CSA of your CPC. The supplier does know, however Ze and the supply voltage, so they could provide you with the prospective short circuit current.

 

[dmxtothemax]

B,C &D can all be "determined by calculation, enquiry or measurement?" .
A cannot.

 

[The Ghost]

I think the most salient point to draw from this is that there will be those moments of "Ehh??? that doesn't make sense" Between the tutor and C&G it is inevitable that this will happen. I am afraid you will have to take this in your stride and focus on the goal rather than get bogged down in the detail. One question wrong does not a failure make in the exam. You will have to learn from other sources and your tutor right or wrong is like the old saying the boss isn't always right but the boss is always the boss.

 

[Devonchris]

I think there is some confusion between maximum demand and maximum current carrying capacity.

The current carrying capacity is fixed by the physical characteristics of the circuit. The maximum demand is the use to which the circuit is put, and is variable. I think this is made clear by reference to Appendix A (maximum demand and dversity) in the OSG.

When designing a new circuit the only way to determine the max demand is by enquiry ie. ask the customer what it is to be used for, and then design the circuit accordingly.

With an existing circuit you can (1) ask the customer what is connected. Enquiry

(2) check the rating plates on the appliances and work it out. Calculate

Or (3) stick a Clamp Meter on. Measure

So the correct answer is A

Disclaimer.
I am not a qualified electrician.

 

[The Ghost]

You mentioned prospective fault current, that is not an option, only prospective short circuit current is an option. You could not obtain prospective fault current by enquiry because the supplier does not know the CSA of your CPC.

I might beg to differ there on a small detail. Pfc is a generic term for PEFC or PSCC whichever is the higher noted on you cert. However I have asked the DNO and they usually rate the max pfc as 16ka based on "at the point of origin" of the supply this is normally the case for domestic supplies. I have seen certs filled in with this figure as well, rightly or wrongly albeit.

 

[Percyprod]

In large power metering, which would probably use the metering you would have 1 check meter per phase, a 3 phase reactive meter, a main meter and an MD meter. The maximum demand meter has a register which resets every 30 minutes. If the agreed MD is exceeded then a higher charge is applied. So I suppose that may be what they are referring to.