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?

 

[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.