2391 Question, Dec' 2010

[Jurasic Spark]

State the max' permitted percentage (%) voltdrop for each of the following circuits supplied from a public low voltage distribution system:

a) 230v lighting circuit

b) 3p 400v motor circuit

c) Distribution circuit supplying a single phase D.B

 

[Sparky Ninja]

Is this an example for others to play with, or are you seeking help with it?

Appendix 12 either way.

 

[Jurasic Spark]

Is this an example for others to play with, or are you seeking help with it?

Appendix 12 either way.

I'd be interested in what you and others think the answers are

 

[johnboy6083]

1)3%
2)5%
3)3%

 

[Sparky Ninja]

I'd be interested in what you and others think the answers are

a) 230v lighting circuit = 3%

b) 3p 400v motor circuit = 5%, however a greater volt drop may be acceptable for startup.

c) Distribution circuit supplying a single phase D.B = +10% to -6%

 

[johnboy6083]

when i answered, i made an assumption, that the DB would have to feed lights. Can you explain your answer further please widdler?

 

[Sparky Ninja]

when i answered, i made an assumption, that the DB would have to feed lights. Can you explain your answer further please widdler?

Well c) states only that it is for a distribution circuit and not a final circuit.
Large installations cannot be designed to a total voltage drop of 5%. If the voltage at the origin is within designer's control, as when the supply is from an 11000V/415V transformer, use can be made of the Electricity Safety, Quality and Continuity Regulations allowed voltage range of +10% to -6%. BS7671 assumes this in setting the deemed to comply 3% or 5% for a supply from a public low voltage system.
Assuming a nominal 230V line to earth at the farthest consumer, the open circuit or highest voltage could be 230V plus 10% that is 230V + 23V = 253V and the lowest 230V minus 6% that is 230 - 13.8 = 216.2V, a total voltage drop of 36.8V, 14.5% of 253V. These would be voltage drops to the origin of the final circuits allowing the use of standard final circuits with a further 3 or 5% voltage drop.


Taken from chapter 5 of the Electricians design guide.

 

[johnboy6083]

would this be the same for a submain, ie feed to garage, or commercial outbuilding?

 

[Sparky Ninja]

depends on whether you can determine if it's a distribution or final, that's the key. In the initial question it's worded as distribution circuit.

 

[johnboy6083]

i think that maybe common sense has to apply. as long as at the final distribution board, the volatge is withing ESQCR regs, then your ok.
would you agree?

John

 

[jappattackprice]

Has anyone got a copy of dec2nd exam
Would be interested in looking at it again

 

[Jurasic Spark]

State the max' permitted percentage (%) voltdrop for each of the following circuits supplied from a public low voltage distribution system:

a) 230v lighting circuit 3%

b) 3p 400v motor circuit 5%

c) Distribution circuit supplying a single phase D.B. I would say this is a typical ambiguous C & G question. There is no stated volt drop figures for distribution circuits. The volt drop of the distribution circuit would be part of the volt drop of any final circuit from the sub board which would be either 3% or 5% depending on what the final circuits from the sub board are.

. ...

 

[CRAGS]

Question C typical city and guilds question possibility of 2 different answers don't ya just love em

 

[Guest123]

In my opinon the C&G are trying to catch you out with option C as they are looking to see if you are thinking outside to box by assuming the distribution/submain circuit to the single pahse DB could be supplying lighting circuits. The issue I see is that the VD of this circuit would need to be less than 3% to allow for the final circuit to the lighting points connected to it.


Got to love C&G.