College Theory Question

[JohnW2]

I believe that it's quite common to use ......

It's just a little unconventional.

Oh dear - are you perhaps having a quiet morning?

I was merely saying, as a minor pre-amble to my main points, that "it's quite common" and "just a little unconventional" are not necessarily all that far apart!

Kind Regards, John.

 

[ban-all-sheds]

However, I still wonder whether a lighting ring is often going to be the best solution.

You don't think that it would be at least reasonable to assume that if the original professional designer thought that it was, he was right?

Voltage drop would become much less of an issue if one split the circuit into two or more radials (each carrying less current),

And space in the DB would become more of an issue.

and I suppose one could contemplate going up to 2.5mm² (subject to termination being OK) if it were still an issue even then.

"Subject to..."? There's a good chance it won't be.

10A of lighting is quite a lot for one lighting circuit (about 40 55W fittings)

Have you seen the size of some offices?

and, apart from anything else, convenience (hence possibly 314 of regs) could be improved, and possibly cost of wiring reduced, by splitting the circuit into more bite-sized pieces.

How do you know that there aren't multiple rings? In many open-plan offices I've worked in there were multiple circuits, and they served alternate luminaires, so that the loss of one circuit didn't plunge a whole section of the floor into darkness. To do the same with radials would probably not halve the length of the circuits.

However, it's obviously all down to the designer to decide what is best in a given situation.

Indeed, and the only point I was trying to make was that, contrary to Paul_C's assertion that it's a little unconventional, designers quite often (or often enough for it not to be unconventional) decide that a ring is best.

 

[JohnW2]

I think you're definitely having a quiet morning (as am I) ....

However, I still wonder whether a lighting ring is often going to be the best solution.

You don't think that it would be at least reasonable to assume that if the original professional designer thought that it was, he was right?

As you've quoted (and consideing that I have said in other messages that I'm learning here, in relation to a matter of which I have no experience) I merely said that I "wondered" .... and then (as again you've quoted), I went on to write:

However, it's obviously all down to the designer to decide what is best in a given situation.

Kind Regards, John.

 

[holmslaw]

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[Paul_C]

I suppose it just depends upon at which point one considers the transition toward "fairly common" coincides with the point at which is can cease to be regarded as "unconventional."

To throw something else into the picture, I don't know the rules regarding paralleled conductors in other parts of the world where rings are not generally used, but the American NEC has a long-standing prohibition on the use of parallel conductors in the smaller sizes, allowing them only in sizes 1/0 and larger (> 50 sq. mm). And then they are required to be of the same conducting and insulating material, the same length, terminated in the same way, etc. In other words, they're really only permitted for the convenience of splitting larger feeders into more manageable smaller conductors.

 

[JohnW2]

To throw something else into the picture, I don't know the rules regarding paralleled conductors in other parts of the world where rings are not generally used, but the American NEC has a long-standing prohibition on the use of parallel conductors in the smaller sizes, allowing them only in sizes 1/0 and larger (> 50 sq. mm). And then they are required to be of the same conducting and insulating material, the same length, terminated in the same way, etc. In other words, they're really only permitted for the convenience of splitting larger feeders into more manageable smaller conductors.

Yes, I touched on this before. Although it encourages equalisation of currents between parallel conductors, BS7671 seems to accept the concept of unequal currents, since this is mentioned in several places (without any caveats about conductor size). Although I haven't looked very deeply into this (since it will almost certainly never be relevant to me) it looks as if it could require separate (but presumably linked) OPDs for the two ends of the ring. Is that your understanding?

Kind Regards, John.

 

[RyanM]

Thank you all for your replies! It has definitely given me something to think about and research!

If ignoring my original question then (some of the figures were incorrect, as I guessed them while at work), what is the most common practice for an office/commercial lighting circuit?
Say 16 light fittings, each with 4 55w tubes in - so a total of 3520W. Would this be best on two radial circuits? What would the issues/negatives really be of putting it on a ring? Obviously in either circumstance, 2.5mm cable would be used?


Thanks a lot

 

[RyanM]

I've drawn a picture, as I work better with pictures!

So in theory is this what you mean?

One or the other?

 

[EFLImpudence]

Well, definitely not the first option as one fault will plunge the office into darkness.

The second may be alright but I would alternate, at least, the columns if not the lights.

The design depends on a lot of variables so, without the distances involved and the installation methods, it is not possible to say which would be best - 2 radials, 2 rings, 4 radials, 1mm², 1.5mm², 2.5mm².


Also, what was the correct answer to the overheating MCB college question?

 

[ricicle]

If they are looking for a single answer, then its because the MCB is too small and fluorescent lighting starting currents are causing the over heating

That would only possibly be true if the fittings were being constantly switched on and off. Hardly likely in an office.

 

[ban-all-sheds]

Best thing is random interleaving with switches in illogical places so that it's never possible to control the lights in a neatly defined section of the office and certainly never from just one place.

Employees love that when they work late and are expected to turn the lights out when they leave.

 

[ericmark]

There are two things that spring to mind.
1) Loss connections
2) Overload
Loss connections has been covered but overload is not so straight forward. Pre-HF fittings the florescent lamp was very dependent on voltage and if one takes a 60W fitting the current should be 0.26A but in practice it is over that value and as the voltage raises it could reach 0.36A very easily with a fitting designed for Europe 220v and used in UK with 250v.
So I would measure the amps rather than calculate amps and if I found it was over the rating then measures would need to be taken.

There are a number of ways to reduce the power taken starting at points closest to the supply one could change the ballast for higher voltage versions or HF type.

This may have already been done and splitting the circuits could be another approach but the ring helps to maintain the voltage required to strike the florescent so splitting may cause problems.

But as already said jumping in and doing something without finding root cause may hide faults so step one is always find out why.

 

[holmslaw]

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[ricicle]

Its very likely that the lights in a modern office could be constantly on and off, due to lighting controls eg occupancy sensors etc, as required by part L since 2001.

Then I would seriously be looking at the settings on the occupancy sensors
because even a fairly low on time of, say, five minutes wouldn't be enough to cause overheating of an MCB.

 

[holmslaw]

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