Lampholder Overcurrent Protective Device.

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As mentioned in another thread and Regulation 559.5.1.204:

"Lighting circuits incorporating B15, B22, E14, E27 or E40 lampholders shall be protected by an
overcurrent protective device of maximum rating 16 A."


Why would or should this be so?
 
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... Why would or should this be so?
We've discussed this before, and I don't think anyone has ever come up with a rational explanation/justification - least of all why it should be restricted to circuits with certain types of lampholders.

Mind you, it has never been particularly restrictive - even in the days of incandescents, >16A would imply more than about 36 x 100W bulbs and, today, it would 'limit' one to about 245 x 15W LEDs per circuit :)

Kind Regards, John
 
Possible the writer considered a need to limit the power that could be dissipated in a prolonged plasma arc in a faulty lamp or/and lamp holder.
In what way does such a fault differ from a very low impedance fault due to any other cause, in any type of circuit (and, if a 'lighting circuit', one using any type of lamp holder)?

Kind Regards, John
 
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Oh, I don't remember it before.

It's not the restriction of total load that made me ask but the comment on the other thread that, in effect, a ring circuit spur cannot be used for one light without fusing down (even if appropriate cable csa were used).
 
Oh, I don't remember it before.
We've discussed it frequently, often with eric going on about the fact that many/most lighting accessories are only 'rated at' 6A - and telling the story of his lamp/bulb that got welded into a socket!
It's not the restriction of total load that made me ask but the comment on the other thread that, in effect, a ring circuit spur cannot be used for one light without fusing down (even if appropriate cable csa were used).
Well, 559.5.1.204 only applies to "lighting circuits", whatever they may be, so maybe does not apply to circuits that are primarily supplying sockets (given that it would be a bit odd to regard such as a "lighting circuit"!) ?

Kind Regards, John
 
Well, 559.5.1.204 only applies to "lighting circuits", whatever they may be, so maybe does not apply to circuits that are primarily supplying sockets (given that it would be a bit odd to regard such as a "lighting circuit"!) ?
Ah. Good point.

So - even less sensible.
 
It doesn't differ. But in a few case a very low impedance fault may develop into a zero Ohm fault when a short circuit through plasma is created.
Two conductors unintentionally touching (i.e. the 'usual' sort of fault) can also represent a near-zero-ohm fault. I really don't see why a path through a plasma is any different from a copper-to-copper path.

Kind Regards, John
 
Ah. Good point. ... So - even less sensible.
Maybe, if "less sensible" is even possible :)

As we've often discussed (in relation to 'minimum conductor sizes' ** ), BS7671 does not actually define (at least, not in Part 2) a "Lighting \circuit\2. However, the latest versions of Tablr 52.3 have carried a footnote which says\;
NOTE 4: For lighting circuits and associated small items of current-using equipment, such as a bathroom extractor fan.
Whilst hardly a definition, that certainly could be taken to imply that a circuit primarily supply sockets is not a "Lighting circuit", even if it does supply some lights. If that's the case then, as I said, 559.5.1.204 would not apply to ring finals.

[ ** of course, if we are talking about 'less sensible', it does appear that one can wire a 6A circuit (whatever it may be called) with 0.75mm² cable, provided that it is flexible cable :) ]

Kind Regards, John
 
Copper to copper fault will melt and vaporise the copper at the contact point copper until the gap is wide enough that the fault connection is broken.

But if the gap never gets wide enough to break the arc then the plasma arc can be created.

A short Live to Neutral or Earth conductor in a lead sheathed cable can be a melting pot of copper, lead and carbonised insulation that flares out like a roman candle ( firework )
 
Copper to copper fault will melt and vaporise the copper at the contact point copper until the gap is wide enough that the fault connection is broken. But if the gap never gets wide enough to break the arc then the plasma arc can be created.
All true, but provided only that the Zs of the circuit is compliant (i.e. cable has large enough CSA), then an MCB with any In would, in either case, clear the fault within milliseconds. I therefore still see no reason for restricting the In of the OPD.

As another aspect of the silliness, the reg only restricts the In of the OPD on a "lighting circuit" to "16A", without any mention of the type of device. It would therefore seem that protecting a "lighting circuit" with a D16 would be compliant with that reg, even though the magnetic trip threshold could be as high as 320A, as compared with only 100A for a ('prohibited') B20 or 160A for a ('prohibited') B32.

Kind Regards, John
 
As another aspect of the silliness, the reg only restricts the In of the OPD on a "lighting circuit" to "16A", without any mention of the type of device. It would therefore seem that protecting a "lighting circuit" with a D16 would be compliant with that reg, even though the magnetic trip threshold could be as high as 320A, as compared with only 100A for a ('prohibited') B20 or 160A for a ('prohibited') B32.

True enough, but the reg would have been from a time when fuses were all you had (and it did use to be 5A/6A for SBC, SES*) and simply got retained from those days, but 15 did become 16 and 5 became 6 in general updating at some point, presembly they thought it seemed sensible to retain the limit and probably never bothered to consider trip curves, perhaps they should have done!


*That only changed in the 17th in 2008
 
True enough, but the reg would have been from a time when fuses were all you had (and it did use to be 5A/6A for SBC, SES*) and simply got retained from those days, but 15 did become 16 and 5 became 6 in general updating at some point, presembly they thought it seemed sensible to retain the limit and probably never bothered to consider trip curves, perhaps they should have done!
All true, but it was really only 'in passing' that I mentioned that apparent 'secondary silliness' in terms of the present day.

The primary question ('silliness'?) relates to why the restriction was introduced in the first place. Why did they impose (regardless of cable CSA) a limit of a 15A fuse specifically for a "lighting circuit" - particularly since they did not impose any corresponding "maximum fuse size" for any other type of circuit|?

Kind Regards, John
 

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