I think it means "farthest" but apart from that I cannot add anything.
I'm just surprised I hadn't realised before how silly the was requirement was.
I'm confused about junction box rules
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I'm just surprised I hadn't realised before how silly the was requirement was.
I would be very surprised if you had not made that comment, but I can assure you that I copied/pasted the exact text from the OSG - so don't blame me
It does seem totally ridiculous, but I can't think of what on earth could have been going on in their minds to cause them to write that - and even more surprising that, in more recent times, no sensible person has come along and removed it. There surely must be at least one sensible person involved with the writing/revising of the OSG, mustn't there?
I think it's probably that a lot of those people behave like sheep, rather than actually thinking (if they can)No one bothers when a ring is extended but people don't like multiple spurs in say 4mm, strange ay
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I haven't seen anything on my travels through all the old regs and guides I have. Having said that, I haven't by any means read all of them cover to cover.
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That doesn't make sense?Exactly - as I said, that's what I've always assumed - but, as I pointed out, it leads to ridiculous situation
Yes - but, as I wrote, even if it is a very long ring, spurs originating anywhere near the midpoint of the ring would be allowed to have little (or, ultimately, NO) length
Oops. my interpretation was the longer the ring length the longer the permissible spur lengthExactly - as I said, that's what I've always assumed - but, as I pointed out, it leads to ridiculous situation
Yes - but, as I wrote, even if it is a very long ring, spurs originating anywhere near the midpoint of the ring would be allowed to have little (or, ultimately, NO) length
IE 1/16 of 100m = 6.33m anywhere on the ring... which I what I believe I've seen before and therefore only skimread your post
but
My gut feeling without any calculations or fag packet sketches is:
At 75% round the ring a spur of length 1/16th of the ring will result in the same Z at the centre
At the origin a spur of length 1/4 of the ring will result in the same Z at the centre. However adding that at the 50% position will double the Z
Apo;logies if mythoughts are too irrational
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Thinking about it, the obvious thing is to limit Zs.
Indeed, it's crazy. Interpreted in that way (which seems the most obvious/likely), it means that if, say, one took a spur from a socket which was right in the middle of the ring (the 'best' place to originate a spur), the maximum permitted length of that spur would be zero !! ... and not a lot better for spurs originating fairly close to the middle of the ring!
As I've been writing it (and its consequences) seem pretty crazy to me!
It will (for the spurred socket) and one obviously theoretically ought to determine (by measurement or calculation) the Zs at the end of the spur to make sure that it is low enough. However, this arbitrary rule of thumb (which means that one is not allowed to have a spur originating at, or close to, the midpoint of the ring) is surely not an acceptable alternative to actually measuring or calculating the Zs?
It's as good a theory as any other I can think of but, as I've just written to Sunray, I really don't think that this arbitrary 'rule of thumb' is a particularly acceptable approach, particularly given that its effect appears to be to prohibit spurs which originate at, or close to, the middle of the ring!)
FWIW applying the 1/8 from the centre point rule: In order to make calculations easy I've assumed a ring resistance ofOops. my interpretation was the longer the ring length the longer the permissible spur length
IE 1/16 of 100m = 6.33m anywhere on the ring... which I what I believe I've seen before and therefore only skimread your post
but View attachment 396100is very different to that and quite frankly meaningless other than the L/N impedance is highest at midpoint and as such adding a spur will increase that further.
My gut feeling without any calculations or fag packet sketches is:
At 75% round the ring a spur of length 1/16th of the ring will result in the same Z at the centre
At the origin a spur of length 1/4 of the ring will result in the same Z at the centre. However adding that at the 50% position will double the Z
Apo;logies if mythoughts are too irrational
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Gosh this thread has run and run
Back to the OP - I too believed what was written and that the spark was told what the plans were
So I just think the spark used what he had to hand and nobody would question it rather than using the correct part from the get go
I am now retired BUT always carried traditional JBs and MF JBs in my van for these very situations
Back to the OP - I too believed what was written and that the spark was told what the plans were
So I just think the spark used what he had to hand and nobody would question it rather than using the correct part from the get go
I am now retired BUT always carried traditional JBs and MF JBs in my van for these very situations
Yes, on reflection, as you and Sunray have said, Zs is fairly obviously what it was about.
However, one cannot say anything about the maximum permissible length of a spur from a ring (Zs-wise) without knowing, or making some assumption about, the Zs of the ring itself (highest at mid-point of ring).
It seems that, at least for some purposes, they have assumed that Zs at the mid-point of the ring was already the 'maximum permitted' - in the sense that that explains why the stated 'rule of thumb' does not allow a spur of any length at the midpoint of the ring (and makes permissible spur lengths impractically short near to that midpoint).
However, what I still don't get where their "one-eighth" comes from. I haven't got time at the moment to do the proper sums, but will later. However, for starters, assuming that the mid-point of ring is already at 'maximum Zs', it would seem that a spur originating very close to (or even 'at') the origin of the ring (in same size cable as ring) could have a length equal to one-half (not one-eighth) of the distance from that point to midpoint of ring and still have a 'just OK' Zs, couldn't it?
More later, when I have a little time to look in more detail at the numbers.
Furthermore, and more to the general point, if it were a small ring, with a Zs at midpoint well below the 'maximum Zs', it would be possible (Zs-wise) to have a much longer spur than the rule-of-thumb indicates - ultimately potentially longer than (let alone 'one eighth of') the distance from spur to midpoint of ring!
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I think that is where we are all going wrong.
Stop thinking about it and just throw it away.
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