CHESTNUT

[JohnW2]

I can't add anything at the moment but would just ask - On a ring with no spurs, surely you would agree, the farthest point would be half way round from the CU. Therefore, is it plausible that the farthest point can move?

Well, 'farthest' is clearly a relative term, and therefore has no true meaning in the absence of a refernce point (i.e. 'farthest from what'). In the absence of any context, I agree that one would probably assume that the reference point was the CU.

However, in the situation we are discussing, there is another point of interest, namely the point of connection of the spur - so maybe they did mean the farthest point from there. As you said, if that's the case then it would be just another way of saying that unfused spur lengths should be less than 1/16 of the total ring length. That would, in fact, make a lot more sense than your 'obvious' interpretation, since at least it would not result in the maximum recommended spur length varying according to where it was attached to the ring. I suppose the general concept would then also make a bit of intuitive sense, since I think most of us would question the sanity of a design which had a tiny ring (say just a couple of metres) with countless very long spurs attached to to it. Unless/until I can think of any more sensible interpretations, I'm inclined to think that this interpretation was probably what they had in mind - but goodness knows where the seemingly arbitrary 1/8 (or 1/16) would have come from!

Kind Regards, John.

 

[ban-all-sheds]

I had to look.

The tabulated maximum length in the OSG for a ring final on a 30A BS 3036 on a TN-S supply with no RCD is 49m.

 

[JohnW2]

I had to look. The tabulated maximum length in the OSG for a ring final on a 30A BS 3036 on a TN-S supply with no RCD is 49m.

Hmmm - so I guess that if my suggestion as to the most likely meaning of the OSG's 'rule of thumb' is correct, they're recommending a maximum unfused spour length of 3.0625m - 'orribly close to a figure I've seen somewhere else

Kind Regards, John.

 

[ban-all-sheds]

Mmmmm

 

[EFLImpudence]

The tabulated maximum length in the OSG for a ring final on a 30A BS 3036 on a TN-S supply with no RCD is 49m.

So, there would be no 'room' (Zs-wise) for a spur at the mid-point (farthest from CU).

so I guess that if my suggestion as to the most likely meaning of the OSG's 'rule of thumb' is correct, they're recommending a maximum unfused spour length of 3.0625m - 'orribly close to a figure I've seen somewhere else

I see the maths. but why would that be ?
Wouldn't it depend on how far from the CU the spur was?

 

[JohnW2]

The tabulated maximum length in the OSG for a ring final on a 30A BS 3036 on a TN-S supply with no RCD is 49m.

So, there would be no 'room' (Zs-wise) for a spur at the mid-point (farthest from CU).

I guess that could be what they're thinking - but that tabulated max of 49m applies for Zs up to 0.8Ω, so if Zs is actually lower the max ring length increases (e.g. they quote 111m for a 0.35Ω TN-C-S Zs, which would give a 'rule of thumb' spur length of up to 6.9m per my guessed interpretation).

so I guess that if my suggestion as to the most likely meaning of the OSG's 'rule of thumb' is correct, they're recommending a maximum unfused spour length of 3.0625m - 'orribly close to a figure I've seen somewhere else

I see the maths. but why would that be ? Wouldn't it depend on how far from the CU the spur was?

No, that results from the interpretation of the 'farthest point' being the farthest point from the spur (i.e. half of the total ring length, hence the 1/16 figure, regardless of the position of the spur).

Kind Regards, John.

 

[EFLImpudence]

I guess that could be what they're thinking - but that tabulated max of 49m applies for Zs up to 0.8Ω, so if Zs is actually lower the max ring length increases (e.g. they quote 111m for a 0.35Ω TN-C-S Zs, which would give a 'rule of thumb' spur length of up to 6.9m per my guessed interpretation).

True, but isn't the OSG what can be done without thinking?
I.e. worst case scenario and bound to be safe.

I see the maths. but why would that be ? Wouldn't it depend on how far from the CU the spur was?

No, that results from the interpretation of the 'farthest point' being the farthest point from the spur (i.e. half of the total ring length, hence the 1/16 figure, regardless of the position of the spur).

Yes, but couldn't a spur taken from the fuse be 12m. long?

 

[JohnW2]

True, but isn't the OSG what can be done without thinking? I.e. worst case scenario and bound to be safe.

One would have thought so - but surely the true worst case is, as you suggested, when all of the permissible Zs at a particular point on the ring has been 'used up' by the ring itself - in which case any length of spur cable connected at that point would result in Zs measured from the end of that spur being too high?

Yes, but couldn't a spur taken from the fuse be 12m. long?

If the spur comes from the fuse, then surely the two interpretations we've been consdering become the same, and both limit the spur length to 1/16 of the total ring length, don't they?

Kind Regards, John.

 

[EFLImpudence]

True, but isn't the OSG what can be done without thinking? I.e. worst case scenario and bound to be safe.

One would have thought so - but surely the true worst case is, as you suggested, when all of the permissible Zs at a particular point on the ring has been 'used up' by the ring itself - in which case any length of spur cable connected at that point would result in Zs measured from the end of that spur being too high?

Yes, so if the farthest point is always the mid-point of the ring, then 0/8=0 hence no spur allowed.

However the eighth (sixteenth) doesn't apply the nearer to the CU the spur is.

Yes, but couldn't a spur taken from the fuse be 12m. long?

If the spur comes from the fuse, then surely the two interpretations we've been consdering become the same, and both limit the spur length to 1/16 of the total ring length, don't they?

They do if that were a universal rule but there is no reason why a spur from the fuse should be limited to 3m.
Again, is it just the OSG stating that a 3m. spur is always safe without calculation.

As far as Zs is concerned, the spur would comply as long as the end of it were no more than 12m. (cable run) from the CU from wherever on the ring it was taken.
It could actually be a little more than 12m. because of the other leg lowering the resistance so 12m. would always be safe.

So, wouldn't it make (more) sense if the figure used was a half to the farthest point - from CU (a quarter of the ring length)?

 

[JohnW2]

Yes, so if the farthest point is always the mid-point of the ring, then 0/8=0 hence no spur allowed.

Again true, if they are measuring 'farthest' from the CU.

They do if that were a universal rule but there is no reason why a spur from the fuse should be limited to 3m. .... As far as Zs is concerned, the spur would comply as long as the end of it were no more than 12m. (cable run) from the CU from wherever on the ring it was taken. .... So, wouldn't it make (more) sense if the figure used was a half to the farthest point - from CU (a quarter of the ring length)?

I think you've convinced me that, if it's worst case Zs they are thinking about (which initially didn't even occur to me), then it makes mores sense if 'farthest' is being being measured from the CU (which is what you suggested in the first place) - but I still can't imagine where the seemingly aribitrary 1/8 came from. Such a 'rule of thumb' would also be ridiculously conservative in most real-world (i.e. not 'worst case') scenarios, since I think it would effectively make it impossible to have spurs anywhere near the mid-point of a ring.

Kindest Regards, John.

 

[JohnW2]

Haven't forgotten your post of 01:37 - been busy and haven't had time to give it the necessary attention.

BAS, I was wondering whether you've yet had time to give the necessary attention?

Kind Regards, John.

 

[JohnW2]

Haven't forgotten your post of 01:37 - been busy and haven't had time to give it the necessary attention.

Still giving the necessary attention?

Kind Regards, John.

 

[ban-all-sheds]

No - I've had time but have misapplied it.

I will see if I can apply myself.

Dodgy 'or' instead of 'and', wasn't it?

 

[JohnW2]

No - I've had time but have misapplied it. I will see if I can apply myself. Dodgy 'or' instead of 'and', wasn't it?

Yes, sort of - and, as with most mistakes with OR and AND operators, it had a crucial effect - in terms of whether or not a 3m limit applies with the two 'regulatory approaches'.

Kind Regards, John.