Should a ring main be fused??

[OwainDIYer]

May not be ideal, but surely a ring with 15A fuse on each leg is actually safer than one with 30A fuse covering both.

Indeed it is, in terms of cable protection;

I would say this is not necessariy the case.

Assuming the load is evenly distributed, each fuse will carry half the overload current. Thus the first fuse to blow may blow more slowly.

The overload current will then be fully carried by the second fuse until that blows.

The total energy let through by the two fuses with the delay is likely to be greater than would be let through by one fuse.

Someone with the time-current curves can probably calculate it, but I certainly woudln't say it was *safer*

 

[westie101]

Someone with the time-current curves can probably calculate it, but I certainly woudln't say it was *safer*

Agreed, in certain circumstances we put two circuits in parallel to fault find, relying on the larger let through energy.
I know this seems a strange way of working but as the cables are buried (and we haven't blown any premises up - apart from neutral faults) it is one of the best ways of fault finding - cause as much damage as possible at the fault position to give us a good chance of finding it!

 

[JohnW2]

I would say this is not necessariy the case.

It certainly depends upon the exact circumstances.

Assuming the load is evenly distributed, each fuse will carry half the overload current. Thus the first fuse to blow may blow more slowly.

One would have to look at the curves, but I rather doubt that it would be significantly, if at all, slower - since half the overload current going through the half-rated fuse would represent exactly the same percentage of In (i.e. the same proportionate degree of over-current) in both cases.

The overload current will then be fully carried by the second fuse until that blows.

True, but that fuse should blow very quickly, since a current which overloaded the circuit as a whole would be way over the In of that second fuse.

The total energy let through by the two fuses with the delay is likely to be greater than would be let through by one fuse.

Probably, yes (as you say, one would have to look at the curves and do some sums) but, as above, I suspect only slightly so.

However, it is in situations of seriously unevenly distributed (over)loads (the theoretical Achilles heel of a ring final) that the dual fuses would be likely to make things safer. Although, again, the two fuses would have to blow 'in turn', I suspect that dual fusing might win in this situation - but I'd have to do some work to be sure!

In any event, as I said, the main (potentially serious) hazards of dual fusing are nothing to do with cable protection or let-through energy but, rather, to the risk of people finding themselves unexpectedly working on live circuits.

Kind Regards, John.

 

[EFLImpudence]

Assuming the load is evenly distributed, each fuse will carry half the overload current.

Yes, but that overload current will be enough to blow both fuses. The same overload current that would blow one fuse of twice the rating.
If, theoretically, exactly evenly balanced then both fuses will, theoretically, blow at the same time.

Thus the first fuse to blow may blow more slowly.

Not quite sure what you mean here - more slowly than what?

The overload current will then be fully carried by the second fuse until that blows.

If the fuses don't blow at the same time then the current will double and blow it 'instantly'.

The total energy let through by the two fuses with the delay is likely to be greater than would be let through by one fuse.

If we are talking "theoretically equally" balanced load and fuses there should not be a delay.

Someone with the time-current curves can probably calculate it, but I certainly woudln't say it was *safer*

The main drawback with a Ring Final Circuit is that the load is very rarely balanced.
Therefore, if the overcurrent occurs nearer one fuse than the other, the current would blow the nearer fuse slowly and then the other 'instantly' while that current would not have been great enough to blow the fuse with twice the rating at all but may have been too much for the conductors.

Even if you are correct about the 'exact mid point' overcurrent the unlikelihood of that being the case would still lead me to deduce that two fuses would protect the cable more.

I was not advocating the arrangement but pointing out that it was not dangerous and, as far as the cable was concerned, actually better.

 

[JohnW2]

<quite a few things>

For once, I actually managed to type faster than you tonight

Kind Regards, John.

 

[EFLImpudence]

 

[Chri5]

Might you have another fuse box in the area? plenty of older installation have more than 1 fuse board.

Check around the area of the meter and look for more than one set of mains tails extending from either the meter or a black square unit (called a Henley box).

Some small fuse boards don't always look like what is commonly considered to be a board.

Here's a picture pulled (randomly) from the net.