2 rings from one fuse

[Wattsy]

Hi all,

Just changed a heater for an old couple and when i went to test the circuit it became apparent that someones added another ring to the same fuse. Prop the people that put did the conservatory. :run

I've advised them that it needs to be put on its own but thinking about it is it doing any harm?

Thanks

 

[ianmacd]

As long as they are both rings then its fine. Not brilliant but one circuit.

 

[PC Electrics]

Hi all,Just changed a heater for an old couple and when i went to test the circuit it became apparent that someones added another ring to the same fuse. Prop the people that put did the conservatory. :run

I've advised them that it needs to be put on its own but thinking about it is it doing any harm?

Thanks

Provided the second "ring" conforms to design requirements for a ring and isn't a load of spurs on spurs, then in all reality it probably isn't unsafe per se. Probably the simplest solutions are to either crimp two legs together to end up with one larger ring (max length permitting) or put it on it's own MCB (a 20A perhaps). But since all you've done is some maintenance and it's not unsafe then just leave well alone.

Unless you think stuff like this really is unsafe, I am wary of mentioning it to older people. Stuff like this causes them worry, it preys on their mind and they lose sleep over it. Next thing, you've got some hyped up daughter on the blower demanding to know why you've upset her Mum! On the other hand, the worry thing can be a source of business. I've changed a few CUs round our way after being called out to a blown 3036 that the old dear couldn't fix ("can you fit me one of those nice boxes with the switches, Dear?"). Just be diplomatic with your advice.

 

[Wattsy]

Thanks for the replys. I think i'll put it in the comments and explain it to them.

Slightly off topic i'm pretty sure it has 1mm cpc.

r1 = 0.71ohms

r2 = 1.76ohms

0.71 * 2.5 = 1.775ohms

When did the 1mm cpc go out and does any one know what conditions it could be problematic. I know theres a section in part P that says on a ring final circuit with rewireable 30A fuse?

Thanks again

 

[Andy™]

in some cases, 1mm earth cannot take the fault current required to blow a 30A 3036. not really an issue with a 60898

 

[Wattsy]

Its ok with 1361's then?

 

[ianmacd]

What were your Zs's for each ring?

 

[Wattsy]

I only tested the ring that i installed the panel radiator on.

Zs 0.6 ohms

 

[ianmacd]

Its one circuit though so shouldn't you have tested the other ring too?

 

[Wattsy]

I hadn't really thought about it as being one circuit.

It goes to the conservatory and supplies two sockets but its new 2.5/1.5.

 

[kme]

Its one circuit though so shouldn't you have tested the other ring too?

Now I`d be tempted to call it 2 circuits.

If the upstairs and downstairs lighting are into one 5A 3036, and I changed a light in the lounge, I wouldn`t necesarily go pootling about in the bathroom to test the "other circuit" that is in the same breaker. In fact, come to think of it, if it IS one circuit, what would it be called?

`cos if its 2, then they`re either ring or radials. but 2 radial circuits classed as one circuit cannot be "a radial", unless you`re suggesting one is "spurred off" the other at the fusebox?

opcorn

KME

 

[Andy™]

Now I`d be tempted to call it 2 circuits.

1 MCB 1 circuit

 

[ianmacd]

Read the dentition of circuit in BRB. Same MCB same circuit.

 

[kme]

Right - I`ve just read the (definition?).

" An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)".

So, you can`t call it a circuit if it gets OCP from 2 devices ; but it does NOT say that 2 circuits cannot be fed from one MCB. Sorry.

 

[kme]

ok - 314.4.

Though how you can then have, for instance, addition of smoke alarms which are connected, at the CU, into a "regularly used lighting circuit", is beyond me. The smoke alarm install is a connection into an existing circuit, but doesn`t create either as radial or a ring.

We need a new circuit type name - perhaps a "duadial"????

 

[ianmacd]

Right - I`ve just read the (definition?). " An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)".

So, you can`t call it a circuit if it gets OCP from 2 devices ; but it does NOT say that 2 circuits cannot be fed from one MCB. Sorry.

Yes it does. Same Device=circuit not 2 circuits.

 

[AIM]

If two circuits [legs even if they are RFCs] on one MCB. Could you show them as Circuit xx A and circuit xx B on the schedule of test results against the same MCB? At least then all relevent details could be specified for both legs?

 

[PC Electrics]

Thanks for the replys. I think i'll put it in the comments and explain it to them.Slightly off topic i'm pretty sure it has 1mm cpc.

r1 = 0.71ohms

r2 = 1.76ohms

0.71 * 2.5 = 1.775ohms

When did the 1mm cpc go out and does any one know what conditions it could be problematic. I know theres a section in part P that says on a ring final circuit with rewireable 30A fuse?

Thanks again

That's okay as long as it's on a 60898, 61009 or 1361. It's only a problem with rings on a 30A 3036, where 1mm cpc does not meet the requirements of the adiabatic equation (543.1.3).

2.5/1.0 was only around for a short while in the very early 70's. It was very quickly discovered that it doesn't comply and was dropped. It was a time of high building activity though, so there's plenty of it still about.

 

[Voltimax]

Adiabatic equation time

 

[ianmacd]

Adiabatic equation time

Any particular one? Seen a great one about measuring the stress and strain surrounding a crack moving at a contstant velocity and have been dieing to use it.