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Posted By Topic: Ring fed circuits

Nov 23 2018 19:03

I've searched old threads here but can't find anything.

Ring circuits don't seem to be mentioned at all in 3000.

But there also doesn't seem to be anything to say they are not allowed?

Presumably, these would not be considered parallel conductors? As they are't technically parallel.

I would have thought if they were allowed, there would be information on how to calculate the rating of the cables, how to protect spurs etc.

Would they be a part one "solution"?

When does something that's not in the book, actually become not permitted?

Nothing that I can find in ESRs, 3000, 3008..

Nov 23 2018 20:25

It amazes me why NZ and Australia decided to part with UK electrical regs way back when and go our separate ways. Ring circuits make so much sense (saving cable and labour). Also, fused plug tops are a brilliant idea. (Our crappy PDL plug tops are a nonsense.)

Nov 23 2018 20:32

Doug P
The ring circuits I think you are referring to is a practice used in the United Kingdom for the supply arrangements for socket outlets in domestic installations. BS 7671 (the UK Wiring Rules) has all the details, and all the socket outlets are rated at 13 amps, and all plugs that plug into the socket outlets have a fuse inside the plug to protect the supply cord to the appliance from overcurrent.

Currently, the use of the ring circuit system in NZ is restricted by the requirements of Electricity (Safety) Regulations 2010 and AS/NZS 3000 requiring the use of 3 pin flat pin 10 A socket outlets and plugs complying with AS/NZS 3112.

In brief, the ring system uses a 2.5 mm2 TPS cable and starts at the switchboard and loops into and out of all of the socket outlets in the ring circuit and then returns to the same switchboard and in the same terminals as the start of the ring circuit.

A ring circuit cannot supply an area greater than 100 m2 in domestic dwellings. The overcurrent device is typically 30 or 32 A.

Testing after installation and during periodic inspections is complex, and the continuity of the rings conductors (phase, neutral and protective earth) is also required.

Many years ago several attempts were made in New Zealand by a well-known electrical manufacturer to see if a 10 A HRC fuse could be fitted inside a 3 pin flat fuse plug to AS/NZS 3112, but the wattage loss in the HRC fuse inside the plug at 10 A exceeded the max temperature rise permitted by AS/NZS 3112 for the whole plug. This heat rise limitation of AS/NZS 3112 is to ensure good connection of the plug pins to the socket outlet contacts. If it had been possible to get a compliant 10 A plug, the use of ring circuits might have been introduced into New Zealand from 1960 to 1970.

In the UK 13 A plug to BS 1376 have very large brass pins which help to reduce the temperature rise in the plug.


Nov 23 2018 20:40


A couple of other things spring to mind,

Any part 1 solution would have to be inspected.

Domestic installations must be part 2.


Nov 23 2018 21:19

correction to my post UK socket outlet are BS 1363 not BS1376.

Ih you want more UK discussions of the advanatages and disadvanatages of Ring circuits google BS 1363 or ring circuits.

Nov 23 2018 21:54

All good points and interesting information...

But, I'm not sure how the NZ 10A plug wouldn't be suitable?
e.g. there's nothing to stop us running a 32A 6mm2 radial circuit or a 25A 4mm2 radial circuit with 10A outlets right? We aren't limited to 20A on our power circuits?

I just suggested that it may only be possible to use a ring as a part 1 solution, as how to install a ring circuit isn't detailed in part 2.

So my real question is, what is telling us we can't do a ring fed circuit?

I can't see how it's the 10A plugs, or a 100m2 limit which I presume is the British standard..

Nov 23 2018 23:30

Our rules say cables must be protected against overcurrent in accordance with 3008. I don't think you can use the parallel conductors rules, because of differing lengths.

So you could do a ring circuit, but would still have to put a 16A MCB on it...

Nov 24 2018 02:21

The way I see it (and I may be wrong!):

A ring circuit is, by definition, a parallel circuit, and therefore by regs must be wired in 4mm minimum.

The regs also require that a thing with a 10A plug on it must not draw more than 10A through the plug, so there is no need for a current limiting fuse as is used in UK practice. (This is one of the big differences between UK and NZ regs)

There is no reg directly preventing the use of a MCB greater than 20A to a 10A GPO. The MCB is there to protect installation wiring only. (Another difference between UK and NZ practice)

This is all Part 2 stuff.

The only actual real issue is 3008 issue, that 0.75mm flex must not be overheated during a fault condition.

So, two x 4mm into the terminals of a GPO...


Nov 24 2018 08:24

Be aware that my earlier post had only intended to point out that the UK does use a ring circuit and it has special requirments in reply to a genral inquiry about ring circuits.

Of interest to note that the UK (and some old countries which the UK looked after for many years (Hong Kong and Singapore are typical examples)) are the only places in the world that you will find the ring system with the BS 1363 fused plugs.

There is currently no reason that a ring circuit could not be used in NZ.

For example, if you had a very long final subcircuit and needed a low voltage drop to meet the overall voltage drop of 5% from the point of supply to the most remote point in the electrical installation.

One of other factors to consider is the maximum cable hole size in the socket outlet and I recall that AS/NZS 3112 socket outlets must be able to terminate 2 x 4mm2 cables.

It is true that when NZ joined with AU to use AS/NZS 3000:2000 the need for the final subcircuit overcurrent device changed to protecting the final subcircuit wiring only,
What it supplied ceased to be an additional consideration in the fixing the overcurrent device current rating.

The use of a AS/NZS 3112 socket outlets in an NZ electrical installation was to force a high degree of standardization on type of socket outlets used and this is also linked to the supply voltage window which in earlier times was called "Standard Low voltage".

This approach was used to tie in with the electrical appliance requirements that if the appliance was fitted with a AS/NZS 3112 plug a simple and safe method of connection of the correct supply voltage from an electrical installation was always assured.

This simple concept has fallen into disuse whenn NZ joined with AU and the joint use of AS/NZS 3000 as a common set of wiring rules.

Nov 24 2018 09:01

Pluto. Thanks for the details. But I'm not sure where you come to the conclusion that a ring circuit could be used?

dbuckley. I don't see it as a parallel circuit. As for the 0.75 flex.. I haven't calculated it and I'm not sure where there would be a limitation on the short circuit protection rating for appliances. I guess in the appliance standards.

Someone. Where did you get the 16A MCB requirement from?

I'm still at a loss to see where a ring circuit fits in our standards. If it was allowed, electricians would be trained on how to install it, and there would be references to cable calculations and protective devices (MCBs and RCDs etc), and how to deal with spur sections etc.

"Radial branched distribution" is only mentioned a couple of times. In and appendix B - But, appendix B also states "alternatives are permitted".


Nov 24 2018 11:23

Perhaps you are looking at this from the wrong end DougP. If there's Nothing in the regs or standards specifically banning it then how can it be illegal?

The fact that it doesnt perhaps fit your particular interpretation Doesnt exclude it from use.

Never actually done it myself, except perhaps from making up formica switchboards in the 70's, but ive come across it plenty of times over the years and never have an issue with it.


Nov 24 2018 11:43

So I've found some historical information.
In the 1976 wiring regulations, ring circuits are detailed. Including an exemption for fuses to be larger than the CCC of the conductor for ring circuits. And maximum number of points, maximum current rating for the subcircuit, and the cable size.

Some of the details are interesting. Like for normal (non-ring) FSCs with 10A outlets, the maximum rating of the subcircuit shall be 20A. For a ring circuit, it specifies the use of fused plugs. Protection of 30A max and cable size not smaller than 20A.

In NZS3000:1997 it has a couple of clauses about the use of ring circuits to overcome voltage drop. Must be on an appropriately rated circuit breaker. And both ends of the circuit must be connected to the same terminals on the switchboard.

Then in ASNZS3000:2000 and later versions, there's no mention of ring circuits.

Nov 24 2018 12:11

DougP queries:

> I don't see it as a parallel circuit.

It mos definitely is. Try drawing it out. Though thinking a bit more about this at a more awake time, the length of the conductors is unequal, so that could be a stumbling block.

In the wider context, the regs shouldn't be a design manual: they should provide rules that ensure a required minimum degree of safety. Unfortunately, part 2 in particular does get awfully specific about many things.

Nov 24 2018 13:04

Yes a ring circuit is a form of parallel circuit,so they are covered by 3.4.3. Also by 3.6.3 for volt drop. Note there's no actual requirement to have equal length, that bit is in an explanatory Note and is not an enforceable requirement. Just because ring circuits aren't specifically mentioned doesn't mean they can't be done under these clauses. But it does suggest that maybe they need to be treated with extra caution.

The idea that "if it was allowed, electricians would be trained on how to install it" makes me smile (grimly). After all there are plenty of actual requirements that electricians don't seem to have been trained on; some of them quite basic, like how to test. Or how to read a Standard.

Agree with dbuckley that Wiring Rules are not supposed to be a textbook, they are a set of minimum requirements that must be met, plus a bit of additional guidance (because experience shows that it;s necessary, largely due to lack of understanding within the trade (training again?).

Ring circuits are very useful for switchboard wiring (eg feeding circuit protection); but there are good reasons not to encourage their use for final subcircuits, or even for submains. Chances are very high that some fool would break the ring, thus rendering the installer's careful CCC & V-drop calcs invalid and the circuit non-compliant (and possibly unsafe). Different in the UK, where everyone expects a ring configuration.

The only time I've struck a ring circuit outside of a switchboard were in military contexts; Kensington army drill hall had ring submains supplying the DBs (one ring for ltg DBs, one for power); fed from 2 sets of 3-phase fuses for each ring. Nasty when you're not expecting it.
The other was a 1960's era frigate, where the main power is ring-fed around the ship between the 4 principal switchboards, so that in the event of battle damage they can isolate any section and feed around the other way. Similar to a bus-tie arrangement but brought back to close the ring; and in normal service not used in ring config.


Nov 24 2018 18:03

Ok so it sounds like we're all good to go then.
4mm2 isn't a problem.. we're pretty close to needing that on a lot of 20A circuits anyway.

By my calculations, for 4mm2 T&E partially surrounded, we're at around 34m for 2.45% VD @ 16A per side of the ring (32A MCB). Or can we also consider that as a distributed load on each section of the ring, and use half the current rating (effectively 1/4 of the rating) for VD calculation? 3.6.2 Exception 1. So that would give us around 70m per side of the ring, @ 2.4%VD.
Allowing for 5-6 droppers per side of the ring, that should give us probably 40m to the farthermost point?

The ring circuit would make it much easier to balance the load across outlets for the kitchen and laundry etc as well, and reduce nuisance tripping being on a 32A circuit breaker or RCBO.

We can probably do most houses with two ring circuits, which will cut down on the number of RCDs and MCBs and a smaller switchboard size as well. An 10 or 12 pole board should be big enough using single DIN RCBOs.

Who's going to be the "first" do re-implement the ring main circuit to NZ?

Nov 24 2018 18:25

@DougP re "Someone. Where did you get the 16A MCB requirement from?"

If we can't treat it as a parallel circuit (due to unequal current sharing), then effectively it's a radial circuit with a bit of extra copper thrown in. So we have to treat it as whatever cable size it's run in - I was assuming 2.5.

Common practice now seems to be to protect that at 16A, which is an argument I'd rather not get into and irrelevant...

Nov 25 2018 11:45

No objection to treating it as distributed load; but for V-drop I always assume worst case ie 10 A at furthest socket & 10 A at 2nd-furthest.
Anything less than that I regard as not fit-for-purpose, even if technically compliant.