help with wiring regs - dispute with land lord

[meow2222]

2 cables in parallel, not one. Hob is not a continuous load anyway, its intermittent.

NT

[tom.harrigan]

is likely to exceed the cable rating, _all_ of the ring is non- compliant. People tend to get confused by rings, thinking that doubling up the wire doubles the capacity. In general it does not. Someone is likely to pipe up that under certain circumstances you can ensure (so long as no circuit modifications are made in the future) that a particular ring can cope with 32A. That is not the point.

T.

[Dave Plowman (News)]

Define load. Fixed as via an FCU. Continuous or peak? Define the cable rating. At the point where the load is applied? Elsewhere?

[Tony Bryer]

Depending on where this is in the ring you might have one 5m and one

25m leg with the current flows being proportionate

[John Rumm]

Not quite sure I follow... you seem to be saying what I just said, at the same time as querying it.

Rings are protected at 7.6 kW with the assumption that the load is diverse and distributed round the ring. The regulation to which I think you allude is [433-02-04] (copy at end). which basically says the minimum cable size is 2.5mm^2 (MICC excepted), and the particular installation conditions should leave it with at least 20A nominal capacity, and that the load in any part of the ring is should not exceed that nominal capacity for long periods. Note the word "part" - it does not say the total load on the ring as a whole must not exceed 20A.

As I said in my post, a 5kW point load does exceed what is a reasonable point load, while it is diverse, it is still not the sort of load for which the ring was designed. It is probably highly inappropriate that it is connected as it is.

However there is a reasonable chance that the installation will be "safe" (even if a long way from best practice) in the sense the cables are unlikely to overheat. This is assuming the load is not too close to one end or the other. The nature of the load (i.e. thermostatic control, and a low likelihood of using all rings together at full power) is also such that you may never notice a problem even if it were close to one end. At least until you try cooking Christmas dinner for the whole family.

So all in all not a good situation, especially the lack of isolation facility, which is a serious fault, and a fire risk.

Test of the regulation in question:

"433-02-04 For a ring final circuit protected by a 30 A or 32 A protective device complying with BS 88, BS 1361, BS 3036, BS EN 60898, BS EN 60947-2 or BS EN 61009-1 (RCBO) and supplying accessories to BS

1363 and wired with copper conductors, the minimum cross-sectional area of both phase and neutral conductors is 2.5 mm2 except for two-core mineral insulated cables to BS 6207 or BS EN 60702-1 for which the minimum is 1.5 mm2. Such ring final circuits are deemed to meet the requirements of Regulation 433-02-01 if the current-carrying capacity (Iz) of the cable is not less than 20 A, and if, under the intended conditions of use, the load current in any part of the ring is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable."

[tom.harrigan]

The capacity of a ring circuit is, in general, the cable capacity, just like any other circuit. It is not, in general, 7.6kW as you suggest.

Precisely. If any _part_ exceeds 20A load, the whole circuit is non- compliant. This is for safety reasons. It is an additional constraint applied only to rings, to cope with the fact that in general, rings are not protected in the same way as radials.

A reasonable chance it is safe? I'll let others draw their own conclusions.

T.

[John Rumm]

Might I draw your attention to:

473-01-07 Where a single device protects conductors in parallel sharing currents equally, the value of Iz to be used in Regulation 433-02 is the sum of the current-carrying capacities of the various conductors.

It is deemed that current sharing is equal if the requirements of the second paragraph of Regulation 523-02-01 are satisfied.

(section 523-02-01 begins "Except for ring final circuits," and so can be ignored)

If you want the true capacity of the circuit to be limited to the

*single* cable capacity then you may as well protect it at 20A. In which case you may as well make it a radial. Your suggestion seems to suggest that there is no point in using a ring circuit, since it cannot serve a greater power provision.

The capacity (in the sense I mean) *is* defined by the MCB rating, since that is what the ring will supply. The design is such that it is acceptable to load it to that limit, so long as we are talking about diverse loads (i.e. *non* continuous), and they are distributed around the ring (i.e. not high point loads).

My understanding here is different to yours then.

Yup agree with that. If you proposed a standard 32A ring knowing that it was going to have a fixed 26A load on it that may be "on" for long durations (e.g. underfloor heating - could run at full load for a couple of hours getting up to temperature), then your ring would not not meet regulations for the reasons you cite.

However you can have three 10A loads on a ring evenly spread. Total load

7.2 kW, max current in any cable 15A. Hence compliant with the requirements. What is more, even if the loads were continuous, in this circumstance no damage would occur to the circuit, all cables would run below maximum temperature.

Yes let them conclude away.

You have two cables with a method 1 current capacity of 27A. We don't know the installation method, but unless we are talking about a super insulated modern building we are probably not going to be far from that. The point load that *may* be near the middle of the ring, again we don't know. A load that diversity calculations for cookers tells us can be treated as a 16A load. You do the math.

As I said a "reasonable chance". This was an educated guess based on the above, plus the knowledge that "Bob" had wired all the other flats in the same way, and none, as far as we are aware, has barbecued its tenants yet.

For the avoidance of doubt, let me re-iterate. The installation as described by the OP is substandard. It would not pass inspection, it is dangerous in a number of respects. Causing damage to its supply cables by overheating, all by itself is relatively unlikely although possible if the ring is heavily loaded elsewhere.

[Dave Plowman (News)]

Sorry, but you're talking bollocks. The total continuous load allowed is that of the circuit protection device.

You seem to dislike rings. If so don't use them. Then look at the crap that occurs in other countries *in practice*

[Ed Sirett]

The hob should not have caught fire under any circumstances, a good landlord would WANT to make everything safe. I think we will have to give the Landlord the benefit of the doubt that the hob incident was you/your cleaner's fault.

I know of no 4 plate hobs which have a load of less than 13A and indeed the new one is rated at more than 20A. Even allowing for diversity the load exceeds 13A and so is unsuitable for connecting to a 32A/30A ring final circuit. Whilst there is little chance of the circuit as described actually catching fire it is irregular and therefore should not be. This is not a new standard. This circuit would have been irregular 1947 just as much as 2007.

There is also the fact that a cooking appliance needs an isolator. Whilst an oven alone frequently does not have such a switch a hob should always have one.

There is no compulsion for landlords to have a wiring inspection/certificate but the better letting agencies usually insist to keep themselves in the clear.

Keep paying the rent. Ask the Landlord if he wants to fix the wiring, or will let you do it (rather kind of you). If he is obstinate then you may need to bring the force of law down on him.

I expect that a full inspection will turn up a lot of minor (and perhaps some not so minor) irregularities.

[Peter Ashby]

True, that was a mistake.

Peter

[Duncan]

Buying the new hob was a mistake, but was done in good faith. I intended to avoid a confrontation with a confrontational landlord over a trivial sum (for the new hob). I agree that the most likely explanation is that the cleaner started the fire, but I like him and he works hard and I don't want to start accusing people without proof. Anyway, whatever the cause of the fire, it is clear that the wiring needs to be sorted before the new hob can be installed.

So I've reported the problem to the council's environmental health dept, who also deal with dangerous buildings. Surprisingly, perhaps, they were very helpful and say they will arrange to inspect the wiring. If they find it faulty they can force landlord to repair it.

I'll keep you posted. Many thanks to the people who commented on the electrical installation issues and helpfully suggested possible solutions to my situation.

Duncan

[meow2222]

Of course, but the reuslting ampacity is always greater than one cable alone. By how much varies round the ring.

I could be awkward and point out that as the cable warms up its R rises, improving current sharing. Not sure how much difference that makes though, probably a sixth of a hapenny.

Also rings are 30 or 32A continuous, not peak. Peak loads can be far higher in practice, a ring supplying 45A is normal and common, just doesnt go on for long.

NT

[John Rumm]

It is not that insignificant. If you look at Table 4D2B in the OSG,

then compare with:

^2 rises from 7.41 mOhms/meter to 18 mOhms/meter, so that lengthens the effective length of the (hot) short leg by almost 2.5 times.

[David Hansen]

On 30 May 2007 09:05:01 -0700 someone who may be [e-mail] wrote this:-

Since you asked I will do so. John is correct.

[David Hansen]

On Wed, 30 May 2007 19:08:32 +0100 someone who may be John Rumm wrote this:-

A 16A circuit breaker is the maximum size of protective device for connecting a fixed load to a standard ring circuit. However, I still wouldn't connect this hob to the ring without knowing a lot more about the circuit and how it was to be used.

[tom.harrigan]

You confuse a ring with conductors in parallel. Ponder why rings are not "rated" at 40A for a clue as to the difference.

The regs are crystal clear on the 20A issue. I have been crystal clear that a particular ring can be designed to run quite happily at 32A. In general, the only way to ensure that a the current capacity of the cable is not exceeded on any part of the ring, is to ensure it is not exceeded by the ring as a whole under intended conditions of use. A "point load" as you put it, of 5kW would only be acceptable, even to someone who likes to take risks, under certain conditions.

You are fully aware of the extra caveats applied to rings so that they may be "deemed"* to comply with regulations that require co-ordination between cable and protective device. These constraints are by their nature all to easy to circumvent through use or modification.

*Interesting choice of words.

T.

[John Rumm]

I don't think I would connect it at all. The number of ways it could come a bite you later are too great.

[meow2222]

Never realised it would make so much difference. The whole ring thing was quite a clever idea.

NT

[John Rumm]

A ring final circuit is treated as a circuit which has conductors in parallel. If that were not the case, then you would have no need for statements such as "Except for a ring final circuit where spurs are allowed" in 473-01-06

Agreed.

It is your interpretation that this implies the long term load of the whole circuit must be limited to 20A, that I was disagreeing with.

You have? A while ago you were saying 20A was the limit.

I already gave you an example of how you can load a circuit to 30A and not exceed 20A in any cable. In reality this will by a typical pattern of loading for most general purpose ring circuits - multiple loads spread around the ring. If you know you are designing for a different load pattern, then you would choose your circuit differently as well.

We seem to be in agreement on these points.

I think you will find I said that a point load of 5kW was unacceptable. This was one of the reasons I told the OP that the configuration he has was sub standard. I also explained why with certain prerequisite conditions being met, this load would be unlikely to cause damage in practice.

[John Rumm]

Indeed, what many dismiss as simply a way to save copper, has evolved into a quite subtle and sophisticated bit of engineering design.