Part P / BC question

[ban-all-sheds]

That's why it is a designer's nightmare. However, as above, the existance of an OPD in the circuit prevents danger when (as will sometimes happen - such is a statistical approach) loads 'happen' to exceed the maximum estimated on the basis of applying diversity. The dangerous situation would be to apply diversity but not have an OPD to over-ride it with a 'ceiling'

Is it a nightmare?

Or a pointless concept of so little worth as to make it not worth worrying about, let alone allowing it to take on nightmare proportions?

If, without changing in any way either the number of socket outlets I have or the set of appliances I have plugged into them, I rewire my house to have at least one dedicated socket circuit per room, and end up with 3 or 4 times as many circuits as I have now, what happens to my maximum demand?

AIUI, in France your standing charge would be eye-watering if you wanted a supply of the same capacity which is typically "required" by houses in this country based on their "maximum demand", so people go for a lower MD, which is enforced by the suppliers equipment and managed by the consumer's equipment having load-shedding relays. It would be interesting to find out how French electricians estimate MD to come up with a figure which doesn't result in frequent operation of le disjoncteur or of your appliances switching on and off.

 

[JohnW2]

In response to JohnW, why is diversity a designer's nightmare and what's the proper design you refer to?

'Proper design' is possible when one has hard-wired fixed loads and can tailor all aspects of the circuit design to those loads. It may, in some situations, be possible to apply diversity considerations even then - although, by it's very nature, the concept of diversity implies a degree of gambling. If there is certainty about differential loads (e.g. if it is known that Machine A and Machine B will never be in operation simultaneously) then that is not probabilistic, and therefore is part of the actual design, not a matter of diversity.

In some situations, application of diversity is essential. In particular, it would be totally impractical for DNOs to design their networks to be able to cope with every consumer having their maximum theoretical demand simultaneously. However, as with all things statistical, the gambling inherent in diversity comes closer and closer to a predictable near-certainty as the number of consumers/loads increases.

It's on a small scale (e.g. a single household) that diversity becomes more of a problem, for the very same statistical reason. Whilst it's incredibly unlikely that 1000 homes would simultaneously switch on all their showers, cookers, washing machines, kettles etc. etc. , it's far from an impossibility that all such loads will occur simultaneously in a single dwelling.

In fact a greater "designer's nightmare" (within small installations) than the general concept of diversity is the existance of socket circuits - since the designer has no control over what gets plugged into the sockets. In practice, of course, most of that nightmare is alleviated by well-established conventions and practice (and limitations imposed by OPDs), but it's still far from an ideal design situation.

Kind Regards, John.

 

[Paul_C]

AIUI, in France your standing charge would be eye-watering if you wanted a supply of the same capacity which is typically "required" by houses in this country

It certainly would be. Click on any of the three options to expand the section and see the annual standing charges ("abonnement annuel"):

http://bleuciel.edf.com/abonnement-...s-prix-de-l-electricite/tarif-bleu-47798.html

 

[JohnW2]

Is it a nightmare?

Given that you're the second person who has picked up on my use of the word, I probably have to concede that 'nightmare' was perhaps too dramatic a word to use. My point is that any uncertainty about loads precludes an 'ideal' design process.

Or a pointless concept of so little worth as to make it not worth worrying about, let alone allowing it to take on nightmare proportions?

It's certainly not a pointless concept - as I've just written, at some levels/scales (e.g. DN0) it is an essential concept. It's the failure of statsitical process to be very good on a small scale ('small numbers') which is the potential problem.

If, without changing in any way either the number of socket outlets I have or the set of appliances I have plugged into them, I rewire my house to have at least one dedicated socket circuit per room, and end up with 3 or 4 times as many circuits as I have now, what happens to my maximum demand?

That's not really anything much to do with diversity calculations, but it does illustrate the issue with socket circuits. Even with your re-wired arrangement you (or a subsequent occupier) could, theoretically, decide to move all of the appliances into one room and plug them into the same socket circuit.

If one views the sitauation from a statistician's viewpoint, it's quite gratifying to see how well diversity and socket circuits work out in practice. However, that's in large part due to all the 'slack' and 'margins' that are built into everything. If we had OPDs (including cutout fuses) which operated at 1.01 times their In and cables which melted at 1.02 times their rated current-carrying-capacity, we would have a very very different situation on our hands

Kind Regards, John.

 

[ban-all-sheds]

Does anybody know the time-current characteristics of the load enforcement devices French suppliers use?

 

[ricicle]

Does anybody know the time-current characteristics of the load enforcement devices French suppliers use?

They are probably just (adjustable ?) electronic timers which are triggered by CTs. These timers then operate the load-shed relays.

 

[Paul_C]

Maybe I have the wrong end of the stick, but I was always under the impression that the actual load-shedding devices are owned by the householder. I think BAS was referring to the characteristics of the main "disjoncteur" that EDF uses to enforce the maximum demand which has been paid for, set as per the details in my link above.

 

[Stoday]

Does anybody know the time-current characteristics of the load enforcement devices French suppliers use?

I understand the demand integration period for French disjoncteurs was 10/15 minutes. The timing used a thermal element to achieve the period.

My experience with French distribution is over 10 years out-of-date; I expect the disjoncteurs will now have microprocessor timers.

In the UK maximum demand for distribution purposes is defined as twice the number of kWh supplied in a half hour. i.e. the demand integration period is 30 minutes.

Of course, diversity is significantly affected by the demand integration period. The after diversity maximum demand at the meter of a typical house is 5kVA if the demand integration period is 30 minutes.

 

[ban-all-sheds]

Maybe I have the wrong end of the stick, but I was always under the impression that the actual load-shedding devices are owned by the householder.

That's my understanding too. They're optional, but allow you to exist in a low kW band by having your HVAC shut down while your shower is on, for example, or the oven and freezer go off while you boil the kettle, etc.

I think BAS was referring to the characteristics of the main "disjoncteur" that EDF uses to enforce the maximum demand which has been paid for, set as per the details in my link above.

Yup.

 

[ban-all-sheds]

The after diversity maximum demand at the meter of a typical house is 5kVA if the demand integration period is 30 minutes.

The 9kW rating is usually just about enough for a three-bedroom house, though 12 to 15kw are needed if you have electric heating and electric hot water.

 

[JohnW2]

The 9kW rating is usually just about enough for a three-bedroom house, though 12 to 15kw are needed if you have electric heating and electric hot water.

Good grief! One sort-of expects some quaint English on MIs and websites of Chinese companies - but France?!! ....

This option will suit you particularly if you possess few electric devices and if you wish to consume at any time without caring you of period day, or of the year.
.... Further to a decision of public authorities (Arrested of August 12th, 2010 concerning price lists regulated by sale of the electricity), the powers from 18 to 36 kVA included by the option Base of the Blue Price list for the residential customers were put in extinction and are not available any more on the subscription.

Kind Regards, John.
Edit: Juust to clarify, that quote comes from the French EDF website, not the one BAS linked to.

 

[Paul_C]

It can be surprising how diversity in practice can result in one getting away with a lot, perhaps helped in some cases by having more than a fair share of luck.

I remember seeing a house several years ago where this seems to have been the case. It was one of those Victorian places in a nearby seaside town, spread over three floors with about five or six bedrooms, and had been in use as a guest house for some years. I remember there were two electric showers fitted (not sure of the ratings, but obviously 7kW+ each), and while there was gas central heating I recall there were at least three or four of the bedrooms which had 1kW radiant heaters fitted at high level as well. There was an electric stove, and an immersion heater, although admittedly the latter probably didn't get much use with the gas heating installed. And the whole lot was still running on an old 40A supply. No doubt the actual peak demand at times was quite a bit in excess of that, but obviously not enough that it was taking out the service fuse on any sort of regular basis.

 

[JohnW2]

It can be surprising how diversity in practice can result in one getting away with a lot, perhaps helped in some cases by having more than a fair share of luck.

Indeed - although, as I mentioned earlier, I suspect it's often a case of 'regular but temporary overloading', rather than true diversity, facilitated by the generous magins provided by OPDs!

We shouldn't get too excited by 'diversity' in electrical installations. It's much more dramatic in relation to the design of things like road and telecommunications networks, but they benefit from massive scale which makes the underlying probabilistic principles nearly always work like it says on their tin (except, in the case of road networks, on bank holidays )

Kind Regards, John

 

[ban-all-sheds]

Good grief! One sort-of expects some quaint English on MIs and websites of Chinese companies - but France?!! ....

Probably a Babelfish job.

It looks like site aimed at British ex-pats, run by a British ex-pat electrician ("Doug Irons" doesn't seem a particularly French name), so maybe he's got the traditional British ex-pat attitude to learning the local lingo...

 

[JohnW2]

It looks like site aimed at British ex-pats, run by a British ex-pat electrician ("Doug Irons" doesn't seem a particularly French name), so maybe he's got the traditional British ex-pat attitude to learning the local lingo...

I'm not sure I get that - if he were a British ex-pat, wouldn't you expect him to be able to write proper English?
I've just realised that I probably confused you - that quote didn't come from the site you linked to, but from the EDF site, which is linked to from the other one ('Tarifs').

Kind Regards, John.