Shower circuit design.

We seem to be drifting slightly, but the type testing could be interesting.

There are only two or three sets of dimensions used for busbar and terminal positions among the various makes of MCBs, and the overall outline is very similar. They are almost all made in China from unknown manufacturers, and looking inside there is very little variation either. This is unsurprising as the function is identical as is the specification. The brand and labelling are printed on the outside, but all (presumably) have been type-tested to the same specification.

As many of you probably know, I am fairly cynical about many things, but with very good reason and as much understanding as I can achieve. I wonder how many factories these devices actually come from? I suggest that it may be only one or two, backed up by the noticeable fact that certain components get more difficult to source at the same time for several brands at once! Very curious.

As all of these devices have the same type-test, I cannot see what parameter could be different between brands to cause any problem. Mike says “eject metal” above, but this would surely happen whatever the containment, bus bar exact shape, and outer box material were used? They can all break the test 6kA (or 10kA) satisfactorily without failure, my question, therefore, is “what is the difference which prevents intermixing of brands, on the basis of product failure”? My cynic says “this is to keep the customer from buying spares from another source”, and that it is in BS7671 under manufacturer pressure. Trying to stop aftermarket parts for vehicles has been tested as illegal, surely the risk here is actually less, or potentially zero following above?

What happens to BS EN 61439-3 approval if a non-standard MCB (or RCBO, etc.) is fitted?

I have always been a great believer in fixing rather than replacing.

I would say that it is perfectly reasonable to fit a non-standard part when the alternative would be to replace the whole board (due to obsolescence, for example) provided that it is done with appropriate engineering judgement and skill.

The other proviso is that you have professional indemnity insurance in place for your design work.

davezawadi (David Stone): 
 

 Trying to stop aftermarket parts for vehicles has been tested as illegal,

I wonder whether anything will get drifted into the “right to repair” legislation to make the same apply across all products …

To be fair, aftermarket products argument is a little different … If a product (like a car for example) undergoes extensive and stringent safety testing, it's right to question whether  replacement brake discs or pads have also undergone an equivalent amount of type testing, or some other validation that they are made in a way, and of sufficiently robust materials, to offer at least the same safety as the original equipment manufacturer's brake discs or pads.

For single-phase supplies up to 100 A, you have 6 kA OCPDs that have been tested for 16 kA conditional short circuit rating by the manufacturer.

Quite so. 

Although reading the fine print, this is not really  that the 6kA devices are tested bare on a supply of 16kA, if they were they'd be rated at  16kA and charged for accordingly,  but rather that they are OK downstream of a 100A fuse, on such a supply. This means that above 6kA, the DNO 100A fuse is doing some, or all, of the heavy lifting, and opening, first (after pre-arc) to make a rapidly growing series arc to act as a current limiter, and then soon after going completely  open circuit. The MCB is rated to survive that I squared t , and by the way it will probably trip as well…

(also 100 amp DNO fuses vary quite a bit- the 

I2t for a 100KR85 is 73500 but  the 

100LR85 is only 57300 to fully open - the latter, friskier, fuse is preferred by SSE and maybe other DNOs too. )

The I2t for the MCB is also  a variable thing, as illustrated in these plots from a Dorman Smith paper  The let through energies of  larger value MCB's  and that to blow the main fuse cross over in the high kA regions of interest. Not that we need to care too much, but a really low z fault on the shower or cooker cct may well take the DNO fuse with it from a cold start on a  PSSC >6kA  supply.?

Wylex MCB curves show similar trends but are not quite the same… (note they use the pre-arc not the clearance I2T for their BS88 dotted lines, so these are minima, not maxima.) Note also the theoretical constant opening time for 10msec (1 half cycle) and how they are all get going quite a bit faster than that, at least to start opening, if not to completely break the circuit. (who knows how many hundred amps dribble through to give a relatively long low energy tail while the arc is dying out on the quencher)

Now the MCB may be cheesecloth tested only in it's makers enclosure, but I think an MCB that blew itself  to bits would soon lose its reputation so it is a fair bet it would be fine in almost any similar enclosure, or maybe even none at all, at least up to the 6kA figure. 

Given this is a loop plus fault impedance of 40milliohms ( which is not much more than there and back on 1m of 1mmsq, or 2.5m of 2.5mmsq or 10m or 10mm2 etc..) the proportion of 100A single phase installations where PSSC at origin exceeds 6kA is small, and then the proportion of real faults that exceed 6kA will be smaller still. 

We could handle those separately, and say no mixing of MCBs on installations of PSSC > 6000A .

Mike.

gkenyon: 
 

Zoomup: 
 
 That is a purely academic point. Reality varies with theory in many cases. We are talking about Chint here not Rolls Royce. This is a way to ensure makers' profits. Use our stuff only. I am sure that we are aware of that.

 

We can put our stuff into an empty enclosure or existing low Voltage assembly competently, so why not competently modify an existing assembly. 536.4.203 para 1. It is only a simple consumer unit so this is easily achieved.

 

Note 2. “Incorporated components inside the assembly can be from different manufacturers.”

Z.

Not really.

I would recommend you complete your reading of NOTE 2, because it goes on to say (my bold):

It is essential that all incorporated components should have had their compatibility for the final enclosed arrangements verified by the original manufacturer of the assembly and be assembled in accordance with their instructions e.g. the consumer unit, distribution board manufacturer. The original manufacturer is the organization that carried out the original design and the associated verification of the low voltage switchgear and controlgear assembly to the relevant part of the BS EN 61439 series. If an assembly deviates from its original manufacturer’s instructions, or includes components not included in the original verification, the person introducing the deviation becomes the original manufacturer with the corresponding obligations.

In other words, if you fit someone else's breaker, and the original manufacturer does not endorse it, you become the manufacturer of the assembly. Does your insurance cover for that outcome?

Yes Graham I read all of that. We as electricians all manufacture installations and assemblies, often with stull from different makers with no problems. It is all down to our qualifications and experience. Obviously the makers want us to use their stuff as it makes them income. Personally late on a dark rainy night if I had to get something up and running I would use my van stock and not worry about it at all. I must qualify that statement with the fact that I mainly undertake domestic work and small commercial and industrial work these days. With a new big project things may be different and I would take a different viewpoint. A person like you would specify the correct equipment to use. I respect that.

Z.

Thanks Z … not criticising, just putting the complete picture.

It's a really difficult debate that's been circling round for much of my working life. I openly admit to having owned a CU in which I had a couple of “alien” mcb's in the past … and of course have used adaptable DIN rail boxes.

From a CU the manufacturer's perspective, if anything untoward were to happen in a CU, of course they would be looking at the best legal and insurance position for themselves.


As others have said, it would appear that you can put bits in a box and it's OK … but not when the box is a CU. But is that the entire story?

What standard is used for the assembly you've made when you put an mcb in a box? It can't be BS 7671 … see Regulation 113.1.

gkenyon: 
What standard is used for the assembly you've made when you put an mcb in a box? It can't be BS 7671 … see Regulation 113.1.

Presumably no standard at all - just the ones for the individual components.

Chris Pearson: 
 

gkenyon: 
What standard is used for the assembly you've made when you put an mcb in a box? It can't be BS 7671 … see Regulation 113.1.

Presumably no standard at all - just the ones for the individual components.

If it's serving the purpose of switchgear (like a CU), I would have guessed BS EN 61439 ("Low-voltage switchgear and controlgear assemblies")

  - Andy.

mapj1: 
We could handle those separately, and say no mixing of MCBs on installations of PSSC > 6000A .

A board design of composite boards/components to 6kA includes dimensions and spacings etc., and a type test is carried out. As most probably have, I have had to add/replace a older MCB etc., there are sellers on Ebay. A temporary fix is OK in my book until the correct replacement is found - not guilty m'lord ? 

Jaymack