Shower circuit design.

This is an interesting point, it comes back to the whole idea of standards for everything, and how these are made. BS7671 is in many ways good, it offers minimum standards for many areas of electrical installations, and these must all be different as very few situations are exactly the same. Without it, we would obviously find many “strange” installation practices, some of which would be decidedly unsafe, and no instructions as to how to criticise them.

I don't have easy access to the standards for switchgear, but the one thing I would expect to be absent is a requirement that all components come from a single manufacturer, which would make manufacture very difficult. Such does not apply to anything else I can think of, not planes ships, cars, or any piece of technology. They all have parts meeting particular standards, but none have this single source requirement, made by BS7671. Planes for example are type-tested, but many parts may come from more than one manufacturer.

It is also interesting that BS60898 does not appear to describe mechanical dimensions, and whilst all devices are superficially the same size there are subtle changes obviously designed to PREVENT mechanical interchangeability. This is the opposite of standardisation, and undoubtedly comes from manufacturers, as most standards do. I can see no reason for it, and the reason is far from clear, at least in Engineering terms.

davezawadi (David Stone): 
This is an interesting point, it comes back to the whole idea of standards for everything …

It is also interesting that BS60898 does not appear to describe mechanical dimensions …

I think that Sony learned the lesson of standardization with Betamax even though it was better than VHS.

I have had a quick shufti through BS 60898 and I cannot even find a reference to a width of 18 mm. It must come from somewhere. ?

There's no prohibition on mixing parts from different manufacturers in a single bit of switchgear. However, you then take on the responsibility of ensuring that the whole enclosure complies with BS EN 61439 including (I'm guessing here, not having the millionaire income necessary for easy access to such standards) the ability to not arc on 6 kV spikes, on the enclosure's ability to contain hot gases from CB's breaking of short circuits, that devices adjacent to each other at max load don't overheat, etc.

wallywombat: 
 

There's no prohibition on mixing parts from different manufacturers in a single bit of switchgear. However, you then take on the responsibility of ensuring that the whole enclosure complies with BS EN 61439 including (I'm guessing here, not having the millionaire income necessary for easy access to such standards) the ability to not arc on 6 kV spikes, on the enclosure's ability to contain hot gases from CB's breaking of short circuits, that devices adjacent to each other at max load don't overheat, etc.

Mostly covered by the new metal biscuit tin enclosures with a hinged access cover.

Z.

Sparkingchip: 
 

Why would an electrician install a 10 mm twin and earth circuit protected by a B32 MCB for a 8.5 kW shower?

Twenty years ago I was taught to do circuit design one stage at a time completing the complete process. So I was taught that the very first thing to do is to assess the load and calculate the design current, then select an appropriate circuit protective device that matches or is greater than the load and go on from there completing all the stages of the design process in sequence.

When I did the 18th Edition course we did not do that, we did “bite sized“ chunks of the calculation, we were coached to pass the exam by just doing the bit of the calculation that was likely to come up as a question.

The teaching process is just simply inadequate, as we can see from this discussion people are coming up with circuit designs without following the full process and are failing to design a circuit for a water heater that complies with the Wiring Regulations or is really fit for purpose.

Does it matter  if electricians are just going to use an app on their phone to do the calculation? 

Yes it does,  because apps and software will only give the correct answers if you know what information to input, you still need an appreciation of the process. You  obviously would not get this design using an App or software as they would select a 40 amp device.

With the shower circuits you get to a point where you are just using a standard circuit and installing by rote, ensuring that you avoid running the cables in insulation, after you have done a few shower circuits you just keep repeating the design, unfortunately though if people have got the design wrong they may well keep repeating the mistake.

The train system needs a shake up starting at the top with City and Guilds and the other bodies that award qualifications to electricians working on down through the colleges and training centres to ensure that electricians are taught the complete requirements of the various aspects of the trade and not just bite sized chunks to get them through the exam.

Sparkingchip: 
 

Sparkingchip: 
 

Why would an electrician install a 10 mm twin and earth circuit protected by a B32 MCB for a 8.5 kW shower?

 

Twenty years ago I was taught to do circuit design one stage at a time completing the complete process. So I was taught that the very first thing to do is to assess the load and calculate the design current, then select an appropriate circuit protective device that matches or is greater than the load and go on from there completing all the stages of the design process in sequence.

When I did the 18th Edition course we did not do that, we did “bite sized“ chunks of the calculation, we were coached to pass the exam by just doing the bit of the calculation that was likely to come up as a question.

Just to point out that the objective of the 18th Ed course is NOT to teach you how to do the calculations in any way, shape or form. It's intended as a breeze through the 18th Ed, for those that should already be able to do the calculations in the way you were taught. Another way of looking at it, is the 18th Ed is a bit of Continued Professional Development - it's not, and can't be considered, a substitute for some other form of learning about electrical installations, such as NVQ3.

I think the problem comes when people acknowledge that a “qualified electrician” will have 238x+239[y] - that may be true, but there are pre-requisites for those qualifications that the centre should assess before candidates are accepted for those courses.

The teaching process is just simply inadequate, as we can see from this discussion people are coming up with circuit designs without following the full process and are failing to design a circuit for a water heater that complies with the Wiring Regulations or is really fit for purpose.

The way Centres deliver courses is up to the Centre. I'm sure others will pop in their opinions on this, but I guess as always it comes down to pay in the same way as teaching in school. If you really know your stuff, you can earn a lot more using that knowledge in another way.

Does it matter  if electricians are just going to use an app on their phone to do the calculation? 

Excellent point.

Yes it does,  because apps and software will only give the correct answers if you know what information to input, you still need an appreciation of the process. You  obviously would not get this design using an App or software as they would select a 40 amp device.

With the shower circuits you get to a point where you are just using a standard circuit and installing by rote, ensuring that you avoid running the cables in insulation, after you have done a few shower circuits you just keep repeating the design, unfortunately though if people have got the design wrong they may well keep repeating the mistake.

I think that's a fair statement - who checks

HOWEVER, as others have pointed out in this thread, the circuit design is correct for the mcb and breaker size - it could be considered that the load is too great for the circuit.

Yes, it fails at “step 1” BUT only if you consider the circuit needs to be protected against overload. If not, the Iz>In>Ib (Reg 433.1.1) does NOT apply.
Personally, I don't like omitting protection against overload for concentric mineral insulated heating elements, or open-frame filament heating elements (such as those used in fan heaters and some tumble dryers) because they can short part way up, so you can get a resistive earth fault path. However, on the basis of overload protection alone, it's a known maximum load, so from BS 7671 perspective, it's a common practice.

The only issue in this case, is that In exceeds Ib - this means there's no overload protection (may or may not be a problem as discussed above), but more importantly, the rating of the OCPD for continuous use has definitely been exceeded. To understand whether this is a problem for the particular installation you need to check manufacturer's information for the OCPD and the CU (sorry Dave Z) because the shower is not in continuous use. 
 

The train system needs a shake up starting at the top with City and Guilds and the other bodies that award qualifications to electricians working on down through the colleges and training centres to ensure that electricians are taught the complete requirements of the various aspects of the trade and not just bite sized chunks to get them through the exam.

 

Another discussion thread, I would say.

Andy, you have hit on the point I have been making here for a long time, and that is that all the qualifications are not fit for purpose. The 18th edition exam is not satisfactory as the competence standard for electricians, and the present 2391 is not adequate for inspectors.

You need to realise that the OP question about the CPD rating is not about design, it is about something that is existing. No one competent in design would choose a CPD rating that is less than the load. The discussion here is all about the effect of a reduced rating, which I think is very badly understood, if understood at all.

I'll ask the question, “were you ever taught about the characteristics of fuses or MCBs, outside of their nominal rating? Were you taught about the cable ratings, the thermal time constants involved, or the effects of currents above the ratings? Were you in other words taught why the tables in BS7671 have the numbers you read?" In fact, I am sure that you were not, because of the reaction of many electricians to ring circuits with 20A cable, diversity in real situations, or the OP.

The problem is that exams have to be set so that the target group achieves a reasonable pass rate. This in itself is ridiculous, but it is made worse because half the candidates on many courses do not work hard enough to achieve higher standards. Look at the discussion above, which is very good, and how many points were raised and how complex some of them are. I am sure that most people learned something and the others just looked away.

You are fairly convinced that there is something “wrong” with the situation under discussion, from your remarks. From a design perspective, there is, but the point is that it may well work for most people and is safe. I would give it a C3 under inspection, and it may well continue in use forever. It is likely that the shower has at some point been changed to a higher rating, and once this was working no further consideration was given. If one was doing something else then changing the breaker is reasonable, but it is not absolutely necessary.

There has been considerable pressure to suggest that this is “non-compliant” with BS7671, and one may refer to chapter 43 in the BBB, but strictly speaking, there is no overcurrent. The cable is perfectly safe, and the breaker obviously capable in order to work at all. In fact, this situation is often present in the distribution system, the street-level diversity is very high and for periods the current is often greater than the rating of both cables and fuses, and sometimes transformers. You may feel that this is also unsatisfactory, but it continues to provide electricity to us all. The surprise is perhaps that it can happen on the consumer side of the meter, and I admit that this is not common, because the BS7671 area is probably over-designed because the cost is quite low.

This would be a good example to discuss in a 2391 class. It is also time that a standard is set, because it is now too low. Do not hold your breath!

It’s a new circuit installed two weeks ago.

Twenty odd years ago when I signed up for evening classes the head of department said right from the start I would need to do the C&G2400 Design, Erection and Verification, which I did in the fourth year as we had a term off as a break.

The installation courses did not include circuit design, they were exactly what it said they were, installation courses including theory and associated science, so we drew diagras of magnetic fields around conductors and learnt Flemings Rules, but not how to design an electrical circuit.

Not many electricians were taught to design a circuit other than what they did on the Wiring Regulations course, because they never actually did a design course.

A shower circuit is basically a protective device, cable and a switch, so it a pretty poor show if qualified electricians cannot a circuit design calculation for it and determine what those three components need to be rated at, but if they cannot they simply need to use a standard circuit out of the IET Onsite Guide with the correct circuit protective devices.

davezawadi (David Stone): 
 

 but the point is that it may well work for most people and is safe. I would give it a C3 under inspection, and it may well continue in use forever. 

 

That all depends on how hot the mcb/RCBO gets. I don't have a crystal ball to determine how quickly that will degrade over time in this particular case, and maybe it will “last forever”, but experience in the field tells me that, on the other hand, it could last as little as 2 years depending on how heavily loaded the adjacent circuits are, and for what period, and how frequently the shower is used.