Shower circuit design.

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.

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.