Shower circuit design.

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!

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!