What shower power?

[regsmyth]

OK thanks.

 

[JohnW2]

I wrongly thought when I mentioned the breaker size/type, you would have realised it was one of these:

Indeed, and it was because you wrongly thought that was the case that no-one raised these new issues until recently. Almost every domestic CU in the country must contain "B32 MCBs" and the majority of these CUs are decades younger in design than yours (and the MCBs certainly not retrofitted plug-one ones), so it didn't occur to anyone (although I suppose it should have) that we were talking about such an old CU.

As everyone is saying, all we can do is offer you facts and advice. The decisions as to what you do, having received that information and advice, are obviously down to you.

Kind Regards, John

 

[DetlefSchmitz]

John, 'non-ohmic' is an accepted description of something where the resistance is not constant. Typically something like, say, a diode.

Fair enough. However, despite what has recently been suggested, Ohm's Law always applies, even when resistance is not constant (i.e. when the relationship between voltage and current is not linear) - unless one goes to extremes and (as has been mentioned!) starts considering superconductors etc!

Actually, E=IR is not Ohm's law, it is an equation derived from it.

Ohm's law was that the current through a conductor was directly proportional to the potential difference applied. So it has constant resistance as its assumption. This is why something with a varying resistance is called non-Ohmic. This goes back a long way.

 

[JohnW2]

Actually, E=IR is not Ohm's law, it is an equation derived from it. Ohm's law was that the current through a conductor was directly proportional to the potential difference applied. So it has constant resistance as its assumption. This is why something with a varying resistance is called non-Ohmic. This goes back a long way.

Fair enough. However, I feel sure that we all know what each other mean, regardless of historical accuracy. In practice what matters is whether or not the current through a conductor has a linear relationship to the applied voltage. You are saying that, if it doesn't, then that conductor is not obeying Ohm's Law, which may be technically true. However, for any particular applied voltage (hence the prevailing resistance), the situation will still 'obey' the 'equation derived from Ohm's Law'.

Kind Regards, John

 

[PrenticeBoyofDerry]

A sample of what you would generally find in the MI of an electrical shower

In accordance with BS7671 a 30mA Residual Current Device (RCD) must be
included in the electrical circuit. This may be part of the consumer unit or a
separate unit.

The manufacture has a legal duty to advise you on the safest way that their products should be installed and used.
I was instructed to always follow the MI, I think it damn foolish to ignore the recommendation of RCD protecting electrical items such as electrical showers.

 

[Taylortwocities]

Wait for it.

Oh dear…..

 

[DetlefSchmitz]

Actually, E=IR is not Ohm's law, it is an equation derived from it. Ohm's law was that the current through a conductor was directly proportional to the potential difference applied. So it has constant resistance as its assumption. This is why something with a varying resistance is called non-Ohmic. This goes back a long way.

Fair enough. However, I feel sure that we all know what each other mean, regardless of historical accuracy. In practice what matters is whether or not the current through a conductor has a linear relationship to the applied voltage. You are saying that, if it doesn't, then that conductor is not obeying Ohm's Law, which may be technically true. However, for any particular applied voltage (hence the prevailing resistance), the situation will still 'obey' the 'equation derived from Ohm's Law'.

Kind Regards, John

Since this doesn't want to die...

John. The point is that you criticised a poster for not understanding Ohm's Law, when in fact he had understood Ohm's Law exactly correctly, and it was your misunderstanding which caused your post. I am your greatest supporter when you are correct, but once in a while it is good to admit to an error.

The word 'ohmic' has been used in scientific circles, for at least 50 years to represent a linear resistance. Before that too probably, but my memory doesn't extend that far.

 

[JohnW2]

John. The point is that you criticised a poster for not understanding Ohm's Law, when in fact he had understood Ohm's Law exactly correctly, and it was your misunderstanding which caused your post. I am your greatest supporter when you are correct, but once in a while it is good to admit to an error. ... The word 'ohmic' has been used in scientific circles, for at least 50 years to represent a linear resistance. Before that too probably, but my memory doesn't extend that far.

Indeed - but my mistake/error/whatever is in either having never having been aware of, or having forgotten, that meaning of the word "ohmic" - and I have no problem in admitting that. As for the 'actual' statement of Ohm's Law, I believe that the vast majority of ordinary people (those who are not physicists or engineers) are unaware of that, and I therefore generally assume that when they speak of Ohm's Law, they are virtually always mean "the equation(s) derived from Ohm's Law" - just stop (wo)men (or electricians) 'in the street' and ask then what Ohm's Law is - if you get an answer, it will nearly always be equation-related!

Of course, if one wanted to get really pedantic then, unless one can find a material with literally zero temperature coefficient of resistance, then Mr Ohm's Law, as originally stated, never applies precisely - at least, not beyond the initial infinitely short period of current flow (although the equations derived from his Law always do apply, provided one takes the changing resistance into account).

Kind Regards, John