I've been lurking on this site for some years (thanks to all for the info!) without being brave enough to post - ...
Welcome (as a contributor!) to the forum. One doesn't need to be brave to post - if you're worried about "making a fool of yourself", most of us here are capable of doing that from time to time, so you'd be in good company!
...but this thread is fascinating in terms of the way that earthing is thought of with its primary purpose as operating a safety device (RCDs, fuses, MCBs ELCBs etc). Maybe its through having a different background but could an earth not be considered as a safety feature in it's own right; especially as earthing was introduced before more sophisticated devices were widely available?
That's the very reason why I started this thread because, unless I'm missing something, the short/simple answer would seem to be that (provided only that any required bonding is in place), in the absence of some sort of protective device which could operate as as result of current flowing to earth, I can't see that having a TT earth does represent a significant "safety feature in its own right" - which is why I was a bit confused because I thought (like you) that TT earths pre-dated the appearance of VOELCBs (which, themselves, obviously pre-dated RCDs/RCBOs).
.... I'm using my new immersion heater; there is no company-supplied earth, but I have connected the heater's case to an earth rod for safety (the hemp-and-red-lead plumbing joints to the hot water system are high resistance) Early manufacturing techniques are not perfected and a fault has developed putting the neutral end of the element in contact with the case. (Fault marked F on diagram).
I don't know what diagram you are referring to (did you think you had posted one?), but nor do I understand why you are talking about a faults between neutral of the element and it's case. If the neutral end of the element is still connected to the supply neutral, then it (hence the case, given the fault) will be at close to earth potential, as will everything else connected to the installation's earthing system, with or without a connected earth rod. The worrying situation to consider is surely that of the live/line end of the element developing a fault to the case?
Assuming a (dry) person has a resistance of 50 000 ohms and my TT earth is 50 ohms, even if a second fault occurs and the neutral is lost, of the 8A drawn by the heater, only 8mA would pass through the unfortunate person encountering faulty new technology.
For a start, 50,000Ω is a very high figure for body resistance to be working with. It varies widely, dependent on many factors, but even when the areas of contact are dry skin, it can be as low as 1,000Ω - and that is a (not 'worst case', but 'fairly bad case') figure commonly used for such calculations. The calculation you have done is flawed, anyway (se below) but, even if it wasn't, using a more realistic 'bad case' estimate of body resistance would turn your 8 mA into 400 mA - which (depending on the current path through the body) is very likely to be fatal.
You've confused things a bit by introducing the second fault of a 'lost neutral'. As above, if the neutral is not lost (as, obviously, it usually won't be), your neutral-case fault would not represent a significant hazard, since the neutral, hence case, would be at close to earth potential.
If (playing along with your scenario) there were a 'lost neutral' (as well as the other fault), then there would not be 8A flowing through your heater (and the earth rod), anyway, since, even if the heater element had zero resistance, the most that could flow through a 50Ω earth rod would be 4.6A with a 230v supply, or 5A with a 250V supply.
I'm guessing that your "8A" probably relates to a 2kW element running off a 250V supply. If so, that represents a resistance/impedance of 31.25Ω. The total resistance of element+rod would therefore be 81.25Ω (50+31.25), so that, with a 250V supply, about 3A would flow through both of them. That means that the 'earth rod' (and element casing), would be at about 150V above earth potential, and a person with a 1,000Ω body resistance touching that and some 'true earth' would end up with about 150 mA flowing through them - again, potentially plenty to kill.
If one considers the more worrying live/line-case fault (and no lost neutral
), that 150V would rise to nearly 250V, and the voltage through the ("1,000Ω") person would rise to nearly 250 mA.
Given the simplicity of an earth rod, would it not have been a worthwhile and inexpensive safety measure in its own right at a time when electricity was new?
It would, if it would have achieved anything significant - but, as above, my belief is that it would not have done. Others may disagree, but no-one has voiced any disagreement yet!
Of course, this is an academic discussion because, now, one would (should!) not have a TT installation without RCD/RCBO protection.
Kind Regards, John
