What happened?
Supplies to outbuildings
- Thread starter RF Lighting
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It was causing the OCPD to operate after a few minutes. The L>N part of the element was still intact and consuming 3kW as normal, but there is also the additional load of the water conducting L>E.
If you are not familiar with electrode boilers, have a look at big Clive’s video on the subject. It’s very interesting.
If you are not familiar with electrode boilers, have a look at big Clive’s video on the subject. It’s very interesting.
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Installed a couple in a hospital years ago. Odd things!
Electrode boilers are used in industrial applications for raising steam - usually big items. Very efficient at a tad under 100%. There was a move to have one for domestic applications. Safety is the problem.
Heat pumps make better economic sense for domestic applications for heating.
Heat pumps make better economic sense for domestic applications for heating.
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So regs say an immersion cable need not be protected by fuse or mcb? Foolish not to. It is foolish not to protect the immersion and cable.
BS7671 does not require an immersion heater (or any other load) to be protected by an over-current device, since that is beyond its scope. as often discussed here, if any load (immersion or otherwise) requires over-current protection, that should be provided within the item concerned.
BS7671 is concerned only with over-current protection of cables of the fixed wiring, and the only scope for discussion relates to that. It does include (explicitly) a list of situations in which over-current protection may be omitted, and one of those situations is when a cable "...because of the characteristics of the load or the supply, is not likely to carry overload current" [ 333.3.1(ii) ].
Although, as RF has illustrated, if one scrapes the barrel deeply enough one can come up with examples of situation in which a resistive load, such as an immersion, could, (very very rarely) result (or, as with RF, which have resulted) in an overload current flowing. However, given that "very very rarely", I don't think that one can say the an overload current "is likely to happen" - so that I think EFLI is right in saying that to omit ('adequate for the cable') overload protection for, say, an immersion is, strictly speaking, compliant with the regs.
Indeed, as EFLI might well argue, if an immersion is not an acceptable example of a load which is "not likely to" result in an overload current, then what would be - i.e. what would be the point in having 433.3.1(ii) at all?
Having said all that, in practice I am closer to you - and I don't think I ever have omitted, or ever would omit, adequate overload protection for a cable because I did not believe that the load was likely to result in an overload current - primarily to avoid the need for arguments with any subsequent 'inspectors'. However, as above, if I did believe that and therefore omitted (adequate) cable protection, then it would seem that would be BS7671-compliant [if there is any point in 433.3.1(ii) at all ].
Kind Regards, John
Can we explore this? Genuinely interested.
Assuming it was protected by a 16A MCB (much higher for 15A BS3036) then to trip in a few minutes, say ten?, it would require around 25A (less than ten minutes; obviously much more).
So, that is 12.5A for the normal consumption of the element and 12.5A (also 3kW) through the water which would mean that part of the element was also carrying the 25A.
Is this possible for an element which is designed to get hot (70° or more) in normal operation at 12.5A?
Why don't all immersion heater do that when the the casing splits?
Are you sure the element was not just bending and causing an earth fault after a few minutes.
Ok.
Assuming it was protected by a 16A MCB (much higher for 15A BS3036) then to trip in a few minutes, say ten?, it would require around 25A (less than ten minutes; obviously much more).
Ok.
So, that is 12.5A for the normal consumption of the element and 12.5A (also 3kW) through the water which would mean that part of the element was also carrying the 25A.
Is this possible for an element which is designed to get hot (70° or more) in normal operation at 12.5A?
I suppose a flimsy element wire would do similar to a very small extent but even that (well made?) electrode only got to 1kW in, for it, an ideal situation.
Why don't all immersion heater do that when the the casing splits?
Are you sure the element was not just bending and causing an earth fault after a few minutes.
This happens:
The story behind it:
My tenant moved his girlfriend in to the flat without doing any paperwork, she became pregnant and he left. The rent stopped and the girlfriend complained there was no hot water again, to date boyfriend has sorted it each time it failed.
E7 system in a single CU, immersion on 15A MCB on 24Hr side.
I found tenant had:
Piggybacked immersion circuit [1.5mm²] onto shower 32A MCB.
Bypassed failed timeswitch.
Bypassed burnt out 20A DP switch.
Effectively the faulty heater was connected directly on a 32A MCB with no switchgear.
1.5mm² had overheated and burnt out like a fuse at 20A DP switch.
Immersion heater showed ~18Ω but 4.5Ω & 10Ω to earth.
Several inches of the element had peeled open like a pea pod.
Presumably like you, my guess would have been that it would not have been possible for more than a (once only) very brief period of time - for part of the element it would be equivalent to supplying a "230V" element with ~460V, in which situation I would have expected it to have 'burnt out' pretty quickly.
There's another aspect of this which doesn't seem to add up in my mind ... if as much current is going through the water as through the part of the element downstream of the point of 'water contact', that would imply that the resistance/impedance of the water must be the same as that of the element downstream of that 'point of water contact' - which seems extremely hard to believe (particularly with relatively 'clean' water).
A fairly high proportion are, these days, presumably RCD-protected, so we wouldn't get the chance to see if what RF described might happen if the fault were not cleared in these cases.
That does sound like a credible alternative explanation. As above, I would not have expected the initial part of the element to repeatedly survive double it's normal operating current for appreciable periods of time.
Kind Regards, John
... to which I possibly should have added a reminder that "double it's normal operating current" equates to "four times the normal power dissipation in it".
Kind Regards, John
Only if that double current is flowing through the whole element which would require a 480v source. If it is through part of it that part will dissipate 4 times the power.
Double current through the 'first segment' of the element will result in four times the power dissipation in that segment of the element as would be the case when 'normal' current was flowing through it.
Isn't that what I effectively said: "If it is through part of it that part will dissipate 4 times the power."
You did, and that is clearly the situation we (including myself) are talking about - so why your preceding sentence which refers to a situation which we are not talking about?
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