Electric Shower tripping MCB

WRONG!

The current drawn will INCREASE in proportion as the voltage rises. The

9.5Kw will be its rating at either 230v or 240v depending upon the appliances age. As the voltage increases the current drawn will also increase, as will the Kw.

The current (I) and Kw drawn will only be dependent upon the resistance. The resistance (R) will stay approximately the same.

I= V/R

Reply to
Harry Bloomfield
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From a Triton specification sheet for what they advertise as a 9.5kW shower:

SPECIFICATIONS Nominal power rating at 240V

9.5kW ? (40A MCB rating)

Nominal power rating at 230V

8.7kW ? (40A MCB rating)
Reply to
Toby

Back to the subject, at lower voltages (less than 240V) the stated power rating of showers is reduced. For example a 9.8 kW shower @ 240 Volts is only rated at 9kW @ 230 Volts. A 9 kW @ 240V shower is 8.2 kW @ 230 Volts A 7.5 kW shower @ 240V is 6.85 kW @ 230 Volts My example of constant power was only to demonstrate using ohms law to calculate current and various voltages.

Reply to
ripper

ripper wrote on Saturday (17/01/2004) :

You are working on the false assumption that the 9.5Kw will remain constant irrespective of a change in voltage. It does not remain constant, the only constant is the resistance, but even this is not absolutely constant. As temperature rises, the resistance will also rise very slightly.

Reply to
Harry Bloomfield

I was going to join in and mention motors running at under their rated voltage drawing more current but didn't as you may never speak to me again ~)

Reply to
Chris Oates

You are exactly right Chris.

temperature.

Reply to
ripper

On 17/01/2004 ripper opined:-

No it does not!

For an unvaring value of resistance, half the voltage and the current and the wattage drawn would also be reduced by half.

If the current and the wattage stayed the same, then their would be no need to produce different appliances for different voltages and different markets would there? You could quite happily plug the bulbs from your car into the mains, after all a 50w bulb is a 50w bulb if the voltage rating doesn't matter :-)

Simple proof....

What happens to your lights during a 'brown out'?

During a 'brown out' your supply voltage will decrease, your lights draw less current and less than their rated wattage. You are trying to argue that the wattage would remain constant and therefore your lights would not dim.

Reply to
Harry Bloomfield

Maybe it was, but the thread was about the current drawn by an electric shower, which is definitely not a constant power device.

IMO Your posts will not have helped the OP reach an understanding of why his MCB may be tripping!

Reply to
Alistair Riddell

Reply to
Harry Bloomfield

Yes it does, but the amount by which it varies is tiny by comparison to the other values which vary.

Reply to
Harry Bloomfield

No, please don't, in fact go get your coat :-)

Reply to
Harry Bloomfield

I was deliberately keeping well clear of that one!

Reply to
Bob Eager

saw that - the accompanying .pdf is slightly different it says 40/45A

Reply to
Chris Oates

ripper wrote on Saturday (17/01/2004) :

Because as V increases, I decreases for an identical load.

But that has nothing at all to do with the present discussion.

Reply to
Harry Bloomfield

the thermal component of the MCB rather than the overcurrent (the bit that's like a fuse) - the 40A rating is really to low for

9.5kw

Reply to
Chris Oates

"ripper" wrote in news:7HgOb.25323$qx2.2799204 @stones.force9.net:

I bet that makes you feel better

mike r

Reply to
mike ring

Harry Bloomfield wrote in news: snipped-for-privacy@tiscali.co.uk:

No, that's not true.

As the filament cools it's resistance diminishes, allowing more current to pass, so increasing the power again.

Stands to reason

mike r

Reply to
mike ring

Piggin 'eck, this thread has generated - to use a painfully appropriate saying - lots of heat and no light: a ridiculous misapplication of a misnamed law, and crap advice. Sorry to be blunter than usual, but there it is.

To deal with the practical aspects first: there is something *wrong* with at least one of: the shower; the cable feeding it; the MCB; the incoming supply - and that's more or less in order of likelihood (with change in the supply being massively less likely than the other three). The original poster's brother could do a lot worse than spend 50 quid on getting a competent local electrician in: the mixture of lots of incoming amperage, wet skin, and well-earthed metalwork is *not* to be treated lightly. Yes, the RCD - *if* it's working - should protect your brother and any other members of the household from the worst consequences. How lucky are you feeling? And replacing the 40A MCB with a higher-rated one, even the slightly higher rating of 45A, could be

D A N G E R O U S

leading in a quite plausible case to a

F I R E.

Again I ask: how lucky are you feeling?

Now, a little more reasoning. To all intents and purposes, the supply voltage across the UK *is* 240V. The FAQ explains that for product design purposes, we've 'harmonised' across Europe on a wider range of voltages for which kit should behave, but the *actual* supply voltages in the different European countries is unchanged, for now and for a number of years to come. So let's stop dicking about with "well if the voltage was X, the current would be Y, but if it was Xprime, the current would be Yprime", 'cos in just about all reasonable foreseeable circumstances, the voltage will be within a gnat's whisker of 240V. (And another reason for dropping it is that one of the incessant responders got it all arse about face, as Bob tried to explain: more on this below).

So, as it's sensible to say "V = 240", what's I for a shower rated at

9.5kW? Well, P = IV, so I = P/V (/ meaning "divided by") = P/240 = 9500/240 = 39.6A near as damn it. Follow? So a "40A" MCB is *NOT*, *NOT*, N O T too low (or even 'to low' [sic]) for a 9.5kW shower. Thus we conclude that either the whole circuit - shower+supply cables - is pulling *more* than the 9.5kW it ought to, or the MCB has lost its marbles, and "become oversensitive". But "becoming oversensitive" is not a particularly well-known failure mode of MCBs. It could be that the 40A MCB is sandwiched between a couple of other MCBs also running close to, or a little over, their rated currents, so the mutual warming among these is making the 40A one trip: but that's not likely.

What else could be causing a fault? Well, maybe the heater element has somehow developed some internal shorts, so its resistance is now lower, making it pull more current. Possible - happens in electric blankets - but not likely in a leccy shower, where there's not much chance of the heating-element wire touching itself inappropriately. (Pauses for the giggles from the back of the class; looks severely over glasses; continues, nary a glimmer of a smile passing across his face). More likely is some other internal fault in the shower, allowing substantial extra current to pass from L to N, which after a while makes the MCB trip. (The marking of 40A on the MCB is a 'nominal' one; it'll pass 42A or 43A for hours and hours - it won't suddenly cut in at 40.001A! You can look up the detailed "overload versus time to break circuit" curves in any MCB supplier's technical data sheet.) Worse, the fault could be a Live to Earth fault, pulling 5A or more (that's about what you'd need for the MCB to trip not at once but after a little while). Surely the RCD would cut in if it was a L to E fault, wouldn't it? Why yes, of course it would... *if* it's still working. Is it?

What else? Well, maybe the cable was put in a while ago, and was sized for the kind of shower more common 10-15 years back - a 7.2kW model, say; with a 30A MCB. Then someone put in a new, more powerful shower - the current

9.6kW one; and read the instruction about "needs a 40A MCB", and put one in. After all, the cable looked pretty beefy, right? What would happen then? Contrary to Hollywood special-effects pictures, the somewhat overloaded cable will *not* instantly turn into a molten plasma, vapourising all within a 10-mile radius. No, it just runs warm; a bit too warm. Hot, in fact; hot enough - maybe somewhere out of sight, with a bit of thermal insulation - to make the PVC insulation start to soften. Somewhere where this is happening, the cable is bent; and the softening insulation allows the live and neutral conductors to come closer together. If they do that suddenly - bang, the trip will short. If they do it gradually, though, you could get just the behaviour you're seeing: L and N nearly touching when cold (shower off), bugger-all insulation between them now, just a small air gap. Then you turn the shower on; the conductors heat up gradually, expand, and now they touch. Off pops the MCB.

And *that* is one of the possible reasons for the MCB going off.

And *that* is the situation in which some posters are suggesting 'whack a (slightly) bigger MCB in'. *Not* all that sound a suggestion, that. The cabling fault - if such it be - need not relate to an undersized cable, as the detailed picture drawn above had it: similar effects can occur from connections which have worked loose, so they're now getting too hot, maybe sparking a bit, again softening/weakening/charring insulation so it doesn't do its job anymore. No, I don't *know* if a worsening cable fault is causing the MCB to blow more often than before; but it *could* be; and you'd be a fool to just cover up the symptom by whacking in a bigger MCB. If you don't have the knowledge to work out what's going on, for pity's sake pay for an hour's time of someone who does.

Oh, the "mains supply" possibility? Well, it's remotely possible that the incoming supply voltage has been increased recently: maybe someone further away from the substation has complained of too low a voltage, and the supply company's shifted up to the next tap on the transformer; or some local heavy user's no longer heavy-using (any cannabis-in-the-loft growers been busted round your way recently? or a little engineering company's gone bust and no longer fires up its heavyish loads at the times the shower's in use?) A higher incoming voltage will (as we work out in remedial-level detail below) cause increased current to flow. But any change in incoming voltage won't be dramatic - otherwise you'd be seeing lightbulbs glowing extra-bright and burning out noticeably more often; so as an explanation for the MCB behaviour it's the least plausible.

Finally, the electrickle theory bit. The two relevant equations are "V = IR" - that's the one we most usually call Ohm's Law - and "P = IV". Each one tells you how *three* quantites are interrelated, so to ask 'how does one quantity vary as the other one varies' is unanswerable, *unless* you can *justify* the assumption that the third one will stay *constant*. Geddit? The guy rabbiting on about the greater current being drawn as voltage fell in order (by some unexplained, and in this case totally non-present) to magically maintain a constant power was talking utter tosh. As Bob tried to explain, the most nearly constant quantity here - as a matter of physical truth, given the way a shower element is built - is the resistance of the element when it's up to its operating temperature. So, knowing the resistance is fixed, we can do some Really Complicated Algebra. Ready?

V = IR

We can actually stop the analysis right here. Given that R is constant, this says that as V goes up, so does I, 'linearly' - i.e. double V, and you double I; knock V down by 5% and I comes down by 5%. End of story.

If we want to go doing Deep Mathematics, we can go on to work out how the power developed across the fixed resistance will change as the voltage changes. Coo, that's going to be advanced mathematical reasoning, since we haven't been given a single formula which has both P and R in it. But we're going to *derive* it, i.e. work it out. You see, we know

P = IV

And we're going to monkey with V. Can we assume I will stay constant? Can we hell - we just reasoned a couple of paras ago that I was linearly dependent on V: that's what "V = IR" means, remember? Now, given that V = IR, we can write V/R = I (/ meaning Division), by dividing both sides of the Ohm's Law equation by R. (It's mathematically and physically valid to do so, since we know R isn't 0: no superconductors in sight ;-). So, armed with the manipulated flavour of Ohm's Law saying "I = V/R", we can replace the I in "P = IV" with "V/R", to get - wait for it -

P = (V/R)V

or as we more usually write it,

P = V*V*R or P = V-squared times R or P = V^2*R

Thus we see that, *across a constant resistance*, the power developed is proportional to the *square* of the voltage. So, if the voltage sags by 10%, so it's only 0.9 of the previous value, the power across constant-resistance elements will be 0.9*0.9 = 0.81 times its previous value, i.e. nearly 20% down. Fascinating? Maybe. Relevant? Not to the original poster's brother, really; to the unteachable who kept on hoping the constant-power fairy was in the room? yes, but will they ever realise its relevance?

OK? Are we settled on what to do? - For the original poster's brother: get someone genuinely competent in to find the fault. There is a fault. Find it before it has more serious consequences than it has till now. - For the wannabe electrickle theorists: - get your theory straight in your head; - align it with physical reality; - consider the consequence of shitty advice when it concerns heavy-duty volts, amps, and wet skin.

Stefek, shirtier than his normal self...

Reply to
stefek.zaba

Well reasoned * well said = well done

Reply to
Toby

thanks, Toby Another possibility is that there's more than the shower running off the single MCB! Of course, there *shouldn't* be; just like the immersion in this place *shouldn't* have been put onto the upstairs ring back at the CU when someone needed a couple of new ways in the only-one-spare-way-left CU. If there was another load sharing the shower feed - some recent hack-it-about alterations - then that too would account for the behaviour reported. And the MCB would be *right* to trip to protect the cable...

Stefek

Reply to
stefek.zaba

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