Electric Shower tripping MCB

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wrote:

Indeed. It goes up a bit. Which reduces the current slightly. But it's not linear, and not enough to give the change you propose.
I despair. Somebody else, please?
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Bob Eager
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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 ~)
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You are exactly right Chris.
"Chris Oates" <none> wrote in message

temperature.
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I bet that makes you feel better
mike r
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On 17/01/2004 Chris Oates a wrote :

No, please don't, in fact go get your coat :-)
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On Sat, 17 Jan 2004 20:10:06 UTC, "Chris Oates" <none> wrote:

I was deliberately keeping well clear of that one!
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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.

[1]
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On Sat, 17 Jan 2004, ripper wrote:

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!
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On 17/01/2004 ripper a wrote :

Yes it does, but the amount by which it varies is tiny by comparison to the other values which vary.
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On 17/01/2004 ripper a wrote :

Yes, but you do need to know how and when to apply it.
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wrote:

No, it doesn't.

LOL!
So if I apply a 1.5 volts from a Duracell AA cell, I can get about 6300 amps? Lovely!
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Better yet, consider the situation if you were to ever cut the supply! to maintain the somewhat dotty theory that the current will vary to keep power constant as the voltage changes (I = P/V where P is constant) the current I must become infinite!!!
Conclusions, either: a) When you turn off a shower (or a light bulb) the power dissipated remains constant and the current is infinite. OR b) The constant power idea is tosh and it is the resistance that stays constant.
Please avert your eyes if you are offended by someone shamelessly waving their qualifications about - I don't like to do it but it seems reasonable at this point... I have a masters degree in electrical engineering, came top of my class, was awarded a prize by the Institution of Electrical Engineering (the IEE) and my grasp of electrical theory leads me to opt for answer "b". Anyone think they have any better ideas?
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On Fri, 23 Jan 2004 10:27:25 +0000, Matt Beard wrote:

Yes. The resistance remains very nearly constant.
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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)
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shower:
saw that - the accompanying .pdf is slightly different it says 40/45A
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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.
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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
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ripper wrote:

[SNIP a load of tosh]
Wrong the power will go up with the suare of the voltage. e.g. voltage goes up 10% poer taken will go approx 20%. Yes, the heater resistance will change with temperature but not to a great extent.
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On 17/01/2004 ripper a wrote :

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
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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

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