Main Fuse - why?

orrla

Hi, Thank you all for your replies!
Yes, fridge and freezer but nothing else was on last night, and just two heaters with one bar each.
First kettle was cheap thing - and coincidence, I think too.
Point is I must write urgent papers, for my exams, and since organising new boiler - because an old was declared unsafe and shut off by plumber who claimed there are no spare parts for it - was taking too much of my time and attention I decided I'll go on on heaters until I'm done. Now this. And snow outside. What else??

Jaffa_cake

macman wrote: »

What 'bar' on an electric fire only draws 400W? Aren't they generally switchable in 1kW increments (1/2/3 kW's?)

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Although 1kW (1 thousand watts) per "bar" (heating element) may be traditional, you should remember that this is not always the rule.

We have 2 electric heaters, each has "2 bars" (heating elements) each bar is just 0.75kW (750 Watts)

So OP could very well have a heater with 2 or 3 x 400 watt "bars" (heating elements)
So her heater total loading could be 800 watts, or 1200 watts, depending on how many 400 watt "bars" (heating elements)it has.

JC

Jaffa_cake

Hominu wrote: »

Watts = Volts * Amps
240V * 30A = 7200 Watts
1 kW = 1000 Watts, so 7.2 kW

Typical kettle = 3 kW
7 * 400W = 2.8 kW
Total: 5.8 kW
Plus all your other equipment (fridge/freezer, computer, etc)

I'd be surprised if your heaters are only 400W though.

You can see why a kettle would throw the fuse if you are already near the maximum.

MCB could be a bit flakey at times near their maximum. Never run them at full load.

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Sorry to correct you,,,
But "1 kW = 1000 Watts, so 7.2 kW" is not correct.

1kW (one thousand watts) divided by the voltage which is 240 will equal 4,16 amps.
We should round it up to say 4,2 for efficiency.

Taking this a step further, if 1kW (one thousand watts) requires 4,2 amps at 240 volts, then 3kW (three thousand watts) will require three times 4,2 amps which will be around 12,6 amps.

So if our 3kW (three thousand watts) heater is plugged into our socket, and all heating elements, if they are selectable, are switched on, then we can assume this heater will use almost 13 amps.

This accounts for the brown 13 amp fuse, we find in the plugs of these type of heaters.
Lets take our 3kW heater, with 3 separate 1kW heating elements.
If we fitted for example a smaller capacity fuse in the plug, (same physical size) of say 10 amps, then we would only be able to use for example 2 out of the three possible heating elements, ie, 8, 4 amps, switching on the third heating element would now make the heater draw 12,6 amps, which would, after a very short time, "blow the fuse" in the plug.

A typical electric jug kettle is now only around 2kW (two thousand watts)
From above, we can see that 2000 watts will want around 8,4 amps.

Ring main circuit breakers are 32 amp rated.
Unless however the circuit in question is a radial circuit, then I expect it to be fitted with not a 32 amp circuit breaker, but a lower capacity 20 amp circuit breaker.

So it was asked earlier, "how many heaters can I have on" ?
Well, now you can work it out.

One 3kW heater on full power will want around 12,6 amps
One 2kW jug kettle will want around 8,4 amps
Your typical fridge is a peculiar machine.
I say this because most of the time it uses nothing, as it has pulled the contents inside down to the temperature you selected.
However when it restarts, the compressor motor asks for around 6 amps, for about 2 seconds, while it starts up, then when the compressor motor is running, it settles down, and uses about half an amp (120 watts) but probably less than 1 amp, (240 watts).

You may find that your amps used, were not the real reason for the trip switch "tripping off"
It may have not tripped off due to overloading amps, but to excessive leakage.

Depending on your CU (consumer unit - [new type fuse box]) you may have an earth leakage device for each ring circuit, or a earth leakage device that covers many circuits.

When the total leakage approaches 30mA (thirty thousandths of an amp) it will switch off, and isolate that circuit, and every thing on that circuit, that is plugged in, will lose the power.

For example,
Your 3kW electric fire may be leaking 10mA, which by itself, that's ok
Your jug kettle may be leaking 10mA, which by itself, that's ok
Your fridge may be leaking 10mA, which by itself, that's ok

But if they all lose that much leakage on the same circuit, then this adds up to 30mA and then the safety device trips off the power.

As you can see, you were no where near the maximum amps for that circuit, but it was the accumulation of the leakages that switched off that circuit.

Any thing else, please ask......

JC

orrla

It makes sense - untill the kettle business I was quite generously using all heaters ... Now, I am confused a to how many I can turn on - these heaters are effective in dealing with temperature but the whole thing makes me nervous...
Thank you all!

macman

How do you know that each bar is 400W? What is the rating of each appliance?

Gromitt

Jaffa_cake wrote: »

.
.
Sorry to correct you,,,
But "1 kW = 1000 Watts, so 7.2 kW" is not correct.

So your saying 7200 watts is not 7.2kW ? or 30A @ 240V is not 7.2kW ?

Then you say 1000 / 240 = 4.16, which is correct, but then 30 * 240 = 7200 is not correct?

Care to explain, your post is very confusing.

We are talking about the ring main here, not a particular 13A socket.

Jaffa_cake

Gromitt wrote: »

So your saying 7200 watts is not 7.2kW ? or 30A @ 240V is not 7.2kW ?

Then you say 1000 / 240 = 4.16, which is correct, but then 30 * 240 = 7200 is not correct?

Care to explain, your post is very confusing.

We are talking about the ring main here, not a particular 13A socket.

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Hello Gromitt,

I underlined your error.
Your explanation to the OP was confusing, as your reply read like
1 kW = 1000 Watts, so 7.2 kW"
To the OP, who is clearly not familiar with this, it would have been more clear to explain.
When did you last fit a 30 amp MCB ?
Do you know why they are 32 amp, and not 30 amp?, I am sure you do, and I am just teasing you, no need to answer.

I actually can't see where I mentioned that 30 * 240 = 7200 is not correct? , as I made no reference to 30 amp MCB.

If you can point out where in my post there is a mistake, I would be quite gratefull.

JC.

orrla

macman wrote: »

How do you know that each bar is 400W? What is the rating of each appliance?

This is what it says on a box and safety instruction - power=1200W, settings= 400W/800W/1200W.; fuse 13AMPS.

Kettle= 1850-2020W.

Cardew

Watts = Volts * Amps
240V * 30A = 7200 Watts
1 kW = 1000 Watts, so 7.2 kW

Jaffa_cake wrote: »

.
.
Sorry to correct you,,,
But "1 kW = 1000 Watts, so 7.2 kW" is not correct.

I think you have misread or misunderstood.

240v x 30 amps is 7,200watts

Now as 1kW is the same as 1,000watts, 7,200watts is indeed 7.2kW..

In winning the 5,000metres at the London Olympics, Mo Farah ran 5km - as 1km = 1,000metres;)

Gromitt

Jaffa_cake wrote: »

.
Hello Gromitt,

I underlined your error.
Your explanation to the OP was confusing, as your reply read like
1 kW = 1000 Watts, so 7.2 kW"
To the OP, who is clearly not familiar with this, it would have been more clear to explain.

I don't know how I could have made it more plain, like Cardew says, it seems quite simple. I explained how I got to 7200 Watts, and then explained that 1 kW = 1000W, so therefore 7200 Watts would be 7.2Kw... How much more simple could you get?

Yes, I didn't use 32 and instead used 30, to make the maths easier. It was better than getting out the calculator