I have an 8.5kW shower on a 32 Amp MCB

[danlightbulb]

As subject, but it doesnt trip.

8500/230 = 36.95 Amps

Measured my voltage and its 242V.

8500/242 = 35 Amps.

So why doesn't the MCB trip?


The reason I have looked is because I was thinking about fitting a 9.5kW shower in the existing ones place. Its fed by 6 mm2 T&E. Whats the maximum rating of 6mm2 when its run in open air (albeit behind panelling but its not clipped or in trunking)? Run is about 10-12m.

Thanks

 

[Softus]

So why doesn't the MCB trip?

Either you have two elements and haven't run it on both, or you haven't run it for long enough for the MCB to trip.

Whats the maximum rating of 6mm2 when its run in open air (albeit behind panelling but its not clipped or in trunking)?

In your installation the maximum possible for T&E might be 46.0A, but you need to clip it so that the surface to which it's clipped can conduct heat away.

 

[megawatt]

A 32A MCB could go for hours at 35A and still not trip Dan, these things are not an exact science

 

[ricicle]

You need to know the resistance of the element to determine its current draw at various voltages.You can do this by measurement, or by calculation.
You need to look at the rating information to see the power consumption at rated voltage.In this case say 8500W@240V.

Resistance=V²/P = 6.77ohms 240/6.77=35.7A 230/6.77=33.97A

 

[mikely]

As subject, but it doesnt trip.

8500/230 = 36.95 Amps

Measured my voltage and its 242V.

8500/242 = 35 Amps.

So why doesn't the MCB trip?

This graph http://www.tlc-direct.co.uk/Figures/3.18b.gif shows a typical characteristic operating curve of type B MCBs, which are most commonly used in domestic situations. Vertical axis is seconds, horizontal is amps. A 32A type B MCB is specified (by BS EN 60898) to carry 36A (1.13 x In) for 1 hour without tripping. A current of 5 times the rated current is required to trip a type B MCB in < 100 ms.

 

[bernardgreen]

8500/230 = 36.95 Amps

Measured my voltage and its 242V.

8500/242 = 35 Amps.

Resistance of element is therefore 6.22 ohms (230 / 36.95)

This remains constant.

current at 242 volts = 36.95 * (242 / 230 ) = 38.88

The wattage is 8500 at 230 volts, it is higher at 242.

 

[plugwash]

but either way it is not going to be high enough to take out a 32A breaker in the time of a normal shower.

 

[DESL]

The wattage is 8500 at 230 volts, it is higher at 242.

Depends on the rating of the shower. Newer showers are now rated at the nominal voltage of 230V

Anything over about 2 yrs old is rated at 240V so a 8.5kW at 240Volt will draw 35.4 A so at nominal voltage 230V it will draw 33.9 A and power will be 7.8kW.

 

[danlightbulb]

Thanks all, especially Mikely, that graph explains really well.

As a bit of a spin off then, the graph shows 10,000 seconds required to trip a 32A MCB at just over 40 Amps. 10,000 seconds = 166 minutes.

So if I was to have a 3 hour shower, and was pulling 40A, apart from getting a big electric bill, that MCB should trip. So how do they actually work then, is there a build up of charge, heat, capacitance?


Back to the problem, am I ok to switch out the 32A and replace with a 40A? Could I upgrade to a 9.5kW shower on 6mm2 cable and what breaker size would I fit for that?

Thanks

 

[Adam_151]

Sustained overloads are detected by the thermal element of the breaker, gradually heats up and causes a bi-metalic strip to bend which operates the trip mechanism.

At that sort of overload the time is large/infinate, as you get a larger current it'll trip faster, at 145% it must trip in the conventional time (an hour normally) (but most breakers will be faster than that), the lower bound is 115% overload, below which the breaker must not operate

If you think this is all a bit risky, then you'll be delighted to know that its merely matching what the cable can take, a breaker that tripped at 32.1A wouldn't be very convenient!

overloads of upto 145% result in faster ageing of the cable and this can be tolerated for short duration, more than this and the copper starts to melt through the PVC and the cable goes bang if its sustained for very long!

The old re-wireable fuses take 200% overload to disconnect in conventional time, this is where the 0.725 correction factor comes from, its half of 1.45

 

[Steve3948]

Thanks all, especially Mikely, that graph explains really well.

As a bit of a spin off then, the graph shows 10,000 seconds required to trip a 32A MCB at just over 40 Amps. 10,000 seconds = 166 minutes.

So if I was to have a 3 hour shower, and was pulling 40A, apart from getting a big electric bill, that MCB should trip. So how do they actually work then, is there a build up of charge, heat, capacitance?


Back to the problem, am I ok to switch out the 32A and replace with a 40A? Could I upgrade to a 9.5kW shower on 6mm2 cable and what breaker size would I fit for that?

Thanks

10-12M 6.0mm2 clipped direct on a 40A Mcb with 9.5kW Shower not a problem.