Disconnection time

[m4tty]

Hi

A 10.5kw shower is on a b32 cpd. The cpd trips on overload when the shower is used.

I'm trying to work out how long it will take to disconnect by using graphs in bgb but need some help. Don't want the answer but want to know how to find answer as I can't remember

Thanks

 

[Andy™]

you want fig 3.4 (249 BRB)

find 43A at the bottom (each block is 10A between 10 & 100). go up until you meet line for B32, then along to give seconds

and a load of 43A doesnt meet B32 ont he graph, so it should not trip...

 

[m4tty]

Think I've remembered. I'm on page 301 BGB.

A 32a cpd will carry 50a for 1000 seconds. Is that right?

Cheers

 

[Sidewinder]

m4tty,

Use the bullrush charts in the back of the book.

Find the one for a b32.

What current is the shower drawing?

Come across the A axis until you intersect the B32 line then drop down to find the disconnection time.

Fresh in my mind funnily enough!

Will be for another 2 days too, then on to something else.

 

[m4tty]

So 10500/240 = 43.75 so it shouldn't trip according to the chart.

Thanks Andy much appreciated

 

[Sidewinder]

M4tty,

it's 230V, not 240 as this is now the design voltage.

But don't tell the DNO's!

You are borderline with trip in 10,000 or no trip.

Maybe another fault?

 

[Andy™]

M4tty,it's 230V, not 240 as this is now the design voltage.

But don't tell the DNO's!

You are borderline with trip in 10,000 or no trip.

Maybe another fault?

and the 10.5 will most likely be at 240. 230 will be something like 10KW

shower has a fixed resistance, lower voltage, lower current flowing

 

[Sidewinder]

Andy, Canoe,

Quite, but, m4tty did not state really if it was 10.5@240 or 10.5@230...

 

[Andy™]

Andy, Canoe,Quite, but, m4tty did not state really if it was 10.5@240 or 10.5@230...

most(if not all) showers are 240v at stated kw, with a lower kw for 230v

 

[Sidewinder]

Andy,

Quite, but, they should no longer be designed for 240V for use in the UK should they now, as the nominal voltage in the UK is no longer 240V is it.

The manufacturers should now be designing their kW ratings for the nominal operating voltage, thus 10.5kW @ 230V.

M4tty does not say if it is a new shower or an old shower.

 

[m4tty]

I didn't fit the shower but it's got 10.5 on cover and the measured voltage was 240v so that's why I calculated at 240v. I know circuit design is at 230v but wanted actual current. Is that not what you would have done. thanks for all replies been ill so head not thinking straight. Cheers

 

[steptoe]

what size cable?

is it 6mm and thats why shower is on a 32?

could a large VD come into play here?

just something else for the mix.

 

[Ash]

I've still yet to come across a home with 230V nominal.

Most of the time my meter reads 245V or around that

 

[Sidewinder]

Ash,

Every home in the UK now has 230V nominal regardless of what your meter tells you.

 

[Evans Electric]

Ashers, you must pretend that the voltage is 230 , stop looking at your voltmeter ,no good will come of it, its the Devil's work .

I plugged my Loop imp. tester in last week and it said 251V but I think I got away with it , no one was looking .

 

[Andy™]

I've still yet to come across a home with 230V nominal.Most of the time my meter reads 245V or around that

nominal voltage is always 230v

actual voltage is rarely 230v

 

[Berlioz]

has smbd the link to BRB? I would like to have a look at it. Regards. Berlioz

 

[apprentice87]

Hi all,

ha ha, Have to laugh at the thing about the voltages!!

The nominal voltage is now 230V no matter what. They are permitted a tolerance though of plus 10% and minus 6% that that MUST provide. This is a legal requirement.

Makes you laugh all this stuff about energy saving light bulbs and all, why do they not just turn the voltage down to 230v like it is meant to be, instead of leaving it at 240v??

...Because they would lose money then.....Oh, and also they might have to reinforce their network as they have a job supplying the correct voltage at the best of times as their network is well overstretched anyway. Having to "turn it down" to 230 would not be helpful to them. Still, all CE marked stuff will now be designed for 230, so at 240 will not the service life be shortened??? It certainly would in the case of incandescent bulbs.....

john...

 

[Nicky Tesla]

if it is 10.5kw at 230 volts then it will be 240/230 x 10.5 = 10.95kw at 240v and 251/230 x 10.5 = 11.45kw at 251v

 

[andy_spark]

You need to find the resisitance of the shower element. Assume 230v & 10.5kW.

Resistance = (VoltagexVoltage)/Power

R = (230x230)/10500 = 5.04 ohms

Use ohms Law I=V/R

@ 250V I = 250/5.04 = 49.6 A

Acoording to the

@ 230V I = 230/5.04 = 46 A

There is a difference of about 4A depending on voltage, although we must assume 230v, in practice it isn't and the MCB trips in practice not in theory.

According to FIG 3A3(B) 50amps will be carried for up to approx 800-1000 seconds, therefore it may not last 1000 seconds. 800 seconds is 13 mins which could easily be the time of a shower if you are mrs andy_spark. If the mcb is sensitive or the voltage slightly higher this could be an even lower time.