ohms law, little confusion, where am i going wrong?

[chopper]

when testing a lighting circuit say we get a reading (r1,r2) of 1.10 the by using ohms law we work out the ampage, we
get 230 divide by 1.10 when gives 209.09 amps. but it is protecte by a 6 amp mcb, where am i going wrong

 

[telectrix]

becauser your R1+R2 is the measurement of the cable resistances. the current drawn in used is dependent on the load resistance, i.e. the lamp. you would get the 209 A as a fault current if you had a dead short L to E.

 

[ipf]

Think about it. What's the earth for... and when does it come into play?

 

[chopper]

so we would get 209a L to E dead short, which leads me to my next little question which also sounds dat but how much current does a 6a mcb allow through it, remember them telling us at college that it wasnt just the which seems obvious 6a

 

[telectrix]

a MCB will generally stand 50% above it's stated rating for about 1 hour before tripping. remember, the higher the overload current, the faster the trip. look at the time/current graphs in app.3 BRB.

 

[chopper]

cheers for clearing that up, something was thinking about earlier as they didnt teach at college or they brushed and i forgot,
so the current of a lighting circuit with 10 x 100w lamps = 10 x 100 = 1000, divide by 230 =4.35a.
guess this is more in design ?,

 

[GLENNSPARK]

well it works chopper unles of course you are dealing with discharge lamps then you X the wattage by 1.8......

 

[chopper]

yeah , would like to get more into the design side rarther then just house bash

 

[ackbarthestar]

a MCB will generally stand 50% above it's stated rating for about 1 hour before tripping. remember, the higher the overload current, the faster the trip. look at the time/current graphs in app.3 BRB.

Interesting. I remember doing some additions on a commercial property where we had installed 24 58w fluorescent fittings and then calculated what would happen when left on the original B6A breaker.
I got a theoretical value of around 10.3A and a clamped value of 9.58A then checked this against the time/current graph. It tripped out at just under 10minutes !

 

[telectrix]

looking at the time/current graphs, that's spot on. at 10A it should trip in 500secs, or 8.33 minutes.

 

[Rockingit]

Interesting. I remember doing some additions on a commercial property where we had installed 24 58w fluorescent fittings and then calculated what would happen when left on the original B6A breaker.
I got a theoretical value of around 10.3A and a clamped value of 9.58A then checked this against the time/current graph. It tripped out at just under 10minutes !

You clearly have too much time.......

 

[ackbarthestar]

You clearly have too much time.......

If only..... Not a difficult problem since the lights were up and while doing a MCB shuffle around I thought it would be interesting to see what would happen if we left the original breaker in place to see how quickly it would disconnect on overload.

Now I'm on my hols I'm able to reflect a little and move into the festive spirit