tripping value

[Farmelectrics]

got a keen apprentice who never stops asking questions which is good but not on a monday morning when you dont feel to clever anyway he was asking me how low would your ir reading have to be to trip breaker i explained that min value was 1M ohm but he wanted to now how low the reading would be to trip it help lol

 

[telectrix]

ohms law

 

[telectrix]

R = V/I

R= 230/0.03 ( for a 30mA RCD

R= 7666.66666666666666666666666 ohms

or if you got a real voltage of 240V. 240/0.03 = 8000 ohms.

 

[Darkwood]

ohms law use it to find current flow depends on mcb size and time curve too as to how long it takes to trip, and also the existing load on the cable needs accounting for-, 1M ohm is very high resistance still but is an indication that the insulation value has been compromised and needs investigating but its not low enough to be a direct issue if the value is held.

 

[johnboy6083]

well it would be the tripping curre, found onm the time/current graphs, or for instataneous values can be calculated. This would then be inserted into the formula R=V/I, so to trip a 10A C type in 0.1 to 5 secs, you need 100A. 230/100= 2.3 ohms maximum, or 0.0000023 Mohms.This is also your max Zs (not corrected for temp).

for 30mA RCD's it is 230/30x10-3 = 7.666ohms or 0.0077 Mohms.

For interest, this isnt the max Zs for a 30mA RCD, becuase you use the touch voltage threshold of 50V, which gives 1667ohms.

 

[Darkwood]

and if an rcd then ..... see post 3 but he bet me to both .... damn my fishfingers needing turning over :/

 

[telectrix]

misread itr was breaker, he said. so time/current graphs will tell.

 

[Farmelectrics]

cheers lads didnt bring thinking cap to work today will explain to the young man in morning

 

[ackbarthestar]

From App. 3 fig 3A4 you can guesstimate at 1.45 times the breaker rating. So a 10A MCB with a 15A load will take anywhere between 16 -30 mins. to disconnect (most likely shorter due to the accumulation of ambient temperature and the heating effect.)
Or you could say a load of 3.5kW (34 fluorescent fittings) which would provide a resistance of 15 Ohms which approximates to 15A.

I did an experiment with 24 fluorescent fittings connected to a B6A before upgrading the breaker to B10A and it disconnected in about 9 minutes

 

[sedgy34]

ohms law

whose he

 

[telectrix]

rumpole in disguise.