17th edition Question ?????

[sparkydan86]

Hi guys,

Been doing revision for my 17th edition amendment 1, come across this Q, but dont how to work it out....

Q. A fault current due to an earth fault of negligible impedance in a 400v, three phase four wire circuit having an earth loop impedance of 0.3 ohms is?

a. 1383 A
b. 767 A
c. 124.5 A
d. 72 A

Thanks

 

[roukel01]

answer (b) I think. An earth fault to a phase would have a voltage of 230V not 400V, hence 230/0.3 = 767A

 

[telectrix]

agree. just the sort of Q. that tempts you into using 400V. then wondering why 1333 is not in the answers

 

[plugsandsparks]

Agreed 767 but has anyone actually found anywhere with 230v, i normally find 238-251v it annoys me when the regs keep saying nominal is 230 when it quite plainly is not....... - red wine time...

 

[trev]

Key word is nominal mate, it's just a baseline for the theory. We all know it's like Santa and the Loch Ness Monster.

 

[telectrix]

just another case of the meddling, unelected bureau rats in brussels interfering in UK affairs. we beat napoleon and hitler. why do we bow down to these johnny foreigners now?

 

[RogertheBodger]

They supply the Red wine Tel. Sacrifices have to be made - nominally.

 

[plugsandsparks]

I understand that we sort of went from 240 to 230 with some magic dust but it makes a laugh of doing volt drop calcs and upsizing the cable when what you really need is a decent resistor to get the volts down. They are even selling products like "V-phase" i think that drop you down to 220 if you want it so your light bulbs last more than a week.....
I now admit to installing an undersized SWA cable to a summerhouse, well it did not meet the volt drop calcs but guess what i loaded the cable up with a couple of heaters and yes it still would not go below 230v and i was drawing 20amps on a 2.5sqmm cable, its all a load of **olocks - BTW, just had a quick look and the wine in front of me was made in Australia .. HA

 

[needasparks]

Be quite something if you could get 400v go to earth

 

[trev]

Be quite something if you could get 400v go to earth

I'd be willing to bet Tony could do it. That and more

 

[needasparks]

I'd be willing to bet Tony could do it. That and more

You just know he's gonna explain that now to prove me wrong and confuse the poor bloke even more........and me

 

[Deleted member 26818]

hi guys,

been doing revision for my 17th edition amendment 1, come across this q, but dont how to work it out....

Q. A fault current due to an earth fault of negligible impedance in a 400v, three phase four wire circuit having an earth loop impedance of 0.3 ohms is?

A. 1383 a
b. 767 a
c. 124.5 a
d. 72 a

thanks

230v /0.3 = 766.6666666666667