PFC queery

[LVSparks]

Hey, why does gn3 say the pfc of a 3 phase sytem will always be higher on the pscc test than on the pefc?
Also when you have the pfc measurement how do you use it to select your protective conductor size?

 

[telectrix]

because on 3 phase the pscc is measured between phases ( or L/N ) and pefc is to earth.

you select your protective conductor by means of the adiabatic equation ( 543.1.3 )

 

[LVSparks]

because on 3 phase the pscc is measured between phases ( or L/N ) and pefc is to earth.

hey thanks for the reply i understand that the pscc is between L-N or L-L, but why is this reading higher than that for L-E, i thought that L-E would have a better return path with a lower resistance so consequently would have a higher reading of KA to disconnect the protective device. GN3 claims pscc would be higher but doesnt explain why?

 

[telectrix]

lower resistance of line conductors compared to protective conductor. L-L is < L-E.

 

[Deleted member 26818]

With 3phase, you double the PSCC measured between L&E.

 

[LVSparks]

With 3phase, you double the PSCC measured between L&E.

PSCC is not measured between L-E m8 thats PEFC. However why is the figure doubled?

 

[SPARTYKUS]

Rule of thumb. Hard (or impossible ) to derive real figure

 

[Guest123]

If your instrument is unable to measure the PSCC between phases on a 3P system, as per single phase you measure between L&N.

The PSCC will always be higher between the phases of a 3P system than between L&N because the voltage between them is higher 400 v 230.

The actual sum is to multiply a L-N reading by 1.732 to obtain the theoretical maximum PFC between phases, however we are told to multiply by 2 to err on the side of safety.

 

[telectrix]

nicely clarified there, lenny.

 

[LVSparks]

If your instrument is unable to measure the PSCC between phases on a 3P system, as per single phase you measure between L&N.

The PSCC will always be higher between the phases of a 3P system than between L&N because the voltage between them is higher 400 v 230.

The actual sum is to multiply a L-N reading by 1.732 to obtain the theoretical maximum PFC between phases, however we are told to multiply by 2 to err on the side of safety.

that is spot on, thanks for the answer, makes sense now double the voltage the rest of the figures will be larger as per ohms law. sometimes its obvious when you know the answer.

 

[Deleted member 26818]

Always assuming that there's a neutral to measure to of course.

 

[i=p/u]

nothing passes you spinlondon... you should be cop aka polis aka pealer, you miss nothing even better you should be a snifferdog

tell me this some motors dont require a neutral is that because they just want the phases for their flux

 

[telectrix]

all motors have a neutral. its the sloppy bit on the floor shift, or between P and D on an auto box.