Prospective fault current

[uksel]

saying V=I.R is fine, for DC theory

it is 100% correct, but i assure this is the formula for DC. when considering AC it becomes complex when you have to consider much more, ie. impedance, frequency, reactance, inductance etc.

when calculating AC power/current you are looking to give a single state value (of voltage) which is an RMS value (Route Means Squared), because as you know AC voltage is not constant it goes on positive and negative cycles up between approx +/-330V. the formula i think also includes the frequency of the supply voltage (how often the sine wave crosses 0V in a full cycle) and the peak/lowest value of voltage on the sine wave, this is then calculated to give an RMS value. I'm babbling a bit now because i can't find the best way to explain what i'm saying

i'll get onto google and see if i cant get a clearer answer

 

[sivoodoo]

Please do Uksel, I am very interested in this and would like to see some equations

 

[uksel]

AC = Alternating Current,

the voltage is also alternating between approx +330V & -330V

in an AC system the voltage is never constant, which is why an RMS value is applied.

i would say it stands to reason that the maximum actual PSCC and PFC are higher than simply using ohms law to calculate from the value of 230, because you are using the RMS value, which is only about 0.7 times the actual highest value for the voltage

here's a little science lesson for you...

AC, DC and Electrical Signals

 

[DanBrown]

saying V=I.R is fine, for DC theory

it is 100% correct, but i assure this is the formula for DC. when considering AC it becomes complex when you have to consider much more, ie. impedance, frequency, reactance, inductance etc.

VIR is still the same as VIZ (z-impedance) really. Impedance is resistance that has taken into account all this frequency, reactance and inductance etc. Thats as i could see it anyway!
V = IxZ, I = V/Z, Z = V/I

 

[uksel]

In the instance of AC, you are taking the voltage as being a constant 230V, it is not, this is merely an RMS value, it is alternating between ~ +330V and -330V

the voltage is just as often 10V as it is 230V, or just as often -280V as it is 230V in the full sine wave cycle, and by doing AC calculations using DC theory is obviously not going to be entirely correct. it would be only correct at the exact instant where the voltage was on it's +/-230V part of it's cycle.

what my point was, in theory how can you take the PFC on an AC installation by calculation using DC theory. In reality the answer should be reached more like

Zs x 330 = Max PFC

if you dont mind me saying, Impedance has nothing to do with frequency, frequency is involved because it has an effect on the timescale of the sinusoidal waves cycle, which in turn (i think) will affect the RMS value. I also think that the reactance and inductance of the circuit could also play a part because it could cause a sort of dampening effect on the intensity of the wave, similar to rectifiying AC into DC and using capacitors to smooth the voltage, this dampening effect could perhaps also have an effect on the RMS value because of the timeframes and in turn would have an effect on the vectors when you use vector diagrams to work out the waveforms.

in honesty it's been years since college so i'm very hazy on the temrinologies and the workings of AC theory but the above is vaguely how i remember it

 

[EastAnglian]

Uksel - Just as the Nominal Voltage is oscillating then so is the Current - that's why its called "Alternating Current" as it flows in opposite directions each half cycle. It is therefore just as valid to describe the nominal voltage of the UK supply as 230V RMS as it is to refer to an alternating current of 100A RMS.

When using Ohms law to calculate an AC current flowing in a load as a result of an AC Voltage we replace "Resistance" by "Impedance". Impedance is a complex resistance which is related to how the load responds to an alternating current. The load's capacitance and inductance as well as resistance all contribute to its impedance. As capacitive and inductive reactance are related to the frequency of the alternating current/voltage then there can be a frequency dependence on the equation.

For PFC we assume that at 50Hz the impedance of the source (ie Ze) is a simple resistive value.

So Ipfc=Uo/Ze

 

[darran]

you need to disconnect the main incomming earth for a Ze test to eliminate the parallel earth paths, but the PFC (PEFC, PSCC) is carried out with the bonding in place.

 

[cornburn]

Uksel - Just as the Nominal Voltage is oscillating then so is the Current - that's why its called "Alternating Current" as it flows in opposite directions each half cycle. It is therefore just as valid to describe the nominal voltage of the UK supply as 230V RMS as it is to refer to an alternating current of 100A RMS.

When using Ohms law to calculate an AC current flowing in a load as a result of an AC Voltage we replace "Resistance" by "Impedance". Impedance is a complex resistance which is related to how the load responds to an alternating current. The load's capacitance and inductance as well as resistance all contribute to its impedance. As capacitive and inductive reactance are related to the frequency of the alternating current/voltage then there can be a frequency dependence on the equation.

For PFC we assume that at 50Hz the impedance of the source (ie Ze) is a simple resistive value.

So Ipfc=Uo/Ze

Thread is 18 months old, 1-0 to me

 

[adasadas]

Fellas. I thought a PFC was measured with the main earth still connected to the installation, line to earth, and a PSC line to neutral; only with a Ze do you disconnect the earth..

Fellas. I thought a PFC was measured with the main earth still connected to the installation, line to earth, and a PSC line to neutral; only with a Ze do you disconnect the earth..

 

[jamie-spark]

Disconect Main Earth. Put meter across Earth incoming (met) Connect other leads to Phase conductors do test. Then connect Earth and Neutral test leads together and connect to neutral phase conductor, connect the other lead to Line Phase conductor.
Test again. Wich ever is the highest reading in Ka record that on the test results.
Don't forget to re connect the main Met.
All these tests done with power on cons unit OFF

This is a Ze you are doing a pfc Should be done with all earths and bonds connected as more paths to earth may mean a greater fault current as there is less resistance. Then test between line and N and take the highest amount. if you dont have that function do a loop test take the reading and divide 230 by the result. ie 230/ 0.26 = ka =0.884 ( 884 amps)

 

[Fred West]

Well I was going to offer him this

[ame=http://www.youtube.com/watch?v=54C_FLSZaCY&feature=relate]YouTube - Prospective short circuit current test[/ame]

But then I noticed the date he asked,if he has not found out by now he has no hope

 

[trebor]

can somone tell me how to mesure pfc for a domestic property need this asap!!!!!
thanks in advance

Set meter on pfc

1 test between incoming line and neutral = PSCC

2 test between incoming line and the earth bar with all earths connected to include all parallel earth paths, not disconnected as suggested = PEFC

highest of 2 is recorded as pfc

can somone tell me how to mesure pfc for a domestic property need this asap!!!!!
thanks in advance

Set meter on pfc

1 test between incoming line and neutral = PSCC

2 test between incoming line and the earth bar with all earths connected to include all parallel earth paths, not disconnected as suggested = PEFC

highest of 2 is recorded as pfc


NB only Ze is measured with the main earth disconnected and installation isolated

 

[Guest123]

I'm hoping the OP has finished his test cert by now.......the thread is 18 months old.

 

[jamie-spark]

Aye better late than never

 

[tonynulty]

Thanks Laney06, never done it this way before, always used tester, how do you know this is correct, if it is this is easier and saves the cost of a loop impedance tester. tony