Phase-Neutral fault vs Phase-Phase fault!

rob w

Just wondering if it is possible that a two phase fault would produce a smaller fault current than a fault between phase-neutral?

I've been doing some calculations for a college project Im working on to work out PSCC at different points in an installation. Ive been getting results showing slightly lower current from a 2 phase fault compared to a phase-neutral.

For the phase-neutral fault;

I have calculated the total impedance to be the sum of one supply transformer winding, the impedance of a phase conductor and the impedance of a neutral conductor. I have then divided this into the phase voltage (230v)



For the 2 phase fault;

I have calculated the impedance to be the sum of two supply transformer windings and the impedance of two phase conductors. I have then divided this into the line voltage (400v)


Pretty sure thats the correct way to go about it, if not correct me, but Im getting a slightly higher current for the ph-n, though I thought it would be the opposite!!!

Thanks!

2011

rob w wrote: »

Just wondering if it is possible that a two phase fault would produce a smaller fault current than a fault between phase-neutral?

Possible, but not likely I would think.
EDIT: I just tried this with and got the same answer.

I've been doing some calculations for a college project Im working on to work out PSCC at different points in an installation. Ive been getting results showing slightly lower current from a 2 phase fault compared to a phase-neutral.

Basically the prospective short circuit current is calculated by using Ohm's Law.

Isc = V/Z

V for Ph to N = 230VAC
V for Ph to Ph = 400VAC

For the phase to phase fault although the inductive componet for the Z value will be double the phase to neutral fault.

rob w wrote: »

I have calculated the impedance to be the sum of two supply transformer windings and the impedance of two phase conductors. I have then divided this into the line voltage (400v).

I would expect the impedance of the two pase conductors to be the same as the impedance of the neutral and phase conductor.

As you stated the difference is 2 windings in the transformer rather than one.

If Z1 = the impedance for phase to neutral
and Z2 = the impedance for phase to phase

Then Z1 = R + Xl

and Z2 = R + 2(Xl)

The impedance will have 3 components, R, Xc and Xl.
Xc will be almost zero, the resistance is likeley to be small also.
However the inductive reactance will be large and for the phase to phase fault it will have doubled.
At the same time the voltage will only have increased by 1.7ish (root 3) so it is not suprising that the phase to neutral current is larger when you think about it.








Post you calculation.

rob w

Thanks 2011, always great for the answers!

Heres a link to the calc, https://dl.dropbox.com/u/47146575/1.pdf.pdf

It's just the fault at point no. 1 where the single phase fault is larger, the other two are ok.....you'll see what I mean!

also,

(Zt1 is the transformer impedance, Zc is cable impedances)

John1999

Hi there I was just wondering if anybody could explain to me how I go about the wiring of an industrial/commercial lighting circuit including your emergency lighting?like from the distribution board how do I go about it,to the light switch h emergency exit etc.
And if anyone could explain how I would wire a dali lighting circuit I'd appreciate it,I just like to be able to walk on to a site and basically get the cable and go and wire it from scratch and what way I do it etc.

Kind regards
John Donnelly.

Conte..

John1999 wrote: »

Hi there I was just wondering if anybody could explain to me how I go about the wiring of an industrial/commercial lighting circuit including your emergency lighting?like from the distribution board how do I go about it,to the light switch h emergency exit etc.
And if anyone could explain how I would wire a dali lighting circuit I'd appreciate it,I just like to be able to walk on to a site and basically get the cable and go and wire it from scratch and what way I do it etc.

Kind regards
John Donnelly.

For 3p short circuit current you multiply the measured SP short circuit current by 1.7, thats the breaking capacity needed

Emergency lighting the basic method I used at distribution boards is each lighting breaker feeds into a n/c contact on a multipole contactor, a test switch breaks supply to contactor, a that's for non maintained,I've had a few beers so stand to be corrected on above

salmocab

John1999 wrote: »

Hi there I was just wondering if anybody could explain to me how I go about the wiring of an industrial/commercial lighting circuit including your emergency lighting?like from the distribution board how do I go about it,to the light switch h emergency exit etc.
And if anyone could explain how I would wire a dali lighting circuit I'd appreciate it,I just like to be able to walk on to a site and basically get the cable and go and wire it from scratch and what way I do it etc.

Kind regards
John Donnelly.

From the way your question is phrased your not an electrician?

2011

salmocab wrote: »

From the way your question is phrased your not an electrician?

That’s allowed.
Many people aren’t electricians.

salmocab

2011 wrote: »

That’s allowed.
Many people aren’t electricians.

Yes but the things they are looking for information to do themselves is very much an electricians job, it’s not the usual can I change this or add that that we see around here. It’s someone looking to be told how to wire lighting start to finish including contactors, emg ltg and even Dali stuff. This is not to be encouraged in any way it’s not debatable whether it needs a qualified and registered electrician like so many of the things that get asked.

2011

salmocab wrote: »

Yes but the things they are looking for information to do themselves is very much an electricians job, it’s not the usual can I change this or add that that we see around here. It’s someone looking to be told how to wire lighting start to finish including contactors, emg ltg and even Dali stuff. This is not to be encouraged in any way it’s not debatable whether it needs a qualified and registered electrician like so many of the things that get asked.

There is no problem with highlighting the risks.
Electrical information is available all over the internet and in many publications. This forum is not here to police what people do however often when the process is explained properly DIYers realize they are out of their depth.