Please Explain: Symmetrical fault current

[daveede]

Im doing my 2391 course and it was quickly mentioned about symmetrical fault current when talking about PFC testing.

It was just mentioned and not explained so can anyone explain what this means.

My first thought is that 3 phases shorted together would cancel out and leave 0 volts??
But this cant be correct or cant be the whole story as its must still produce a fault current or it wouldn't be in the course.

I have tried to google it but not really getting anywhere cant find the answer and what i am finding is just crazy looking math and talking about transformers and motors

Thanks for the help guys

 

[mpspark]

Does this help?
http://www.powerstudies.com/articles/ImportanceofX-over-RRatios.pdf

 

[IQ Electrical]

Im doing my 2391 course and it was quickly mentioned about symmetrical fault current when talking about PFC testing.

It was just mentioned and not explained so can anyone explain what this means.

My first thought is that 3 phases shorted together would cancel out and leave 0 volts??
But this cant be correct or cant be the whole story as its must still produce a fault current or it wouldn't be in the course.

I have tried to google it but not really getting anywhere cant find the answer and what i am finding is just crazy looking math and talking about transformers and motors

Thanks for the help guys

A symmetrical fault is a fault that affects all phases in an equal fashion.

An asymmetrical fault is a fault that only affects say 2 phases of a 3-phase system.

The prospective short circuit current line to line will be 1.732 X the line to neutral value in a 'text book' 3-phase/N installation.

As a 'rule of thumb' Guidance Note 3 recommends multiplying the measured line to neutral prospective short circuit current by 2 to obtain a line to line value that errs on the side of caution.

 

[Guest 004]

A symmetrical fault is a fault that affects all phases in an equal fashion.

An asymmetrical fault is a fault that only affects say 2 phases of a 3-phase system.

The prospective short circuit current line to line will be 1.732 X the line to neutral value in a 'text book' 3-phase/N installation.

As a 'rule of thumb' Guidance Note 3 recommends multiplying the measured line to neutral prospective short circuit current by 2 to obtain a line to line value that errs on the side of caution.

very good indeed IQ
what I dont like is GN3 note to double pfc, I always record pfc x 1.732

 

[IQ Electrical]

very good indeed IQ
what I dont like is GN3 note to double pfc, I always record pfc x 1.732

It's like we're not trusted with that complex mathematical calculation!

 

[Guest 004]

^ yeah had many an argument about that one as well

 

[daveede]

Thanks for the replies

The bit im wondering about is if you short all 3 phases wouldn't the sine wave cancel to zero, and if this is correct how do you get a fault current. i guess you get a fault if the phases weren't balanced somehow but this would seem to be small at first thought because they would have to be pretty unbalanced to make a large fault almost like only 2 phases shorted etc

ps. This may be why the pages i googled were talking about motors and transformers, is it anything to do with voltage and current being out of phase

 

[topquark]

Definately not double unless the phases are seriously out of wack! Double is, I think, just overstating it for caution IMHO.

 

[Guest 004]

doubling can cause more problems than what it is worth, especially if you have a Ze of 0.01 and fuses that can only handle 40 Ka fault current

 

[topquark]

Thanks for the replies

The bit im wondering about is if you short all 3 phases wouldn't the sine wave cancel to zero, and if this is correct how do you get a fault current. i guess you get a fault if the phases weren't balanced somehow but this would seem to be small at first thought because they would have to be pretty unbalanced to make a large fault almost like only 2 phases shorted etc

ps. This may be why the pages i googled were talking about motors and transformers, is it anything to do with voltage and current being out of phase

If you picture the timing of the 3 phases P1, P2, P3 (that's the implied sequence). When P2 is at it's peak, where would P1 and P3 be in their cycle?

 

[daveede]

hey topquark,

Thats pretty much what i mean, across 2 wires or across a load the difference is the peak of the sine wave gives you the potential / voltage.

But 3 phases in 1 wire would sum to zero as in your example if:
L2 is at peak
L1 is rising from its negative peak
L3 is going towards it negative peak

So its all the 3 waves are added as in 1 wire they would cancel to zero potential

If i remember correctly for sine graphs for non electrical you would calculate the area under the waves and they would be equal and therefore cancel, assuming this works for electrical aswell.

 

[topquark]

For that single wire with all phases yep. However we only ever considering two phases with respect to each other for fault potential. You are always looking at the worst possible fault scenario.

So yes you are correct (and so is the maths).

You can probably visualise that easier if you look at the timing for L2 half way through it's cycle, then L1 and L3 are at opposite peaks.

 

[daveede]

Thanks for the reply

What does this mean in terms of Symmetrical fault current which i believe is all 3 phases shorted, so how do you get a fault current etc

2 phase pscc is ok i get that no probs

Maybe i just need to know where the fault current in Symmetrical fault current comes from / goes to, or is it just an exchange between all 3 phases but wouldn't ever go to earth or neutral

 

[topquark]

That would depend on the nature of the fault, consider an over voltage fault affecting all 3 phases. You are still then considering the fault between two phases as the highest PD and hence current.

 

[topquark]

I think, what may be confusing here is that with a symmetrical fault you are not considering all three phases for what current could flow, still only two. The nature of the fault itself is still symmetrical, in that it affects all 3 phases.

 

[daveede]

That might be it

How does it effect all 3 phases if you consider only 2?
and why is it then considered different to the normal 2 phase pscc?

Thanks for helping me

 

[topquark]

Remember it's prospective (IE what could the maximum be). The minimum will be 0 (or as close to it as the real system would allow for). Remember we're considering a mathematically perfect system here (always will be some "real world" error, maybe one of the phases is slightly out etc).

So your fault could be a short between all 3 phases, so far so good. If you (during investigation, ignoring ESC) were to grab two of those phases thats what would flow.

 

[topquark]

How about this as a (maybe better) scenario. You have a fault involving all 3 phases (a short between all 3 phases/cables). The supply then drops a phase whilst that fault exists. That will then give you your maximum current of square root of 3 X (assuming minimal resistance for the short etc).