Unless you write DSP software (or need to approximate things on a very small integer microprocessor) you don't need to worry about it!
In the vast majority of cases the "wiring regulations" only deal with two effects:
- Voltage drop (L-N)
- Fault current (L-N and L-E)
Usually you simply have a value for PSSC & PFC at the source on an installation and accept it (or measure it if already installed) and from those you can set limits for final circuits to meed VD limits and to clear faults (and along the way not to blow up breakers).
If you get a copy of the IET's "Electrical Installation Design Guide" (ISBN 978-1-78561-471-2 for the 4th edition sitting here on my desk just now) it has some worked examples in chapter 6.
Again as this is looking at LV supply to final installations (not the HV or DNO's side) they are only really looking at an earthed star-point transformer.
An as aside, the IET book shows the "typical" let-through energy I2t of a fuse and a breaker in Figure 4.1, but the breaker's cure is somewhat over-optimistic! Compare it with the real world MCB characteristics from, say the Hager commercial catalogue here:
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Page 96 for MCB let-through curves.
Page 127 for MCCB curves
No! Just compute it the obvious way:
Z = sqrt(R^2 + X^2)
For all numeric range most folk work with that is perfectly good (again, not an issue for this forum, but you can have issues of the square terms overflowing/under-flowing so doing Pythagoras
correctly is subtly more difficult).
Alright, but my understanding is that transformer reactance will change the numbers in reality, so Z = sqrt(R^2 + X^2) is not technically accurate for mains and sub board faults.
I will use your equation of 1/2 the smaller number for now.
But still struggling to find the X value of trafos- I guess this is my main concern now.
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You didn't, you converted all the sections from R and X into individual Z values, and then added the vector/phasor values using their magnitudes only.
Pc is stating the same as me, add all the Resistances to each other, and all the Reactances to each other and THEN convert to the Z value as sqrt(R^2 + X^2).
Ah! Ok, that makes more sense now!
The resistance gives a voltage drop in phase with the current, but the reactance gives a voltage drop at 90degree to the current
Agree. And a DC off set during a fault if I'm on the right tack?
The maths for the most part is just simple stuff, for the lv side of transformers you would just use normal resistance and reactance addition that is basic ac theory you would have already done during basic training.
No basic training in this regard. As I said, electricians in the US do not calculate earth fault loop impedance as its not required or mentioned in the NEC. We do not carry a multi function tester and most of us have no clue what Ze, Zs or R1+R2 even means. A lot of us couldn't even tell you the per foot impedance of #12 (3.31mm2) or #10 (5.26mm2) even though we run yards of it every day.
We only size the wire based on code rules governing ampacity, terminal temps (60*C vs 75*C) and de-rating factors. We consider voltage drop after that fact, based only on our discretion. Voltage drop limits are not mandated by NFPA 70. We can legally have 20% voltage drop to any load if we deem it acceptable. Further there are no disconnection time requirements present in the NEC. If were to run 1000+ feet of 2.08mm2 wire to a swimming pool or light post we could (and can) legally do so...
In fact up until 10 years ago most US electricians mistakenly believed that a ground rod could trip a standard thermal magnetic breaker which resulted in countless tragic electrocutions and injury law suits- one example of a fence that kept becoming energized:
View: https://youtu.be/C5EiSEtxRKU?t=620
We now know better- ground rods don't clear faults- however that is only half the story. A very small minority is starting to realize that having an equipment grounding conductor isn't a guarantee. Things like railing, pools, fences, light poles, water slides, farm equipment, ect is remaining energized because the loop impedance of the circuit is to high to trip an ordinary breaker. If electricians knew about loop impedance and disconnection times these incidents would not be happening.
Lastly this was also what got AFCIs into the NEC, when it was theorized that disconnection times in excess of 6 half cycles were leading to fires. Older residential breakers had a magnetic pickup of at least 20x, some without it entirely.
Hence why I'm inquiring here with questions that are rather elementary and obvious to IEC folks. I mean no irritation hard headiness
You only need to use symmetrical components if you want to calculate the unbalanced fault levels across transformers including the high voltage network.
They would not normally be used. Typically you would just accept the fault current for an earth fault direct on a transformer is the same as the three phase fault current, however you specifically included a network with the HV included.
Good to know. I'm willing to assume infinite for the MV network.
However, I'd like to know if I need sequence components for a setup where 480 volts is going to 120/208Y, as is typical in buildings.
For a transformer, all the sequence components are fundamentally the same as the voltage impedance. The only difference is the zero phase sequence one, and the inclusion of this along with how it should be included in the calculation is dependent upon the winding group and connection.
This can either be worked out by understanding the connections or by reference to sequence diagrams such as these:
View attachment 58306
View attachment 58307
In the US the calculation processes are described in the IEEE red book and the IEEE buff books (at least they are from my mid '80s versions)
I would assume they have typical values for the specific equipment you use there.
Thanks! But they are missing one of our most common pole top transformers
View: https://Upload the image directly to the thread.com/a/PAkh5XJ
T-T, which gives 3 phase power via only two core and coils- lighter and less material over delta-wye from what I'm told.
I know, we like to be different!