single phase transformer in arc flash study
single phase transformer in arc flash study
(OP)
Hi everyone, It's my first post here.
I have to do an arc flash study where the utility have a lot of 1 phase transformer 25kV to 120/240V. The client want us to calculate the short circuit current and the arc flash hazard at the secondary of theses transformers.
I red that the calculation must be done for an equivalent 3 phases system for the arc flash, but what's the equivalent? If I want the same short circuit current as the single phase, i must take a transformer with a bigger kVA, but is it realistic? Does the equivalent system is the one with the same short circuit current or the same power?
And for a 3 phase system, you can stop the arc flash if the transformer is 125kVA or less (at 240V), but if it's a single phase, do you stop at 125 kVA or 125/3 = 42 kVA?
Thank you
I have to do an arc flash study where the utility have a lot of 1 phase transformer 25kV to 120/240V. The client want us to calculate the short circuit current and the arc flash hazard at the secondary of theses transformers.
I red that the calculation must be done for an equivalent 3 phases system for the arc flash, but what's the equivalent? If I want the same short circuit current as the single phase, i must take a transformer with a bigger kVA, but is it realistic? Does the equivalent system is the one with the same short circuit current or the same power?
And for a 3 phase system, you can stop the arc flash if the transformer is 125kVA or less (at 240V), but if it's a single phase, do you stop at 125 kVA or 125/3 = 42 kVA?
Thank you






RE: single phase transformer in arc flash study
For three-phase systems, IEEE 1584 says calculations should be done for ALL 240 V systems.
For 208 V systems, it recommends calculations be done for systems 125 kVA and larger.
RE: single phase transformer in arc flash study
But i would like to do it via the arc flash software we are using (who work in 3 phase system only). I red in "a practical solution guide to AFH" that IEEE-1584 recommend that calculations be done for an equivalent 3 phases systems and states that this will yield conservative result.
But i have some difficulty to figure what's the equivalent system. Is it three single phase transformer with the same rating of the one i am studying who will form a 3-phase transformer.
If i have a single phase transformer 14.4kV/120-240V 167kVA, then i would take a three phase transformer 25kV/416V 501 kVA for the study? (that the 416V which make me perplex)
Anybody have an idea?
Thank
RE: single phase transformer in arc flash study
RE: single phase transformer in arc flash study
If you are looking at equipment some distance from the transformer, the three phase analysis may not be acceptable. Zero-sequence cable impedance will be significanty higher than the positive-sequence impedance. A three phase analysis will result in fault currents that are too high and fault clearing times that are too low.
RE: single phase transformer in arc flash study
i take a 167kVA 14.4/120-240 V with 5% impedance
the nominal current of the transformer is 167kVA/240V = 696 A
And the fault current will be 696 A / 5% = 13 916.66 A
is it right?
RE: single phase transformer in arc flash study
That method may give you a single-phase fault current that matches the three-phase, but what about the arc-energy?
Just setting the single-phase fault current equal to the three-phase fault current does not mean the energy will be calculated correctly, since all IEEE equations are based on three-phase faults.
This is really a moot point anyway, since IEEE-1584 explictly says it does NOT apply to single-phase systems.