Transformer Rating

[Grouch1980]

What's the reason a transformer doesn't have an SCCR rating?

I understand its impedance limits the short circuit current going through it, and the equations involved, and that it provides an available fault current on its secondary... but shouldn't the transformer still have an SCCR rating?

 

[jim dungar]

A transformer doesn't have an SCCR similar to the reasons conductors and motors do not also.

Power transformers are able to the magnetic forces of fault current based on the way they are wound. Their thermal mass typically allows them to to handle large amounts of current for relatively longer than the conductors feeding them.

Smaller transformers such as Current Transformers may have limitations on the amount of fault current they can handle without damage.

 

[Grouch1980]

Power transformers are able to the magnetic forces of fault current based on the way they are wound.

Sorry... able to what... withstand?

 

[Grouch1980]

Their thermal mass typically allows them to to handle large amounts of current

But shouldn't this large amount of current have an upper limit, and therefore be labeled as the SCCR rating of the transformer?

 

[petersonra]

But shouldn't this large amount of current have an upper limit, and therefore be labeled as the SCCR rating of the transformer?

When they test transformers they short circuit them. It is how they determine Z.

 

[Grouch1980]

I still don't get it.

 

[petersonra]

I still don't get it.

The point is transformers are pretty hard to kill.

 

[Grouch1980]

The point is transformers are pretty hard to kill.

Ah.

 

[LarryFine]

The point is transformers are pretty hard to kill.

At least the Steven Seagal models are.

 

[ron]

Conductors have withstand ratings (short circuit ratings). The transformer primary and secondary winding conductors should likely have a withstand rating, but it is relatively high so in the same way you by conductors without knowing the SCCR, it is similar for transformers.
In reality, if there is a switch and / or fuse in the transformer, that has a SCCR, which would establish the rating for the assembly.

 

[jim dungar]

Conductors have withstand ratings (short circuit ratings)

Conductors have damage curves, not really SCCR values.
However conductor assemblies, such as bussing and lug terminations, do have SCCR values typically dependent on magnetic forces causing movement.

 

[electrofelon]

When they test transformers they short circuit them. It is how they determine Z.

But only up to a voltage that causes rated current to flow on the shorted secondary. Unless there is another test where there crank 'er up.

 

[Julius Right]

There are standards requiring short-circuit test of transformer as IEEE C57.12.90 for instance .
The type of fault to be applied will be dependent on the available energy source. Any of the following types may be used (given in order of preference for three-phase transformers):
a) Three-phase source: three-phase short circuit
b) Three-phase source: single phase-to-ground short circuit
c) Single-phase source: simulated three-phase short circuit
And are standards requiring the level of this current as
for two-winding transformers, the required value of symmetrical current for any test shall be determined from the equations in Clause 7 of IEEE Std C57.12.00-2006.
In IEC World IEC 60076-5 standard requires short-circuit test for
ability to withstand short circuit .

 

[Grouch1980]

so in the same way you by conductors without knowing the SCCR, it is similar for transformers.

In the same way I 'specify' conductors?... oh nevermind... I think you meant 'buy'.

 

[Grouch1980]

Power transformers are able to the magnetic forces of fault current based on the way they are wound. Their thermal mass typically allows them to to handle large amounts of current for relatively longer than the conductors feeding them.

So as you mentioned in Post #2... this is the same reason motors don't have SCCR ratings as well?

 

[Julius Right]

Generally, at low voltage system, it is not necessary to check
ability of a cable to withstand the short circuit current. [Available Fault Current AFC] except for the tap conductor [see NEC art.240.92].
For medium and high voltage cable it is required.
However, since it is not a constant value you cannot attached to cable properties as a SCCR.

NEC requires SCCR for breakers and panels only.

 

[jim dungar]

NEC requires SCCR for breakers and panels only.

For many decades NEC 110.10 required all equipment to be applied within its short circuit capability, not just OCPD mentioned in 110.9.
Because this section was so commonly ignored more specific language has been added to additional code sections.

 

[Julius Right]

Thank you, Jim, for your remark. However, this article is referring to the equipment not for the cables. For any equipment, for which the code requires to check the SCCR, you need to calculate the AFC. Then you need total impedance , to select the overcurrent protective device and coordinate this with up-stream protective devices.

 

[jim dungar]

Then you need total impedance , to select the overcurrent protective device and coordinate this with up-stream protective devices.

NEC 110.10 is about equipment and conductors being applied correctly based on the available fault current it is exposed to 110.10 has nothing to do with coordination.