Transformer Overload Protection

[timm333]

NEC Table 450.3(A) requires transformer secondary fuse for 600V or less to be 125%, which makes sense because the 125% secondary fuse will provide overload protection to the transformer primary circuit. However for more than 600V, this table allows the secondary fuse to be 250%. How shall the transformer get overload protection if the secondary fuse is set to 250%?

 

[mayanees]

XF Overloading and protection

XF Overloading and protection

Timm,

That's a good question, and your observation is accurate.
Per 450.3(A):
Protection for the low-voltage secondary transformers (<1000V) is limited to 125% of the rated current.
Protection for transformers with primary and secondary voltages > 1000 Volts can be as high as 300% of the nameplate rating.

My rationalization is that the transformers with low-voltage secondaries are at the utilization voltage where sizes are calculated based on known loading, and are naturally smaller than the higher voltage units. The transformers with >1000 V primary and secondary voltages are larger and are upstream of these "utilization voltage" transformers, and as such, are subject to more load diversity so they need to be able to tolerate load swings.
Transformers are dynamic in nature, in that they can tolerate overloads until they overheat. If you look at a typical thermal damage curve, a dry-type transformer can handle 200% loading for 15 minutes before the windings overheat and you start to shorten the life of the transformer. A medium-voltage oil-filled Utility transformer could handle a higher overload level for a longer time before exceeding the temp rating and compromising its windings, so as such, we're allowed to drive it at a higher level.

I think the practice goes hand-in-hand with 240.100(C) and 240.101(A) which permits conductor protection at 600% ampacity, but dictates that the conductor must have coordinated protection from thermal damage. You need to plot the cable damage curve on a TCC and verify that that time/current point is protected.

The allowance of higher protection for the larger transformers helps in coordinating protective devices for low-voltage faults.

 

[lielec11]

NEC Table 450.3(A) requires transformer secondary fuse for 600V or less to be 125%, which makes sense because the 125% secondary fuse will provide overload protection to the transformer primary circuit. However for more than 600V, this table allows the secondary fuse to be 250%. How shall the transformer get overload protection if the secondary fuse is set to 250%?

Per the NEC 2008 Handbook, "...and secondary overcurrent devices prevent the transformer and secondary conductors from being overloaded." I do not believe the 125% secondary OCPD provides any protection to primary side shorts, overloads, etc. That is why there is no "secondary protection only" in this table. Am I correct?

 

[jim dungar]

There is also a difference due to the performance of protective devices (e.g. actual clearing time) based on voltage as well as current.

Note, how for MV systems the maximum rating of of fuses is different than for breakers, however at <600V there are the same.

 

[timm333]

So the ampacity of the cable can be three times less than the fuse rating, as long as the curve of fuse is to the left of cable damage curve and transformer damage curve?

 

[kingpb]

Article 450 specifically covers OCP for transformers ONLY. Conductors are covered elsewhere in the NEC. The OCP for the transformer may be adequate for the conductors too, it can only be properly determined by looking at conductor requirements.

The size of the OCP given in the NEC is maximum, not the specified value. Specific conditions will dictate actual settings.

As soon as you connect conductors to the transformer, the setting maximums in 450 essentially go out the window and 240 takes over.

SqD has a really good paper that discusses this, Bulletin No. 0110DB0201; it is available on internet.

 

[lielec11]

Article 450 specifically covers OCP for transformers ONLY. Conductors are covered elsewhere in the NEC. The OCP for the transformer may be adequate for the conductors too, it can only be properly determined by looking at conductor requirements.

The size of the OCP given in the NEC is maximum, not the specified value. Specific conditions will dictate actual settings.

As soon as you connect conductors to the transformer, the setting maximums in 450 essentially go out the window and 240 takes over.

SqD has a really good paper that discusses this, Bulletin No. 0110DB0201; it is available on internet.

Thanks for this ... http://www.schneider-electric.us/documents/support/codes-and-standards/0110DB0201-transform-tap.pdf