Transformer damage curve
- Thread starter binwork91
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mayanees
Senior Member
- Location
- Westminster, MD
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- Electrical Engineer and Master Electrician
No, you only need to protect within the parameters of tables 450.3 (A) & (B)
Do you know the reason why we don't need to protect transformer from damage?
I think we need to protect the equipment and conductor.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
Ideally your primary device slices to the right and your secondary device shades to the left of the ANSI curve to provide complete protection, but the code requirement is 450.3(A) and (B). And setting of devices is always limited by the upstream allowance and by the devices in the primary and secondary.
Conductor protection is required as well per the 310 tables.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You should protect the transformer, however that is not an NEC requirement. The NEC is about protecting conductors not equipment, see 90.1.
Do you mean we don't need to care about damage curve base on NEC code?
Is there any code require to fully protect the transformer, and make primary side OCDP curve on the left side transformer damage curve?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The NEC has requirements for transformer protection based solely on 125% of the full load (either on the primary or the secondary), no other transformer characteristic is mentioned or considered.
Thank you.
Why are you worried? Transformers can be overloaded by a lot with little to no harm done. Industrial plants rarely do this but utilities routinely do.
Whoever asked about primary/secondary there are a couple advanced tricks I’ve done. One is with a relay/breaker on the primary side you can use a looser curve when closing the breaker in to avoid the magnetizing curve then switch to a tighter setting once it calms down (current under name plate). Another option is a virtual breaker since the secondary side current is high and it’s hard to eliminate the zone between the secondary breaker and the transformer secondary. Place bushing CTs on the secondary lugs and feed these to a protection relay to trip the primary side. Then you can trip as low as you want with primary side protection “only” although technically you have both.
Whoever asked about primary/secondary there are a couple advanced tricks I’ve done. One is with a relay/breaker on the primary side you can use a looser curve when closing the breaker in to avoid the magnetizing curve then switch to a tighter setting once it calms down (current under name plate). Another option is a virtual breaker since the secondary side current is high and it’s hard to eliminate the zone between the secondary breaker and the transformer secondary. Place bushing CTs on the secondary lugs and feed these to a protection relay to trip the primary side. Then you can trip as low as you want with primary side protection “only” although technically you have both.
xptpcrewx
Power System Engineer
- Location
- Las Vegas, Nevada, USA
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- Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
The answer is yes. Refer to NEC section 110.10 (Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics).
If the TCC is on the right side of the damage curve, the transformer will not be protected. There two parts to a transformer damage curve - thermal and mechanical. The ratings/settings in Article 450 are maximum ratings/settings and may not adequately protect equipment since they do not account for thermal and mechanical damage characteristics. You can refer to the IEEE Buff Book for some guidance on transformer protection.
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