Bayonet (Bay-O-Net) style fuses are the go-to protection device for most padmount transformers. Read more about transformer fusing. There are several fuse link options for different transformer designs and applications. Bayonet style fusing lets you easily change these links in the same bayonet housing. Today we are going to look at a few of these different links.
Types of Bayonet Fuses
The transformer’s size, voltage, and use dictate which fuse link works best. This article will look at five fuse links: current sensing, dual sensing, dual element, high-amp, and 38kV. Check out the table below to see the key differences between these fuses.
* Eutectic is an alloy material that melts easily
Current Sensing Fuses
(Also called fault sensing fuses)
The current sensing fuse element is made up of a thin strip of tin or copper. This fuse gets its name from the element’s higher melting point. Current sensing fuse elements are usually resilient to temperatures over 230°C. These are far above typical oil temperature levels for in-service transformers. This means the element tends to melt only under the higher resistive heating of high currents. Hence the name current sensing.
Because of their higher melting point, current sensing fuses are more resilient against extended overloading. They sense faults on the secondary side of the transformer only. As such, they also go by the name fault sensing fuses. Current sensing links are typically suitable for voltage applications up to 23kV.
Current Sensing Fuse Usage Table
Table Notes:
- Application based on fuse melting at 3 to 4 times transformer rated current in 5 minutes.
- Application meets inrush requirements of 12 times transformer rated current for 0.1 second.
- Isolation link or CLF must be used with selected Bayonet fuse link.
- * Application provides more than 4 times transformer rated current for five minutes.
- ** Must be GNDY/GNDY system with less than 50% delta loading.
Dual Sensing Fuses
(Also called Load Sensing)
Unlike current sensing links, a dual-sensing (or load-sensing) element operates under over-temperature and overcurrent. The eutectic (“easy melt”) element material allows melting around 145°C. This lower melting point protects the transformer insulation system from harmful overloading and overheating.
Loading a transformer above its rated kVA for an extended period can harm its insulation and reduce transformer lifespan. When the oil around the fuse element overheats from overloading, the element melts. This protects the winding insulation from more excessive heat. In this way, dual sensing fuses help protect and extend transformer lifespan. Dual sensing links also go by the name load sensing.
Like current sensing fuses, dual sensing links operate under secondary faults. Fuse coordination favors dual sensing links since they can operate under high temperature overloads and secondary faults. Fusible kVA ranges for dual sensing links are slightly lower than current sensing options.
Dual Sensing Usage Table
Table Notes:
- Applications based on 200% of transformer loading for 2 hours and 160% loading for 7 hours.
- Application meets inrush requirements of 12 times transformer rated current for 0.1 second.
- Not to be used at voltages greater than 17,100 for delta configuration, or 14,400/24,900Y with neutral internally grounded.
- Isolation link or CLF must be used with selected Bayonet fuse link.
- * Use will result in some loss of overload capacity.
- ** Must be GNDY/GNDY system with less than 50% delta loading.
Dual Element Fuses
The dual element link employs two separate materials. One easy melt section like the dual sensing link. The other part of the element is nichrome or copper. This second part of the element has a much higher melting point (> 1,000°C). The two-part element gives the fuse link a slightly different operational profile than the dual element. But it is very similar in its functionality. Like the dual sensing link, dual element fuses sense both transformer overloads and secondary faults.
Dual Element Usage Table
Table Notes:
- Applications based on 200% of transformer loading for 2 hours and 160% loading for 7 hours.
- Application meets inrush requirements of 12 times transformer rated current for 0.1 second.
- Not to be used at voltages greater than 17,100 for delta configuration, or 14,400/24,900Y with neutral internally grounded.
- Isolation link or CLF must be used with selected Bayonet fuse link.
- * Must be GNDY/GNDY system with less than 50% delta loading
High-Amp Fuses
The High-Amp fuse link has an element made up of eutectic material and silver. Its overall melting point is 145°C. Like the dual sensing and dual element links, it provides protection against high oil temperature from overloading. This link is for higher kVA applications, which the other links cannot effectively handle. The High-Amp fuse link also has a different appearance. The fuse link is housed inside a cartridge which attaches to the end of the bayonet assembly. High-Amp fuses protect against secondary faults and overload like the dual sensing and dual element options. They provide protection for voltages up to 15.5 kV.
High-Amp Usage Table
Table Notes:
- Coordination recommendations are based on 12X transformer full load for 0.10s, 3X transformer full load for 10s, 200% of rated load at 2 hours and 160% of rated load at 7 hours. Overload protection based on 65 degrees rise transformers.
- * The use of these fuses will provide 175% of rated load at 2 hours and 150% of rated load at 7 hours. The use of the next larger fuse is recommended for greater overload capacity.
- ** The specified fuse provides slightly less than 175% of rated load at 2 hours and 150% of rated load at 7 hours.
38kV Fuses
The 38kV link handles higher voltage applications between 24kV and 38kV. The fuse link and silver-plated cartridge are all one piece. This fuse link is the only option for 25kV and 35kV Delta or ungrounded wye primary applications (with bayonet style fusing). The 38kV link is also a dual sensing type fuse which operates for both high oil temperature and secondary faults.
38kV Usage Table
Table Notes:
- Coordination recommendations are based on 12X transformer full load for 0.10s, 3X transformer full load for 10s.
Conclusion
Bayonet expulsion fuses are simple, reliable, and convenient. They continue to be the go-to for primary overcurrent protection in padmount transformers. If you have any questions about what fusing you should get for your transformer please fill out the form below.
