Has anyone installed fuses on secondary conductors inside a padmount transformer to reduce incident energy at the service equipment? This is for 300kVA and 500kVA 208/120V transformers.

If so, what fusing did you use?

Barry,

Mersen used to market a AFIE reducing fuse, which had a fiber optic communication module that would tie into an arc flash relay specifically for this application (primary fusing tho'). I was going to pull the trigger on going w/ this solution but when I went to purchase they had mysteriously removed the product from their line up. I never found out why it was discontinued...

Sorry this doesn't help much. I have looked into different fuse sets as you are attempting to do, but did not have much luck primarily due to the fact my application was a primary fuse application.

Mike

Check out Eaton transformers. They have an option for a vacuum interrupter on the primary, that is monitoring CT's on the secondary, and will trip on secondary short or overload.

Eaton makes a "breaker integrated transformer" up to 300kVA in 3PH and 100kVA in single-phase.

If you look at the NEC rules it is difficult to comply with 450 and 240 once the secondary conductors are longer than 25 ft if you do not have secondary OCP. In addition, the equipment on the secondary is required to have a main to apply the tap rules. There are exceptions, however, for three phase transformers the general rule requires secondary protection.

The simplest/economical way is to install a safety switch on the secondary. For resistive/non-inductive loads the best choice is a UL Class J, either an A4J or JKS sized at 125% of the FLA and not larger than the the wire size. All this is assuming 600V or less. If the load is inductive (inrush) the fuse type would be AJT or LPJ.

Slightly different topic - To go one step further, if you size the primary OCP on a drive isolation transformer (DIT 480-480) correctly (large enough to ride through the inrush) you will find the secondary incident energy is many times the primary. This is often overlooked in arc flash studies, in most cases both the primary and secondary conductors enter the drive cabinet, however, in many cases the drive enclosure incident energy is based n the 480V cabinet input and not the 480V DIT secondary input.

This is a case where the secondary fuse can in most cases drop the incident energy to under 1.2 cal. It is important to note that Boltswitch has fuse adapters for their switches that allow the use of semiconductor fuses in their switches. This can be done all the way through 1600 amperes. For large drive cabinets this is a great tool for mitigation.

If anyone wants a picture, I have several showing this applicaiton

25ft rule generally won't apply for padmounts, as the padmount is the main building transformer and its secondary feeders are consided 'service conductors' for which there is no length limitation before encountering the Main OCP.

Yes, go to Mersen's website and I believe they make fuses that bolt directly to buss. In a pinch, you could install fuses in a junction box mounted on the side of the transformer. The fuse block would mount inside a J box large enough to comply with any bending radius issues, or you could use 2000 volt locomotive cable (always a good option) where bending is a problem. This way, the hazard is moved to this location and out of the main breaker or disconnect required by code. I've done this once and it worked fine. The hazard was very high at the line side of the main so putting fuses in this way, reduced the hazard to a reasonable level for future maintenance.