What is the right answer?

There are several questions that must be answered in order to properly select fuses. First of all, what voltage rating on the fuse is required? If the transformer is singlephase and connected line-to-ground, the fuse must be equal to or greater than the line to-ground voltage. If the transformer is single-phase but connected line-to-line, a fuse that is equal to or greater than the line-to-line voltage is required.

For three phase transformers, the voltage question has even more possibilities. If the transformer is wye connected to a grounded wye source, and the secondary load is less than 50% delta connected, then you can use a fuse that is only equal to or greater than the line-to-ground voltage of the source. If any of the conditions are not met, or the transformer has a delta primary, then a fuse with a voltage rating equal to or greater than the line-to-line voltage must be used.

Once the voltage question is answered, the selection of fuse size is the next step. Fuse application charts are available from the fuse manufacturer to determine the correct ampere rating of fuse required to protect a transformer. The charts on pages 1.15 and 1.16 are provided for selecting fuse amperage ratings.

Selecting the right fuse mount

After the size of fuse required is determined, the type of fuse mounting can be selected. Conventional drywell fuse holders mounted in liquid insulation offer the advantage of having a dry fuse to replace and the ability to include switches in the same tank. The disadvantage of this type of mounting is that you are limited to 50 Amp fuses. We recommend Cooper X-Limiter, or ELX fuses for this application.

Another possibility is to use a Trayer Universal Fuse Well, which is a wet well fuse holder, that accepts liquid-immersible fuses from several manufacturers, with up to 200 Amps of continuous current. This holder is inherently interlocked because the 200 Amp elbow must be removed before the fuse can be pulled. It can also be mounted in the same tank as switches and other devices. Squeegees can be specified on the fuse holders, to remove the oil on the fuse surface as they are removed from the fusewell. We recommend Cooper/Combined Technologies, Inc. SX fuses for this application.

The final option is the Trayer Drywell, which accepts air-insulated fuses up to 100 Amps. The fuse is inherently interlocked because the 200 Amp elbows must be removed before the fuse can be pulled, and there is no liquid insulation on the fuse. The only disadvantage is that you are limited to 100 Amps. We recommend the Cooper/Combined Technologies, Inc. type X-Limiter fuses for this application.