This was an interesting challenge for VT design, as the client wanted a HV fuse incorporated into an outdoor VT with oil insulation.

WHY? The original design (+30 years ago) incorporated the fuse in the upper oil expansion membrane, but this meant dropping the oil and extracting the fuse and changing it; time consuming and cumbersome!

So, I came up with the idea of an external fuse separate to the HV VT transformer and mounted such that it was quick and easy to change. But, the fuse mounting had to be strong enough to handle the canter-lever loading due to the HV connections and be flexible to be mounted at 0/90/180/270 Deg. on the VT top housing and then the actual fuse housing be rotated to accept the HV connections at 0 or 90 Deg. on top of the fuse. This design requirement was met and very acceptable to the client. The housing of the fuse is made from cylco-aliphatic epoxy resin which is resistant to UV and solar energy degradation.

The VTwas impulse tested at 170kvp +/- ve polarity; 15 shots each.
The induced HV test level was 70kv rms
The partial discharge was < 10PC at 43 KV rms