MOV & Fuse Sequencing

[BrianMuir]

Hi,
I have a design involving a 69KV to 120V direct step down. We are protecting the primary with a pair of MOV arrestors and fuses.

I have drawn the MOV across the fuse and transformer primary, but the electrical contractor suggests the fuse should be ahead of the MOV.
See attached

I don't think I agree with him but this isn't the kind of project I have a great deal of experience with and would welcome additional opinions.
thanks!

 

[mxslick]

Utility company practice here is to indeed have the fuse ahead of the LA so if the arrestor fails (which it will eventually) the fuse will blow instead of creating a violent fault on the primary. But, that is ahead of a feeder with a fuse usually greater than 1 amp.

A call to your local power company's engineering department can provide you with the best answer.

Or, you could fuse the feeder right after the GOAB at say 25-100 amps, which will protect the LA's without nuisance blowing of the 1 amp fuse protecting your transformer. (As I can see a surge within the LA's capabilities easily blowing a 1 amp fuse.)

And for those who are wondering: :grin:

GOAB = Gang Operated Air Break (switch)
LA = Lightning Arrestor

 

[BrianMuir]

ok, thanks for the excellent advice.
this all will be reviewed by the utility company but their policy is to "review rather than recommend".

The 1A fuse is a bit strange, I admit. The secondary load is something like 40A maximum, which on the primary equates to 70mA. This is why I have specified the smallest standard fuse rated at primary voltage. This said, I think we can increase the rating on that fuse if there is a benefit to doing so.