CCVT Fusing 1

[cbradley75]

This may be a stupid question, but it's one of those things that back when I started; it was "always done this way" and I just never questioned it. Back when I was a Relay Test guy, I noticed that all the CCVT circuits were slugged at the safety disco and #10 circuits were run back to the line relays (no fuses). The same CCVT potential was usually daisy chained over to a meter of some kind and through fuses.

I never really questioned it before, but now someone asked me and I'm not sure what the reasoning is. Why are the circuits to the line relays not fused and the meter circuit is fused? I also noticed that all the CCVT safety discos are slugged but the safety discos where PTs are involved are fused. This may be common knowledge, but can someone enlighten me.

Thanks

 

[Marmite]

Where I come from being "slugged at the disco" would have an entirely different meaning altogether. %-)
If you mean fitted with solid links in the secondaries rather than fuses, then it would seem an odd thing to do, as a secondary wiring fault could cause destruction of the VT. In my experience they are fused as near as practicable to the VT with separate fuses for metering and protection.
Regards
Marmite

 

[cranky108]

We fuse the relays, but then again we use redundit relays. We also fuse the meters.

However the question maybe what size fuse do you use? Some CCVT's just don't have enough capacity to operate very big fuses. And 3 amp fuses are typically the smallest fuses we have in stock.

Maybe it's just a risk they are willing to take.

 

[scottf]

The reason (not that I agree with it) is that a short on the CCVT secondary will cause damage to the unit, but will not result in explosive failure, i.e. not a safety issue.

Also, CCVT fused secondaries are more prone to nuisance tripping than with wound-type VTs, due to a variety of issues.

Note sure why the meter circuit would be fused and the protection circuit not be. I suspect it's probably due to the meter group generally having different practices/standards that the protection group. Also protection group might not want a failed meter to take out the protection.

Cranky108-

Normal fuse size recommendations for CCVTs range between 9A and 25A, depending on type and if it is the full winding or the tap winding.

Lastly, with a wound-type VT, a short on the secondary causes a much higher current to flow (available fault current from VT is a factor of 10 or 20 times higher than a normal CCVT). A VT most definitely will fail explosively due to such a short, if not cleared quickly.

 

[cbradley75]

Thanks, scottf, that makes a lot of sense to me. Most likely someone thinking along those lines a long time ago started doing things this way, and we just have always done it that way ever since.

Thanks again for everyone's feedback.