OCP of outdoor potential transformers 1

[GOTWW]

According to NESC C-2 instrument transformers are not required over current protection. The NEC indicates all transformers need OCP. For medium voltage systems, in switchyards and on poles, is fusing of metering potential transformers optional?

 

[rbulsara]

There are two types of Instrument xfmrs.

Current (CT) and Voltage (VT or PT)


CT do not require an OCP....VT/PT does...

At least I have not seen a VT/PT withoout a fuse on MV systems.

 

[GOTWW]

I have seen pt's with no fused cutouts etc., leading me to believe that the fusing may not be manitory. This is why I ask.

 

[peebee]

A complete guess here -- instrumentation transformers saturate easily, don't they? Would that tend to make them inherently current limiting? In that case, the fuses would still be a good idea, as a saturated PT is a trashed PT, but there would be no fire hazard. . . .

I've never seen a PT that wasn't fused, though. And I have no idea if the unfused PT's you're dealing with are legal or not.

One more thought on this -- if a PT was manufacturer installed, they would not be held to NEC. But if it was field installed, then NEC would apply. There might be some different requirements if you're not building to NEC. . . .

 

[busbar]

In many cases for revenue-metering applications, metering components are under exclusive control of utilities, so are not bound by the NEC.

NEC 450-3(C) implies that outdoor VTs may be permitted without primary overcurrent protection. NEC 408.32 requires overcurrent protection for instruments served by VTs

Secondary fusing offers limited protection to VTs. ANSI C57.13-1993 §7.7 requires that one-second secondary-side short circuits will not damage VTs.

Some discussion in General Electric GEP-9186-6 — Section 5 Applications Information Primary Fuses For Voltage Transformers

 

[jghrist]

See thread238-40900 for a discussion of VT fusing on HV buses. Many utilities do not fuse VTs on HV buses but I think most do on MV buses. Cost is a factor, bus protection is a factor, and security of the VT for relaying is a factor.