mbrooke
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Why aren't potential voltage transformers fused? Am I correct there is no requirement in the NEC or NESC?
Why aren't VTs fused?
- Thread starter mbrooke
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JoeStillman
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Do you mean utility metering transformers? Because every time I've used VT's for voltage sensing (25, 27, 47 etc), they've been fused.
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Yeah, I agree, VTs are always fused, CTs are NEVER fused.
big john
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Only time I see unfused PTs are at transmission levels.
CTs are unfused because they are current limited by design and because of the open-circuit dangers that would be created were the fuse to blow. Further, as they are often a critical element in protection the idea would be to sacrifice the CT if it meant reliably tripping the protection.
CTs are unfused because they are current limited by design and because of the open-circuit dangers that would be created were the fuse to blow. Further, as they are often a critical element in protection the idea would be to sacrifice the CT if it meant reliably tripping the protection.
mbrooke
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Nearly every MV and HV VT I have seen outside of switchgear has been unfused.
mbrooke
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mbrooke
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Ok, got found this in the NEC at 450.3 (C):
So anything outdoor in the open can be unfused. But my question is why would the NEC allow this?
So anything outdoor in the open can be unfused. But my question is why would the NEC allow this?
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Two things come to mind:
1. POCO equipment is not covered by the NEC, and
2. An overloaded PT outside in the open is not likely to start any fires or be an injury hazard. Like secondary wires on a service transformer, it will just burn out and stop the fault current that way.
I don't know what your code requirements are but I've always fused them.
mbrooke
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Tony S
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For outdoor use the VT is liable to be a capacitive voltage divider. As for fuses, look at the size of them.
mbrooke
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Does the capacitive component make them inherently (self) protected? (genuinely don't know)
GeorgeB
ElectroHydraulics engineer (retired)
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No power consumed (in the ideal case) unlike resistors where you would have I^2R losses
mbrooke
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The high side, low side always has fused protection.
junkhound
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Have seen folks bypass PT fuse on big gensets because they forgot to set the system up to disconnect voltage reg when the engine is shutoff. Voltage stays up while frequency drops, saturates PT and blows fuse and they cannot figure out why th;e fuse blows so they just bypass it !
mbrooke
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That sounds painful for the VT. At least (usually) during serve frequency drops in the grid are often accompanied by a large voltage dip.
If the vts are directly connected to the bus, then, the protection lies of the line protective relays.. If the connection uses a string of conductors normally it has fuse cut outs.
mbrooke
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Wouldn't buss differential cutout first?... At least it would here in the US where nearly all POCO substations have either low impedance or high impedance buss differential relays dedicated to bus protection. Remote zone 3 or a reverse MHO curve is used as a very last resort.
epimetheus
Member
PT (wound) and CCVT failures are usually explosive in nature due to the voltage and fault current available. As had been mentioned above, the zone of protection provides the primary protection for the VT. Also, usually if the VT fails it means your protection is degraded with it and it's not desirable to have the element in service without the VT also in service. Thus, no real reason to go to the trouble of primary fuses at transmission and sub transmission voltages.
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