Potential Transformer Primary Fuse Basis for Sizing
Potential Transformer Primary Fuse Basis for Sizing
(OP)
Greetings:
Would be grateful for your thoughts regarding the origin of any bases used for the inrush capability of primary fuses for potential transformers.
I have a GE JVM-3 PT (2400-kV, 750-VA) with an EJ-1 fuse. Also an ancient ITE PT-6 PT (600-VA, 480-V) with Gould A6Y7E-96 fuses......
Can anyone recall the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?
Thanks for your help.
Would be grateful for your thoughts regarding the origin of any bases used for the inrush capability of primary fuses for potential transformers.
I have a GE JVM-3 PT (2400-kV, 750-VA) with an EJ-1 fuse. Also an ancient ITE PT-6 PT (600-VA, 480-V) with Gould A6Y7E-96 fuses......
Can anyone recall the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?
Thanks for your help.






RE: Potential Transformer Primary Fuse Basis for Sizing
A transformer may take up to 25 X FLC during the energization inrush. Many fuses rated at 12 X FLC will carry through an energization inrush due to the thermal lag, but some designers may prefer the added safety of 25 X FLC rated fuses.
Bill
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"Why not the best?"
Jimmy Carter
RE: Potential Transformer Primary Fuse Basis for Sizing
Any change in fuse setting if Pts are of capacitor voltage type instead of electromagnetic?
RE: Potential Transformer Primary Fuse Basis for Sizing
Possibly some others will post in here.
As to interturn faults, the purpose of the fuse is not to protect the transformer, it is to protect the wiring feeding the transformer.
When an inter turn fault does develop into a short, the fuses will disconnect and prevent the primary conductors from vaporizing and destroying the cubicle.
Interturn faults tend to be short lived. The heat generated causes the fault to deteriorate quite rapidly.
Even a short on the transformer secondary should not blow the primary fuses on a PT.
Consider a PT fed with #10 AWG conductors. The thousands of Amps developed by a fault will exceed the withstand capacity of the #10 AWG conductors and most likely vaporize the conductors.
The purpose of the fuses is to avoid this possible damage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Potential Transformer Primary Fuse Basis for Sizing
Further, if the primary fuses DO blow, my maintenace staff is told "DO NOT replace the fuses and re-energize the PT until tests have been done but a competent and knowledgeable testing technician."
Internal faults in a PT are often of a nature that does not show up on the typical insulation resistance (DC) tests, no on low voltage ratio tests, a subsequent energization of a failed PT may result in catastrophic (read 'explosive') failure of the PT and much switchgear damage.
old field guy
RE: Potential Transformer Primary Fuse Basis for Sizing
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Potential Transformer Primary Fuse Basis for Sizing
Utilities generally do not fuse high-voltage PTs, at least in my limited experience.
RE: Potential Transformer Primary Fuse Basis for Sizing
I have seen a few installations where the 0.5E fuses blew fairly often on energization of the gear. The last one that comes to mind sits across the fence behind my office, and they used to lose power several times a week. About one in twenty of those events resulted in a blown PT fuse on power restoration. We diligently tested PT's and finally decided to go up to 1E fuse. Problem solved.
This was on an indoor, metal-clad lineup. The practice in the utility I used to work with was mixed. At transmission voltages, no fuses. Of course, do YOU want to find or pay for a 230-kV, one-amp fuse?
At distribution voltages, fuses were common.
old field guy
RE: Potential Transformer Primary Fuse Basis for Sizing
RE: Potential Transformer Primary Fuse Basis for Sizing
It depends.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Potential Transformer Primary Fuse Basis for Sizing
RE: Potential Transformer Primary Fuse Basis for Sizing
The construction of the fuse is such that the burning of the current-carrying element is quenched by non-conductive materials.
old field guy
RE: Potential Transformer Primary Fuse Basis for Sizing
RE: Potential Transformer Primary Fuse Basis for Sizing
Exactly how staged fault testing and arc flash testing is performed.
RE: Potential Transformer Primary Fuse Basis for Sizing
No one has cited this one yet...
From IEEE C57.13 7.7 Short-circuit capability
Voltage transformers shall be capable of withstanding for 1 s the mechanical and thermal stresses resulting from a short circuit on the secondary terminals with full voltage maintained on the primary terminals. "Capable of withstanding" shall be interpreted to mean that, if subjected to this duty, the voltage transformer shall show no damage and shall be capable of meeting the other applicable requirements of this standard.
RE: Potential Transformer Primary Fuse Basis for Sizing
RE: Potential Transformer Primary Fuse Basis for Sizing
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Potential Transformer Primary Fuse Basis for Sizing
As stated, the purpose of fusing the primary of a VT is to keep the system protection from operating in the event of a VT failure, i.e. clear the VT from the line.
RE: Potential Transformer Primary Fuse Basis for Sizing
I thought, may be interturn fault will not create fuse operation immediately.But it will lead to higher currents and then fuse will save from a violent explosion of PT
RE: Potential Transformer Primary Fuse Basis for Sizing
What I'm saying is that a primary fuse will generally not operate until AFTER the VT blows-up/cracks/fails violently. The primary current flow prior to a physical failure is generally not high enough to blow a primary fuse.
In some cases with ferro-resonance, a primary fuse will operate prior to a physical failure, but that depends on the actual failure mode as a result of the ferro-resonance.
RE: Potential Transformer Primary Fuse Basis for Sizing
The HV fuses for PT's aren't there to protect the PT. they're in the circuit to prevent a PT fault from affecting the source switchgear/buswork.
old field guy
RE: Potential Transformer Primary Fuse Basis for Sizing
Thank you for your contributions.
I first set out to answer my original question, "[What is] the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?", but have been delighted in learning more than I have wondered.
After searching, one answer to my original question is found in IEEE-242 (Buff Book), which describes the basis for that requirement as derived from the "hot-load pickup" phenomenon, which requires magnetizing inrush points at 14x FLA at T=1s and 28x FLA at T=0.01s.
A nice article on the subject (but not necessarily for PTs): ht
Now wondering "why not" the same criteria for 480-V PTs (above is all medium voltage).....