I have an inertial flywheel UPS providing backup power. The connected load is about 75 amps. The output breaker is rated at 100 amps (125%). The UPS cannot provide more than 80 amps under any condition therefore it never can clear the 100 amp breaker if there is a fault. In addition, a fault will act an interial brake stoping the unit within a short time. Is there any mandate, code or otherwise, that a source must be able to deliver enough fault current to open its output breaker? Is there a code violation here?
Power limited sources
- Thread starter lbartsch
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Re: Power limited sources
This is strictly my opinion. These are the two Sections that I believe are relevant to your question:
The question is what is "extensive damage"? It is an undefined term. I would submit that it is damage to components of the circuit not initially involved with the fault and not intended to be sacrificial, such as fuses.
This is strictly my opinion. These are the two Sections that I believe are relevant to your question:
I believe what you described meets the intent if "...circuit-protective devices used to clear a fault [...] do so without extensive damage to the electrical components of the circuit. "
The question is what is "extensive damage"? It is an undefined term. I would submit that it is damage to components of the circuit not initially involved with the fault and not intended to be sacrificial, such as fuses.
ron
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Re: Power limited sources
Are you sure the flywheel can't provide more than 80A under a fault condition? I have not contemplated the issue, other than to consider that any generator has an associated sub transient reactance and would have some time of contribution to a fault above its FLA rating.
I do not think that there is a code requirement for a source to provide sufficient fault current to clear a fault. Although the equipment has to be rated to do so if there is sufficient current available.
Another example includes an inverter without a AC bypass, which is somewhat limited in its ability to source a fault to more than approx 300% in many cases.
Are you sure the flywheel can't provide more than 80A under a fault condition? I have not contemplated the issue, other than to consider that any generator has an associated sub transient reactance and would have some time of contribution to a fault above its FLA rating.
I do not think that there is a code requirement for a source to provide sufficient fault current to clear a fault. Although the equipment has to be rated to do so if there is sufficient current available.
Another example includes an inverter without a AC bypass, which is somewhat limited in its ability to source a fault to more than approx 300% in many cases.
Re: Power limited sources
Your UPS generator should have some sort of shunt trip mechanism to protect itself. So if your conductors are sized to the OCPD, it should be impossible for any damage to occur since you generator cannot produce enough current to harm the conductors.
Your UPS generator should have some sort of shunt trip mechanism to protect itself. So if your conductors are sized to the OCPD, it should be impossible for any damage to occur since you generator cannot produce enough current to harm the conductors.
Re: Power limited sources
Output conductors are rated for the OCPD (100A). Esentially, I see this as an intrinsicly safe system. Although the OCPD can never be tripped because the flywheel can't generate enough power to exceed the trip levels, neither can the flywheel exceed the rating of the conductors. I would guess that the flywheel system is a current limited source.
I beleive that the same would apply for a DC-DC converter system that feeds a single breaker rated at or above the total system ampacity. there's never sufficient current to trip the OCPD.
Output conductors are rated for the OCPD (100A). Esentially, I see this as an intrinsicly safe system. Although the OCPD can never be tripped because the flywheel can't generate enough power to exceed the trip levels, neither can the flywheel exceed the rating of the conductors. I would guess that the flywheel system is a current limited source.
I beleive that the same would apply for a DC-DC converter system that feeds a single breaker rated at or above the total system ampacity. there's never sufficient current to trip the OCPD.
Re: Power limited sources
On another note, I have been told by the manufacturer that the device cannot generate more than 80A. I can only assume this to be correct and when you think about teh fact that it is an enertial flywheel, the fault will provide braking action since there is no active prime mover.
On another note, I have been told by the manufacturer that the device cannot generate more than 80A. I can only assume this to be correct and when you think about teh fact that it is an enertial flywheel, the fault will provide braking action since there is no active prime mover.
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