Is the up-over-down method still allowed when protecting equipment in regards to fault-current? Specifically HVAC equipment that is rated lower than what the actual fault current is calculated to be? I've heard/read differing opinions if this method is still allowed.
up-over-down method still allowed?
- Thread starter bwyllie
- Start date
- Status
AC\DC
Senior Member
- Location
- Florence,Oregon,Lane
- Occupation
- EC
Please explain what UP-over-down is. I have not clue
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
This has not been an acceptable practice for some 40 years.
It's purpose is for things like busbars and cables.
Old bad habits are hard to unlearn.
It's purpose is for things like busbars and cables.
Old bad habits are hard to unlearn.
Last edited:
I second this
AC\DC
Senior Member
- Location
- Florence,Oregon,Lane
- Occupation
- EC
Do you mean Taking conductors up and then back down to increase the length to add more impedance to help lower the Ark fault current.
I can't believe that is what you mean but after Jim comment that is how I took it.
I can't believe that is what you mean but after Jim comment that is how I took it.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
It is a method for determining the let through current for fuses. It has a limited range of applicable usage, but it was/is frequently taught by a major fuse manufacturer.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
I'll go out on a limb to say that an "up-over-down" method must involve simultaneously graphing two functions Y vs X1 and Y vs X2 on a common set of axes. Then starting with the a given value of X1 on the common x-axis, you go up to hit the first graph to find the Y-value. Then you go across (constant Y) to hit the second graph. Then you go back down to the X-axis to find the value of X2.
That gives you the X2 that corresponds to the Y that corresponds to the starting X1.
Cheers, Wayne
That gives you the X2 that corresponds to the Y that corresponds to the starting X1.
Cheers, Wayne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I think it would be best to discuss this methodology in a separate thread rather than causing a derailment of the OP.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Seems like you answered the OP's question in post #3. So I would think an explanation of the method for "busbars and cables", if someone is so inclined, would be a reasonable extension of the thread.
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I've never had to do this. But I did stay at a holiday inn....
The basic idea is that you have a current limiting fuse in a circuit, and want to calculate the available fault current downstream of that fuse. You start with the available fault current, and using a graph specific to the fuse trace a path which calculates the fault current effect downstream of the fuse.
This technique is not acceptable for determining the reduced fault current presented to things such as circuit breakers which exhibit 'dynamic impedance'. This 'dynamic impedance' is the changing resistance as the breaker is trying to open. I don't know enough to understand the details, but the downstream device trying to open can 'see' more of the fault current stress then the up-over-down method might suggest. This is why series rating testing is required when using combinations of OCPD to handle fault current greater than the capabilities of the downstream device.
Components which do not exhibit dynamic impedance such as bus bars and cabling also have maximum fault current limits. During a fault the magnetic forces on a bus bar could rip it from its mounts, for example. The up over down method is considered acceptable for determining the fault current (and fault current stresses) that would be imposed on these static components.
-Jon
The basic idea is that you have a current limiting fuse in a circuit, and want to calculate the available fault current downstream of that fuse. You start with the available fault current, and using a graph specific to the fuse trace a path which calculates the fault current effect downstream of the fuse.
This technique is not acceptable for determining the reduced fault current presented to things such as circuit breakers which exhibit 'dynamic impedance'. This 'dynamic impedance' is the changing resistance as the breaker is trying to open. I don't know enough to understand the details, but the downstream device trying to open can 'see' more of the fault current stress then the up-over-down method might suggest. This is why series rating testing is required when using combinations of OCPD to handle fault current greater than the capabilities of the downstream device.
Components which do not exhibit dynamic impedance such as bus bars and cabling also have maximum fault current limits. During a fault the magnetic forces on a bus bar could rip it from its mounts, for example. The up over down method is considered acceptable for determining the fault current (and fault current stresses) that would be imposed on these static components.
-Jon
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I agree with Jon. Basically no one has been able to figure out how to "calculate" the design of series rated devices that work. Thus the NEC requirement for the tested combinations. Now where there does seem to be some disagreement, is using the up-over-down method or manufacturer let thru charts for downstream SCCR purposes. I have had debates in the past with some of the EE's on here and some say you cant even use such methods for static downstream devices/SCCR purposes. Personally, I do not see any restriction in the NEC, especially considering the 240.2 definition of "Current-Limiting Overcurrent Protective Device." Perhaps the NEC needs a definition for "available fault current".
- Status