Selective Coordination to 0.01

[lielec11]

I did a google search and came across a similar thread from last year but it had been closed...

NEC 2008+ requires selective coordination for 700 and 701 systems. Everything I read mentions they need to be coordinated down to 0.01. My main question is where does it specifically mention 0.01s? Also, is this for the emergency (generator) side only, or do I need to coordinate with all my normal side ATS feeders as well.

Thanks,

 

[ron]

700 and 701 is the emergency path only. Once you get to the load side of the ATS, you need to keep going down to the 700 or 701 loads.

Some jurisdictions have clarified to 0.01 and some have clarified to 0.1 (NYC).

 

[lielec11]

So from the load side of the ATS to the branch circuit breakers only... A follow up question I have is do I need to worry about the short circuit current from my utility for this coordination or only the generator contribution when running? If I need to worry about my utility contribution (normal side), it makes coordination a much harder problem unless I oversize every circuit breaker which the client will not be happy with.

 

[don_resqcapt19]

The 2014 NEC has a new definition. It requires coordination to time "zero". Note that Article 517 has its own definition and that one only requires coordination to 0.1 seconds.

Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.

 

[jim dungar]

it makes coordination a much harder problem unless I oversize every circuit breaker which the client will not be happy with.

Yes, you need to consider the utility fault current but only up to the 'normal source feed' to the ATS. Imagine if a single branch circuit fault takes out an entire 42 circuit panelboard.

Oversizing breakers is not always a requirement, we often get 'selective coordination' down to .01s although electronic breakers are usually somewhere in the mix. When possible, use the coordination charts from your equipment vendor.

 

[charlie b]

The 2014 NEC has a new definition. It requires coordination to time "zero".

I haven't bought the 2014 yet, as we will still be on the 2008 for another 3 months or so. But just reading the definition that you quoted, and having no other context to go by, I would venture to disagree that it requires coordination to time zero. For example, if the maximum fault current that is available at one specific point within a distribution system is, let us say, 10,500 amps, and if looking at the OCPD that was actually selected and installed for this particular project and that is immediately upstream of that point is, let us say, 0.025 seconds, then coordination is required to be demonstrated down to that time level, not to time zero. It does not matter that the may be other OCPDs with faster clearing times that you could have chosen and installed instead, what matters is the available fault current and the tripping time of the installed OCPD under that fault current.

Thoughts? Comments? Shall the debate rage on?

 

[charlie b]

OK, on second thought, I must remind myself that requirements are not established within definitions. I would have to see how 700 and 701 state the requirements for selective coordination, in order to see whether "time zero" comes into play there. Can someone post the two articles?

 

[jim dungar]

OK, on second thought, I must remind myself that requirements are not established within definitions. I would have to see how 700 and 701 state the requirements for selective coordination, in order to see whether "time zero" comes into play there. Can someone post the two articles?

You need to selectively coordinate 'to time zero' based on the available fault current not the entire operating range of the devices.

 

[lielec11]

Yes, you need to consider the utility fault current but only up to the 'normal source feed' to the ATS. Imagine if a single branch circuit fault takes out an entire 42 circuit panelboard.

Oversizing breakers is not always a requirement, we often get 'selective coordination' down to .01s although electronic breakers are usually somewhere in the mix. When possible, use the coordination charts from your equipment vendor.

Jim,

So if my ATS feeds a distribution board which in turn feeds panel risers, you're telling me I only need to worry about the available short at the ATS? That doesn't seem right. Am I misunderstanding your point?

 

[charlie b]

You need to selectively coordinate 'to time zero' based on the available fault current not the entire operating range of the devices.

I am not sure we are disagreeing here. What I am saying is that if the available fault current is low enough that I can avoid the overlap in the instantaneous range that is the real issue with coordination, then it would not matter what might have happened with a higher fault current (i.e., one that would have put me into the instantaneous overlap range). Put another way, "time zero" means nothing, if I can achieve coordination at the actual maximum available fault current.

 

[lielec11]

see picture below... if the operating range of my breakers in the instantaneous region extends all the way to 10K and beyond, how will they ever be coordinated? I feel like this is impossible.

 

[jim dungar]

see picture below... if the operating range of my breakers in the instantaneous region extends all the way to 10K and beyond, how will they ever be coordinated? I feel like this is impossible.

The breakers you have chosen will not coordinate.
You are using old generation electronic trip breakers. Some of Square D's PowerPact Micrologic trip breakers have the ability to turn off the instantaneous portion of the breaker.

For Square D breakers, I use this webpage.
http://www.schneider-electric.us/si...nsulting-engineer/selective-coordination.page

 

[jim dungar]

So if my ATS feeds a distribution board which in turn feeds panel risers, you're telling me I only need to worry about the available short at the ATS?

That is not what I said.

You must coordinate based on the fault current that flows through your protective devices.

Coordination is best done working from the load towards the source.
You need to coordinate from the lowest branch breaker up to the normal source feeder of the ATS, it is unlikely you would need to coordinate all the way up to the 'main device' at the service entrance.

I would normally discourage any one from designing a system that has more than three protective devices in series when selective coordination is required.

 

[jim dungar]

Put another way, "time zero" means nothing, if I can achieve coordination at the actual maximum available fault current.

Absolutely.

 

[ron]

The breakers you have chosen will not coordinate.
You are using old generation electronic trip breakers. Some of Square D's PowerPact Micrologic trip breakers have the ability to turn off the instantaneous portion of the breaker.

For Square D breakers, I use this webpage.
http://www.schneider-electric.us/si...nsulting-engineer/selective-coordination.page

Jim,

Most folks are not using 30 cycle rated equipment, so turning off instantaneous is not an option for those people.

 

[dkidd]

The link below is to a good presentation.

http://ewh.ieee.org/r3/atlanta/ias/...e Coordination - Challenges and Solutions.pdf

 

[jim dungar]

Most folks are not using 30 cycle rated equipment, so turning off instantaneous is not an option for those people.

For UL489 breakers, turning off the Instantaneous function does not remove instantaneous protection from the breaker. I am not aware of any UL489 listed breaker that has a 30 cycle withstand rating.

This is from page 15 of Guide to Power System Selective Coordination 600V and Below, on the webpage I provided.

"Of particular importance to the tripping characteristic is the instantaneous override level. For currents above this override level, even if the instantaneous characteristic is turned off, the circuit breaker will trip instantaneously. The override level is factory-set to protect the circuit breaker according to its short-time withstand capability. Therefore, the higher the withstand level, the higher the override is set. This is an extremely important concept and often determines whether two circuit breakers in series selectively coordinate. Note also that the tripping times for the instantaneous characteristic and for currents above the override level are nonadjustable."

 

[ron]

For UL489 breakers, turning off the Instantaneous function does not remove instantaneous protection from the breaker. I am not aware of any UL489 listed breaker that has a 30 cycle withstand rating.

This is from page 15 of Guide to Power System Selective Coordination 600V and Below, on the webpage I provided.

"Of particular importance to the tripping characteristic is the instantaneous override level. For currents above this override level, even if the instantaneous characteristic is turned off, the circuit breaker will trip instantaneously. The override level is factory-set to protect the circuit breaker according to its short-time withstand capability. Therefore, the higher the withstand level, the higher the override is set. This is an extremely important concept and often determines whether two circuit breakers in series selectively coordinate. Note also that the tripping times for the instantaneous characteristic and for currents above the override level are nonadjustable."

I misunderstood your comment.

Instantaneous override is effectively just setting the instantaneous to its maximum value, but it still existing in the protection curve. I had power breakers in mind, where if you turn it off, it is really off.

 

[lielec11]

The breakers you have chosen will not coordinate.
You are using old generation electronic trip breakers. Some of Square D's PowerPact Micrologic trip breakers have the ability to turn off the instantaneous portion of the breaker.

For Square D breakers, I use this webpage.
http://www.schneider-electric.us/si...nsulting-engineer/selective-coordination.page

thanks for the info... I am planning on switchboard construction, not switchgear though, it is only a business occupancy with offices and a couple of basic exam rooms (no life support equipment).

Even with the breakers im selecting, ive seen screen shots of others using the same breaker types and it appears that the instantaneous regions are cutoff after certain points.

 

[steve66]

thanks for the info... I am planning on switchboard construction, not switchgear though, it is only a business occupancy with offices and a couple of basic exam rooms (no life support equipment).

Even with the breakers im selecting, ive seen screen shots of others using the same breaker types and it appears that the instantaneous regions are cutoff after certain points.

It sounds like talking about having the instantaneous region stop at the available fault current, like Charlie was talking about.

If you are using SKM software, I believe there is an option to terminate the breaker curve at the available fault current.

Open a TCC curve, select a breaker curve, right click and select "Selected Device Settings", click the "SC Rag" tab. There you can show the Short circuit current on the plot, and choose how to display the curve for the breaker.