TCC / Coordination

[Mike01]

With the adoption of the latest NEC (article 517) comes selective coordination, how is everyone and the AHJ?s dealing with selective coordination? I herd some states it only applies above a certain time (ex. 0.1 sec) is this correct? If so where could I find this requirement for these states or areas? Thanks.

 

[jim dungar]

With the adoption of the latest NEC (article 517) comes selective coordination, how is everyone and the AHJ?s dealing with selective coordination? I herd some states it only applies above a certain time (ex. 0.1 sec) is this correct? If so where could I find this requirement for these states or areas? Thanks.

Selective coordination entered the NEC in 2005. It applies to legally required standby systems in all locations not just health care facilities.

Wisconsin was considering amending article 517 in NEC2008, but after the public comment period they reversed themselves and decided to accept the wording as it stands.

If I was an owner's agent, I would insist on seeing the selective coordination results prior to releasing/approving equipment for construction. If I have the control to select the exact equipment, I am usually able to achieve selective coordination without much hassle. The areas that cause me the most problem are ATS's located near the service entrance equipment instead of near the loads and feeding transformers, 75kVA and larger, from distribution panels instead of directly from the service equipment (it is hard to coordinate more than 4 devices in series).

 

[Mike01]

How?

How?

Coordination of breakers in the instaneous range seems nearly impossible even with electronic trip characteristics in the instanetous range, fuses typically easier. How do you handle coordination of the system when under generator load? Typically if you have a 2500Kva service, and a emergency generator 625Kva/500Kw the available fault will be much less in the generator mode than in the utility power mode, how do you insure the coordination under both circumstances? This could result in not enough fault current??

I thought I herd somewhere that some states amended the code to selective coordination above .02 seconds (an example not exactly sure of the time specified). Have you herd of this?

 

[MJJBEE]

Coordination of breakers in the instaneous range seems nearly impossible even with electronic trip characteristics in the instanetous range, fuses typically easier.

I believe you may have been listening to a fuse guy for too long. With good relays and breakers Coordination even in the instant range is very possible. Furthermore I would tell you that breakers with Modern electronic trips are more flexible then fuses in coordination it takes some engineering time to generate and looking at the curves, but on a large system that needs to be run all the time it is well worth it. As for the smaller available fault currents from the secondary source there are a few options. one of them is dual range relays based upon ATS position.

 

[zog]

I believe you may have been listening to a fuse guy for too long. With good relays and breakers Coordination even in the instant range is very possible. Furthermore I would tell you that breakers with Modern electronic trips are more flexible then fuses in coordination it takes some engineering time to generate and looking at the curves, but on a large system that needs to be run all the time it is well worth it. As for the smaller available fault currents from the secondary source there are a few options. one of them is dual range relays based upon ATS position.

Right, plus you can test them to verify they still maintain thier T/C charteristics, how do you do this with fuses?

 

[jim dungar]

Coordination of breakers in the instaneous range seems nearly impossible even with electronic trip characteristics in the instanetous range, fuses typically easier. How do you handle coordination of the system when under generator load? Typically if you have a 2500Kva service, and a emergency generator 625Kva/500Kw the available fault will be much less in the generator mode than in the utility power mode, how do you insure the coordination under both circumstances? This could result in not enough fault current??

I thought I herd somewhere that some states amended the code to selective coordination above .02 seconds (an example not exactly sure of the time specified). Have you herd of this?

Fuse selectivity ratios are only good for the specific fuses (and manufacturer) listed in the table. If a down stream fuse is replaced based on the lowest price or easiest availability (a very likely scenario) then selectivity may be lost.

Circuit breaker manufacturers also have selectivity tables that show tested combinations of coordinated devices. These combinations can be fully rated for bolted faults, though they may be at lower SCA levels.

 

[Mike01]

Dual Range Relays

Dual Range Relays

dual range relays based upon ATS position

??

Jim, MJJBEE, Zog, thanks for your responses my question is what exactly is a ?dual range relay? and where can I find more information regarding this? Fuses vs. Breakers do not get me wrong if anything I am a breaker guy however I understand fuses and breakers have good characteristics and bad, and is more dependant on the installation, cost, space, etc. there are many advantages to fuses as well as breakers depending on the situation and use of the building / space,

My recent dilemma consist of a main switchboard with a 600AF/600AT LSIG C.B. (480V) to a MLO distribution panel (600A-480V-3PH-3W) the panel contains multiple 250AF/125AT LSIG C.B. that feed into a 75Kva xfmr, to a single sec. ocpd (C.B. 250AF/250AT) with the conductors going into a wireway than multiple taps to a panel board (125A some 110A MCB) my problem comes in when I leave the switchboard I only have about 65? into an ATS and then 65? to the DP, 150? to the xfmr, 10? disc. Sw. 10? panels. My problem is the fault current (3ph-bolted) on the load side of the breakers is so high the instantaneous always overlap, and the bands (latch/unlatch) are of the same time so it would be a race to see who wins?

 

[jim dungar]

I have a selectivity chart from Square D, that was created by actual factory tests, that coordinates even if the curves show an overlap. For selectivity, with up to 65kSCA @ 480V, their circuit would be:
a PJ 1200AF/100AT breaker to a 75kVA transformer, the secondary main breaker would need to be an LA-MC 400AF/250AT and the branches breaker could be the normal QO/QOB up to 100A.

Have you thought of using a transformer smaller than 75kVA?
Can you turn off the I function of your 600A main?

 

[Mike01]

1200 for 100.

1200 for 100.

I was looking at cutler hammers molded case breakers type ?JG? with electronic trips the 250AF series with lsig ?I? could disable the ?I?portion but the 2000A main the will already be disabled an I did not want to do this on the branches as the arc flash will skyrocket and be unapproachable so trying to keep the arc flash reasonable, but still coordinate as this falls under article 517, another thing is I would have to use a 1200AF breaker and set the trip to 100A the 1200A would take up a lot of space in the distribution panel and seems like a waste of money (8-10K) and space to by 1200A worth of breaker so I can dial it down to 100A sounds ridiculous to me. It might be worth looking into fuses from an economical standpoint. Thanks for all your help.

Just curious where can I find information regarding the ?dual range relays?

 

[jim dungar]

Have you asked C-H if they have any recommendations like Square D does?

If your fault current is so high, how are you addressing the common issue of the ATS's withstand rating when protected by electronic trip breakers?

 

[beanland]

Selectivity versus Cost

Selectivity versus Cost

With molded-case thermo-magnetic breakers, getting perfect coordination, especially in the instantaneous zone, is essentially impossible. If you introduce electronic tripping elements, you have more control and can tweak relay settings to have acceptable performance. However, as stated before, trying to get good coordination with several devices in series is nigh on impossible. And, the costs of extensive use of electronic trip elements may be hard to justify.

In one such case, we discussed coordination with the AHJ and examined the consequences of mis-coordination. It was agreed that perfect selective coordination for all possible fault scenarios was not justified.

How do you coordinate a 480Y/277V GFI breaker with a 20A 277V lighting circuit? Can't be done.

 

[zog]

I was looking at cutler hammers molded case breakers type ?JG? with electronic trips the 250AF series with lsig ?I? could disable the ?I?portion but the 2000A main the will already be disabled an I did not want to do this on the branches as the arc flash will skyrocket and be unapproachable so trying to keep the arc flash reasonable

You can retrofit with Quick Trip, that takes care of both of those issues. What type of breaker and trip unit do you have?

 

[jim dungar]

With molded-case thermo-magnetic breakers, getting perfect coordination, especially in the instantaneous zone, is essentially impossible.

Definitely not true for all situations, especially at low fault current levels.

It was agreed that perfect selective coordination for all possible fault scenarios was not justified.

I absolutely agree. But in the minds of some AHJ's the actual wording of the NEC does not require 100% selectivity which is why they may be willing to consider "variances".

How do you coordinate a 480Y/277V GFI breaker with a 20A 277V lighting circuit? Can't be done.

You do not need to, which is good because it is also extremely difficult to coordinate fuses with GF relays. The selective coordination sections apply to overcurrent only. I can usually get GF devices to coordinate with 30A breakers but that is it.