Selective Coordination

[foghorn280]

Has anyone else had as much trouble as I am having with selective coordination? The Time Current curves that have been supplied for the hospital job I am inspecting are difficult to read. In some instances the fault is cleared in 0.1 seconds and other in 0.01 seconds. Article 517.17(C) states "A six-cycle minimum separation between the service and feeder ground-fault tripping bands shall be provided." Can that be accomplished in 0.1 seconds? How does one go about determining this? I am about to just ask for a stamped statement from the engineer that the system is coordinated and let the liability hang on his license, any comments would be welcomed.

 

[zog]

What software are you using?

 

[nakulak]

60hertz => .1 sec for 6 cycles unless my math is off

 

[brian john]

Did the engineer supply a coordination study? Somewhere in the coordination study there should be settings for all adjustable CB's.

 

[davidr43229]

I am about to just ask for a stamped statement from the engineer that the system is coordinated and let the liability hang on his license, [/QUOTE\A Coordinated system is not a Selectively Coordinated system. Ask him for the curves. If the instantious regions overlap, you are not Selctively coordinated.
Also don't forget any Emergency circuits.
Ever consider fuses, With a 2:1 ratio you don't have to worry.
If not, the degree of liability should rest on the PE.
Just my $.02

 

[kingpb]

It is not possible to clear a fault with a CB in 0.01 secs (0.6 cycles). This has to be a fuse, and yes it is possible in that short of a time. CB's are generally more around the 5, 6, or 7 cycle duration.

Usually the ground fault curves are done as separate curves and are note shown together with the 3ph curves. If they are shown on the same graph, maybe aske for them to be provided as separate curves.

Could you copy one and post it so we can see what your looking at?

 

[jim dungar]

It is not possible to clear a fault with a CB in 0.01 secs (0.6 cycles). This has to be a fuse, and yes it is possible in that short of a time. CB's are generally more around the 5, 6, or 7 cycle duration.

Power/iron frame/air frame circuit breakers (low voltage and medium voltage) are tripped by "solenoids and relays" and so exhibit a multi-cycle opening curve usually in the 3-5 cycle range. So, for coordination purposes we often use clearing times of 5 and 7 cycles.

Molded case circuit breakers and newer insulated case breakers respond much faster, with typical maximum clearing times of about 2 cycles (.03 sec). The speed at which molded case breaker operate is one reason that they are hard (but not impossible) to employ when selective coordination is required and extremely high fault currents (i.e. those that put fuses into their current limiting region) are involved.

 

[foghorn280]

The "study" that has been provided so far consists solely of TC curves. They all appear to overlap in the 0.01 range. The customer, according to the engineer, does not want fuses, they are "inconvenient and difficult to stock". As far as software, I have none nor have I been provided any to use. The time given by Nakulak is what I thought, but with my limited knowledge reading TC curves, they are no where near close enough. I have a PDF but am unable to post it, I cannot find the attachment icon (?). Thanks for the input all, especially the suggestion to ask for the adjustable CB settings. I will keep you posted as I learn more.

 

[jim dungar]

The "study" that has been provided so far consists solely of TC curves. They all appear to overlap in the 0.01 range. The customer, according to the engineer, does not want fuses, they are "inconvenient and difficult to stock". As far as software, I have none nor have I been provided any to use. The time given by Nakulak is what I thought, but with my limited knowledge reading TC curves, they are no where near close enough. I have a PDF but am unable to post it, I cannot find the attachment icon (?). Thanks for the input all, especially the suggestion to ask for the adjustable CB settings. I will keep you posted as I learn more.

Has a short circuit study been performed? Without knowing how much fault current is going to flow in any given circuit you do not know what points on the TCCs to compare. The NEC does not provide any guidelines for coordination non-GFI protection, as long as the curves do not overlap at the available fault current the devices coordinate. GFI devices must have 6 cycles between them.

I do not believe that an untrained person will be able to correctly selectively coordinate a hospital system (from 120V receptacles to service entrance) simply using PDFs of curves.

 

[mo2004]

try the following website it is siemens TCC free software.

Copy the following address it will take you there

http://www2.sea.siemens.com/Product...rent+Curve+(TCC)+Software.htm?languagecode=en

i hope this helps.

 

[davidr43229]

They all appear to overlap in the 0.01 range

If any of the C/B's overlap within the .1 to .01 range they are NOT Selectively coordinated. I would take this back to the PE who authored the drawings, who should have known better than to produce a product like that. I tell all my Design Engineers, the day of the "cookie-cutter" C/B spec is gone. You can go to the C/B manufacturers website and find Selective Coordinaion charts. 1 C/B has "Optimum" co-ordination charts, which are not the same as Selective Coordination.

Molded case circuit breakers and newer insulated case breakers respond much faster, with typical maximum clearing times of about 2 cycles (.03 sec

Jim Dungar is absolutely correct, newer MCCB total clearing times are getting much faster.
If your customer is talking about the resetability of C/B's VS the inconvience of fuses, it is illegal to reset a C/B (OSHA 1910.334)
Just my $.02