# Selective coordination



## Handasee (Dec 18, 2009)

Customer had a 100HP 600V motor short to ground. The motor was protected by FRSR 200A fuses, upstream by 400A fuses, 800A fuses and at the service a
1200A circuit breaker. The circuit breaker tripped, no fuses tripped.
The circuit breaker is a N frame C series Cutler Hammer NDC312T36 with Digitrip RMS 310.
Short delay pickup is set at 2XIr with 200ms delay. Ground Fault pickup is set at 800A with 150ms delay. 
How do I prevent the main circuit breaker from tripping?
Anyone have an reference material on the subject. (I have the trip curves for the fuses and circuit breaker)
The motor fuses will be changed to 150 amps as the original is too high per CEC.


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## wcord (Jan 23, 2011)

Handasee said:


> Customer had a 100HP 600V motor short to ground. The motor was protected by FRSR 200A fuses, upstream by 400A fuses, 800A fuses and at the service a
> 1200A circuit breaker. The circuit breaker tripped, no fuses tripped.
> The circuit breaker is a N frame C series Cutler Hammer NDC312T36 with Digitrip RMS 310.
> Short delay pickup is set at 2XIr with 200ms delay. Ground Fault pickup is set at 800A with 150ms delay.
> ...


There is some software out there that you can buy or even get for free (Siemens)
However, your best choice would be to hire Eaton and have them do the study for you. They need the distances, size of conductors, along with the info you have shown.
There may be someone on here who has the software and could do a study for you. 
Good luck


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## 99cents (Aug 20, 2012)

Handasee said:


> Customer had a 100HP 600V motor short to ground. The motor was protected by FRSR 200A fuses, upstream by 400A fuses, 800A fuses and at the service a
> 1200A circuit breaker. The circuit breaker tripped, no fuses tripped.
> The circuit breaker is a N frame C series Cutler Hammer NDC312T36 with Digitrip RMS 310.
> Short delay pickup is set at 2XIr with 200ms delay. Ground Fault pickup is set at 800A with 150ms delay.
> ...


What are the curves telling you? The fuses should have blown first, I'm sure. You can try LPS fuses but something's wonky here.


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## MechanicalDVR (Dec 29, 2007)

99cents said:


> What are the curves telling you? The fuses should have blown first, I'm sure. You can try LPS fuses but something's wonky here.


According to my calculator the maximum fuse should have been a 150 amp, the FLA being 99 amps.


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## 99cents (Aug 20, 2012)

MechanicalDVR said:


> According to my calculator the maximum fuse should have been a 150 amp, the FLA being 99 amps.


Hardly an difference on short circuit.


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## Lone Crapshooter (Nov 8, 2008)

Do not mess with the settings have a coordination study done there several functions that the 310 Digitrip can do. Most any testing company can can help with the study. Also have a Digitrip tested. 
Digitrip have bands bands not not lines like a protective relay . I think +/- 30 % of the settings. From my experience with the long time settings are a lot closer than +/- 30% but be aware 30% difference is not out of the possibility. 
The Digitrip testers that I have used forget accurate timing in the short time settings . The testers are just not that accurate.
I have probably checked 100 or more Digirtips bit it has been 3+ years now that I have done one.

LC


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## telsa (May 22, 2015)

Down here in the States, a short-to-ground means an INSTANT GFI trip for any Service 1000 A on up at 480Y277 on up.

Pray tell that this Service did not have a similar trip-out logic built in to it ?

From Eaton's web page dedicated to it, it comes with GFI logic built in.

"(7) JG- and LG-Frames are Instantaneous, 120 ms. NG- and RG-Frames are Instantaneous, 100, 300 and 500 ms."

http://www.klocknermoeller.com/EatonMCCB/digitrip.html

Just sayin'...


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## MechanicalDVR (Dec 29, 2007)

99cents said:


> Hardly an difference on short circuit.


Maybe if something tripped sooner......


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## 99cents (Aug 20, 2012)

telsa said:


> Down here in the States, a short-to-ground means an INSTANT GFI trip for any Service 1000 A on up at 480Y277 on up.
> 
> Pray tell that this Service did not have a similar trip-out logic built in to it ?
> 
> ...


So the breaker tripped by design?


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## Handasee (Dec 18, 2009)

MechanicalDVR said:


> According to my calculator the maximum fuse should have been a 150 amp, the FLA being 99 amps.


I did the calculation also and as stated in the original post the fuse has been changed to 150A


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## Handasee (Dec 18, 2009)

telsa said:


> Down here in the States, a short-to-ground means an INSTANT GFI trip for any Service 1000 A on up at 480Y277 on up.
> 
> Pray tell that this Service did not have a similar trip-out logic built in to it ?
> 
> ...


The Digitrip is a 310 not a 310+
The only trip curves that I have found are these,
http://docs.natlswgr.com/docs/W1000140.pdf


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## telsa (May 22, 2015)

Instructions for the Digitrip RMS 310 3-Pole and 4-Pole Trip Unit Installation
and Operation with L-Frame and MDL-Frame Series C Circuit Breakers

Trip UnitTvpe
4-pole Type Trip Unit
3-pole Ground Fault Trip Unit
3-pole Non-Ground Fault Trip Unit


Ground fault trip units are supplied from the factory with a
wire harness with pigtail lead connections for a neutral
current sensor (white and grey wires) and a ground fault
alarm relay (yellow and green wires). A neutral current
sensor is provided with each trip unit, and the ground fault
alarm relay is ordered and shipped separately if required..

.

and so forth.

For us the GFI logic is mandatory -- and has been for quite some time.

Since Canada is within the NEMA universe, I'd not be surprised if C-H shipped EVERY 310 unit in that ampacity with GFI logic.


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## Southeast Power (Jan 18, 2009)

Handasee said:


> Customer had a 100HP 600V motor short to ground. The motor was protected by FRSR 200A fuses, upstream by 400A fuses, 800A fuses and at the service a
> 1200A circuit breaker. The circuit breaker tripped, no fuses tripped.
> The circuit breaker is a N frame C series Cutler Hammer NDC312T36 with Digitrip RMS 310.
> Short delay pickup is set at 2XIr with 200ms delay. Ground Fault pickup is set at 800A with 150ms delay.
> ...


You didn't mention any motor protection beyond basic dual element time delay fuses that were oversized to begin with.
I would have an issue with that as a facility operator. It's rare for a motor to just short to ground with no warning of a bearing drag or overload.
Aside from all of that, I don't think you need much of a third party study to realize that a 100 amp 600volt ground fault will open a 150ms ground fault breaker ahead of shelf stock dual element time delay fuses.
I think the breaker did what it was designed to do and you for sure, with the equipment described, have a motor protection problem and not a circuit breaker problem.


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## Handasee (Dec 18, 2009)

Suncoast Power said:


> You didn't mention any motor protection beyond basic dual element time delay fuses that were oversized to begin with.
> I would have an issue with that as a facility operator.


The motor is controlled by a starter that has overloads correctly sized for the load. 


Suncoast Power said:


> It's rare for a motor to just short to ground with no warning of a bearing drag or overload.


The customer sent the motor out and it was returned with the note "its shorted to ground". The rewind shop was only interested in selling a new motor.
I have asked the owner to send the motor to another shop to analyze why it shorted. I checked the bearings and they seem fine. Overload would of tripped if that was a problem.


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## MechanicalDVR (Dec 29, 2007)

Handasee said:


> The motor is controlled by a starter that has overloads correctly sized for the load.
> 
> The customer sent the motor out and it was returned with the note "its shorted to ground". The rewind shop was only interested in selling a new motor.
> I have asked the owner to send the motor to another shop to analyze why it shorted. I checked the bearings and they seem fine. Overload would of tripped if that was a problem.


Seems out of the norm that a motor would burn insulation and short to ground and not trip a properly sized overload. How long was this motor in service?


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## sbrn33 (Mar 15, 2007)

I had a 20HP motor go to ground just this week and trip a 1200 amp Sq-D main. I had no trouble turning up the GFI settings a bit. The Sq-D site is ****ing horrible if anyone cares.


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## Handasee (Dec 18, 2009)

MechanicalDVR said:


> Seems out of the norm that a motor would burn insulation and short to ground and not trip a properly sized overload. How long was this motor in service?


Motor has been in service for 8 years


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## MechanicalDVR (Dec 29, 2007)

Handasee said:


> Motor has been in service for 8 years


Then I'd be most concerned with why the overloads didn't kick out.


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## telsa (May 22, 2015)

MechanicalDVR said:


> Then I'd be most concerned with why the overloads didn't kick out.


The OP has not addressed whether the MAIN was GFI protected.

I'm not exaggerating when I say that most electricians I've known are surprised that the MAIN has GFI logic.

A GFI MAIN is going to look almost identical to a normal MAIN -- as the GFI logic is a module that can even be installed in the field.

it's not as if the MAIN has a glowing banner that says: "Look at me, dummy !"

As for "instant trip" -- you never saw a breaker move quicker. There is NO WAY that any other OCPD will kick in. The electronics are THAT sensitive, THAT fast.

You won't even hear the gear 'protest' with short-circuit vibrations in the conductors.

The only "boom" is the breaker, itself, flying to an open position. 

That's so loud you can hear it through concrete and half a block away. :thumbsup:

EVERYTHING the OP has posted is consistent with having a GFI'd MAIN.


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## Big John (May 23, 2010)

99cents said:


> So the breaker tripped by design?


Pretty much. 

It can be difficult or impossible to coordinate ground faults with fuses and thermal breakers precisely because it takes so much energy to operate those versus a 1200A maximum pickup on the main ground-fault protection.

Looking at the curve for OP's fuses it appears he would've had to pass at least 3kA of steady fault current to get his fuse to blow first. Even if he cranked his main GFP settings to maximum that number is only dropped to ~2kA. How do you ensure that?

Where coordination is really critical it's done electronically: All the downstream breakers and relays will have their own GFP or ZSI settings that allow them to be programmed to clear the fault before the main.


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## 99cents (Aug 20, 2012)

Big John said:


> Pretty much.
> 
> It can be difficult or impossible to coordinate ground faults with fuses and thermal breakers precisely because it takes so much energy to operate those versus a 1200A maximum pickup on the main ground-fault protection.
> 
> ...


So a stupid question: What is the purpose of the fuses?


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## Big John (May 23, 2010)

99cents said:


> So a stupid question: What is the purpose of the fuses?


Protection against severe equipment overloads and phase-to-phase shorts.

Very often the upstream main breakers are provided without instantaneous trip protection, again because it's very difficult to coordinate with thermal. So the fuses/MCCBs would be responsible for clearing most shorts that didn't involve ground, the main would be responsible for clearing most shorts that did involve ground.


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## Handasee (Dec 18, 2009)

In my original post, I've said that the Main Circuit Breaker had a short delay pick up and a ground fault pick up. Here is what they look like:


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## Southeast Power (Jan 18, 2009)

Handasee said:


> The motor is controlled by a starter that has overloads correctly sized for the load.
> 
> The customer sent the motor out and it was returned with the note "its shorted to ground". The rewind shop was only interested in selling a new motor.
> I have asked the owner to send the motor to another shop to analyze why it shorted. I checked the bearings and they seem fine. Overload would of tripped if that was a problem.


I just commented that it would be rare for a motor to just go to ground. Our rewind shop makes more money rewinding than selling a new motor, especially a 100hp.
If you have the time, send it to another shop and get another opinion.if it's an older motor, an upgrade might be a better choice.


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## Big John (May 23, 2010)

A ground fault is a very easy failure to find. If you have a megger you can check the motor yourself.


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## Southeast Power (Jan 18, 2009)

Handasee said:


> In my original post, I've said that the Main Circuit Breaker had a short delay pick up and a ground fault pick up. Here is what they look like:
> View attachment 88834


Nice but, you would have to evaluate all of the distribution up and down stream of that device to make a decision on those settings.


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## Southeast Power (Jan 18, 2009)

Big John said:


> A ground fault is a very easy failure to find. If you have a megger you can check the motor yourself.


Interesting that a 100hp motor tripped a ground fault but otherwise appears fine.
I would suspect that a lead coming from the windings chafed through.


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## MechanicalDVR (Dec 29, 2007)

Big John said:


> A ground fault is a very easy failure to find. If you have a megger you can check the motor yourself.


Yes Sir, first and foremost. Before removing it.


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## Handasee (Dec 18, 2009)

Big John said:


> A ground fault is a very easy failure to find. If you have a megger you can check the motor yourself.


The customer removed the motor and sent it to a rewind shop before I was involved. As I said before, they sent it back to the customer with a note that it was shorted to ground. I have told the customer to sent the motor to another shop that is willing to disassemble it and find out exactly what shorted. 

I have a megger that gets well used. 

The question here is how to prevent a short like this from opening the main circuit breaker. What I have not said is that this main circuit breaker also gives power to another unit, so this fault caused two companies down time with associated costs.


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