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 Post subject: Time Current Curves of Circuit breakers
PostPosted: Thu Dec 13, 2012 6:46 am 
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[SIZE=14px]Are all circuit breakers designed according to International/National Standards?[/size]
[SIZE=14px]I have to do an arc flash analysis. I have seen a Merlin Gerin compact NS1250 N in a plant. The latest version of ETAP library does not have the same listed. One of my friends says you can take an equal rated breaker from another manufacturer. He says that these breakers follow IEC 60947 and the time current curves will be matching for similar rated ones (circuit breakers). I am not sure about that statement. Please share your experience/views.[/size]
[SIZE=14px]Thank you in an advance. [/size]


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PostPosted: Fri Dec 14, 2012 8:42 am 
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The time-current curves definitely do not match between manufacturers, you have to look at the specifications and model it in your software. For example, the short time pickup of the trip unit may be defined as a factor of frame rating, sensor rating or long time pickup setting, setting ranges and possibilities may be different, the list goes on. There are no requirements in IEC 60947 that limit how the trip unit determines the pickup (and time!).

Unless you are 100% everything is equal you can use a different model from the library, but check it yourself. In case of your Merlin Gerin NS1250 N you could have some luck in finding a model of an equivalent UL rated model, brand labeled as Square D or Federal Pioneer. Just be sure to adjust the right ratings to their IEC equivalents and see if the trip unit does match your own.


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PostPosted: Sat Dec 15, 2012 12:31 am 
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Thanks jvrielink. We use ETAP (as already mentioned) and it does not have the library for many other models of breakers. I came across some digitising software for modelling the breaker in ETAP. I have seen engauge digitizer (its a freeware). I can digitize the TCC of the breaker (get the coordinates of TCC) from the respective manufacturer catalogue/datasheet with the help of Engauge digitizer. I am able to create models of the required breaker. Now the question is - Is this model as good as the ones created by ETAP? If not, where does it lack? Or should I just bother ETAP folks to fetch me the required models.

Sorry I am not allowed to post the links to the software(engauge digitizer, Pl Google).


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PostPosted: Sat Dec 15, 2012 4:21 am 
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What kind of trip unit? I've modeled a lot Merlin Gerin breakers in SKM Powertools so I might be able to help.

To model it correctly you need at least the right tolerances for pickup/clearing time of the individual stages as well as the way the trip unit calculates the pickup setting. For arc flash you'd only need the clearing time, but for protective device coordination you want both. I can't tell you exactly how you build the model in ETAP, but if it is anything like SKM it's going to be a little daunting at first. Basically you construct one or two functions for each setting or stage on the breaker, and put in all the applicable limits and options from the documentation. For overloads you would use I2t or curvepoints type function, instantaneous would be a simple pickup etc. Once you get good at it it's incredibly powerful and fairly quick to do. Especially if you're working with older IEC equipment that's not in the libraries of US software packages it's something I would recommend to spend some time on.

Asking ETAP to do the modeling is an option, but my experience with that kind of thing is that it's garbage in, garbage out. To get a good model you still need to collect all the data for it, and be able to check it afterwards.

Also that Engauge software is nifty, I made something similar but this is a lot better.


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PostPosted: Sun Dec 16, 2012 11:04 pm 
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Trip unit is Micrologic 6.0A. The data sheet can be obtained online. The tolerance is listed as +-15%. I have come across some online tutorials for building our own model of breaker in ETAP. I shall try them. My question now stands answered-That TCC curves vary between manufacturers and there is no such statutory requirement from the International/National standards to use the same TCC while manufacturing any breaker(of some rating).

One final question : Can the TCC of relay (say Alstom CDG 31) be modeled with the same procedure as we model a circuit breaker? Is it as simple as inputting the TCC curve points (which has the pick up settings) and setting the tolerances as is the case for a circuit breaker?

Thank you jvrielink.

http://www.electricalmanuals.net/files/LV-EQUIP/BREAKERS/SQUARE-D/CURVES/613_SetD.pdf.


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PostPosted: Mon Dec 17, 2012 12:38 am 
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The procedure is the same, but tolerances are usually not included for relays (not sure about ETAP). Instead a breaker time is used for getting the total clearing time for arc flash calculation. So you end up with a model that's basically just a line and a fixed time defined for the device.

For relays it's usually easier to adapt an existing model. For a CDG31 just copy the inverse function (BS142 / IEC255) from another relay and tweak the setting options.


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PostPosted: Mon Dec 17, 2012 4:56 am 
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Have you tried to contact ETAP to see if they already have the breaker modeled and are waiting to install in the next upgrade. I have had this problem with SKM, I called them and they emailed me the fix to upload to my library. Give it a try.


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PostPosted: Mon Dec 17, 2012 6:50 am 
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Yes you should contact ETAP first. Do you work for the employer or are you an engineering consultant doing power system studies?


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PostPosted: Mon Dec 17, 2012 11:08 am 
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I work for an employer. My boss visited the ETAP folks in New Delhi, India. He said that the ETAP guys are suggesting to use a different make and change the trip settings to obtain the required TCC, adjust the settings until the TCC matches the required one. I wasn't sure if that was possible (exact match of the required TCC) or for that matter yielded conservative results!


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PostPosted: Mon Dec 17, 2012 11:58 am 
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If you have the latest version of ETAP, then the library already has the Micrologic 6 trip unit, with all the curves you need. All you have to do then is set up a new circuit breaker, enter the NS1250 information in the ratings page, and then associate the trip unit with the circuit breaker.


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PostPosted: Mon Dec 24, 2012 12:09 am 
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Thank you all for your replies.


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PostPosted: Mon Jan 07, 2013 3:00 pm 
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Girish wrote:
[SIZE=14px]Are all circuit breakers designed according to International/National Standards?[/size]
[SIZE=14px]I have to do an arc flash analysis. I have seen a Merlin Gerin compact NS1250 N in a plant. The latest version of ETAP library does not have the same listed. One of my friends says you can take an equal rated breaker from another manufacturer. He says that these breakers follow IEC 60947 and the time current curves will be matching for similar rated ones (circuit breakers). I am not sure about that statement. Please share your experience/views.[/size]
[SIZE=14px]Thank you in an advance. [/size]


LV breakers inverse curve typical are not the same for all manufacturers. It is the HV relay ones (IEC type) which are the same and is based off the IEC equation. Only catch with the HV relay time delay setting is that they can come as either "T" in seconds or "TMS" which is a factor.

To answer your question, no you cannot use a similiar rated one and rename it. But what you can do is find a breaker with the same trip unit (for your case its a Micrologic 6) and use that as a start. More than likely the settings will be the same but your "In" will be based on your breaker size. However, it is always good to check the setting against the manufacturer's datasheet. Get yourself familiar with the breaker is the most important thing and you will find the Schneider range of breakers very easy to model (at least in SKM).


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