Understanding TCC Curves

[DMG_1]

I have a question. I have just recently started doing Coordination Studies and feel that I have a pretty good handle on understanding the TCC Curves. However, I do have a question on how to correctly interpret circuit breakers that only have an Instantaneous Trip. The circuit breaker I have a question about is the Siemensd "ED6-ETI" - Motor Circuit Protector. I understand that it has an Instantaneous Trip only but how to know how much time it will take to trip on the low end. The minimum & maximum trip portions of the curve become parallel with the Time Axis. I can assume 2 seconds but this is not what the curve indicates. Any ideas? I tried to upload a curve from SKM Software but the file was too big.

 

[jim dungar]

Welcome to the forum.

What do you mean by the "low end"?
If you are to the left of the curve, the device will never trip. If you happen to line up with the vertical part of the curve, the opening time is undefined and therefore you can assume the device will not trip.

 

[DMG_1]

Understanding TCC Curves

The area I am talking about falls in the trip area. This is between the line on the left indicating breaker starts to open, and the line on the right indicating breaker completely open. Both of these lines are parallel with the time axis so in my mind it is undefined and I cannot use this portion of the curve to coordinate other devices.

 

[templdl]

Coordinating instantaneous trip settings between series breakers is next to impossible as the magnitude of bolted fault current value will be seen by all breakers and should that fault current be within the pickup range of all of the breakers it would be pot luck which one would trip. With arcing faults you may get some coordination as those value are often much less than bolted faults where the breaker with the lowest instantaneous setting most likely would trip.

But you indicated an MCP mag only (instantaneous) beaker which is intended to take a motor off line should the motor experience a winding failure. When the winding starts to faial in a motor it is often a phase to ground fault which escalates. Before MCPs motor fires were a common problem. With an adjustable magnetic trip it is advisable to set the setting as low as you can but still high enough above the motor's inrush current to prevent nuisance tripping on start up. When set as such a motor which trips the breaker should be investigated for a potential winding failure.

Regarding the TCC curve of a mag only breaker there essentially really is no practical curve. You will find that the curve consists of a vertical line which takes basically a right turn toward the bottom which goes practically horizontal there. This curve represents no time delay is would find with thermal curve.

Considering the fact that you are dealing with instantaneous currents which are unpredictable coordination instantaneous trips is very difficult or impossible at best. Electronic breakers do often have an option of a adjustable short time pickup and delay which give you a better opportunity to coordinate in that area.

 

[jim dungar]

The area I am talking about falls in the trip area. This is between the line on the left indicating breaker starts to open, and the line on the right indicating breaker completely open. Both of these lines are parallel with the time axis so in my mind it is undefined and I cannot use this portion of the curve to coordinate other devices.

You are correct.

 

[Jraef]

Just out of curiosity, if you are doing a coordination study on something that is using an ETI breaker, why did you not mention the thermal curve of the OL relay it is asoociated with? I mention that because there are lots of people who don't realize that there is no valid use of an ETI (IT / MCP) breaker other than as part of a factory assembled listed combination motor starter.

 

[DMG_1]

Jraef.

I am glad you asked that question. I was going to post a seperate thread for the OL but I will discuss here. Let me explain this portion of the system.

1. The ED6 ETI circuit breaker is (1) one of the (15) fifteen circuit breakers located in MCC1.

2. MCC1 is fed from a GE (TP & THP, MVT RMS-9) circuit breaker located in SWBD1.

3. When I run the study, the "Instantaneous Trip" portion of the ED6-ETI intersects with the "Long Time Pickup" portion of the GE circuit breaker in the upper decade. I cannot change the LTPU on the GE circuit breaker because of the rating of the plug.

4. There is a set of OL's associated with the ED6-ETI circuit breaker. The OL curve completely intersects the ED6-ETI curve at approximately the 10 second mark.

My question is, will the OL clear a short circuit prior to it tripping the upstream GE circuit breaker located in SWBD1?

I would think that it would clear the fault first but since the ED6-ETI works based on magnetism and the OL is thermal I am not exactly sure.

Thanks for your help on this.



​

 

[Jraef]

The instantaneous (magnetic) trips are ONLY for short circuit current, and the Thermal OLs are only for long time thermal over current / over load. You need to understand the differences with regards to your coordination study. Yes, you want the fault to clear at the lowest level for the magnitude of the fault, so having an instantaneous magnetic trip of an ETI intersect the long time portion of the upstream breaker should not be an issue. But you don't want the instantaneous trip of the ETI to intersect the instantaneous portion of the main, that would be bad. You also don't want the OL realy curve to intersect with the long time curve of the main either, but I've never seen that happen.

 

[jim dungar]

4. There is a set of OL's associated with the ED6-ETI circuit breaker. The OL curve completely intersects the ED6-ETI curve at approximately the 10 second mark.
​

If the OL relay operates in 10 sec, what difference does it make when the breaker might trip?
Effectively you create a hybrid curve consisting of the breaker and the OL relay, just like is done with a 50/51 relay.

While we refer to things like overload (thermal sensing element) and short circuit (magnetic sensing element), the protective devices only care about current magnitude, not its cause.

Of course a fault between the ETI breaker and the OL relay (i.e. a fault internal to the starter) might continue until the upstream feeder breaker operates. Luckily, except for large motors, the available fault current usually exceeds the 'magnetic only' breaker instantaneous setting.

 

[DMG_1]

The thing I was unsure about was the OL. I was not sure if it would clear the fault. However, after talking to you guys it sounds like it will. Thanks for your input.

 

[templdl]

DMG_1
Regarding your reference to a OL, are you refering to the OLR with is included in a motor starter? OL which I am to assume that you are refering to as an "overload" only has a thermal element where you stated that it would clear a fault. Remember that an OLR has a N/C contact in series with the coil of a contactor. When the OLR opens up that contact the coil of the contactor drops out.
As ,such your reference to OL and fault you may be refering to a TM breaker.

 

[kingpb]

Regarding the OP - the area, between the two bands (of the same protective device) is the error band. Which means that for that relay at a certain level of current for a certain time, the relay will trip within that range.

 

[jghrist]

Regarding the OP - the area, between the two bands (of the same protective device) is the error band. Which means that for that relay at a certain level of current for a certain time, the relay will trip within that range.

And for a magnetic only breaker, the vertical band means that currents in this range will either trip instantaneously or not trip at all. You can't assign an intermediate time value. If the worst case is slow, then you have to assume it won't trip ever. If the worst case is fast (like if you were trying to coordinate with a downstream device), then you have to assume it will trip in zero seconds.

 

[kingpb]

And for a magnetic only breaker, the vertical band means that currents in this range will either trip instantaneously or not trip at all. You can't assign an intermediate time value. If the worst case is slow, then you have to assume it won't trip ever. If the worst case is fast (like if you were trying to coordinate with a downstream device), then you have to assume it will trip in zero seconds.

I think this is what you are saying - The line on the left is where the device could trip, but it may or may not trip in between the left hand line and the right and line. At the right hand side, or above it will trip for sure. So, for coordination you need to look at the left side, and for equipment protection you have to look at maximum of right side.

 

[kingpb]

This shows a typical motor startig curve with OLR and MCP that indicates how the MCP and OLR need to work together to provide the proper protection. Also of note, is the cold start and hot start thermal damage curves that need to be included in the protection, i.e. OLR needs to be under the damage curve. I would have put the GE breaekr on, but don't know ratings.

 

[jghrist]

View attachment 5916

I think this is what you are saying - The line on the left is where the device could trip, but it may or may not trip in between the left hand line and the right and line. At the right hand side, or above it will trip for sure. So, for coordination you need to look at the left side, and for equipment protection you have to look at maximum of right side.

I think you basically understand it. I'm not sure what you mean by looking at the left side for coordination and right side for equipment protection. If you are coordinating with (trying to trip faster than) a downstream device (not likely to have a downstream o/c device for a motor protector), you need to consider that the fast trip will not occur for currents below the right hand side. To coordinate with upstream devices, you need to consider that it won't trip for currents below the left hand side. Motor overload protection depends on the thermal overload characteristic.

Just curious - this doesn't have anything to do with the subject - the 20 A mark on the TCC is shown where 30 A should be on a log-log scale. What software was used to create the curve?

 

[kingpb]

Just curious - this doesn't have anything to do with the subject - the 20 A mark on the TCC is shown where 30 A should be on a log-log scale. What software was used to create the curve?

The program is ETAP, and I checked the graph and everything is in the right place. Perhaps it is the monitor resolution making the 3 look like a 2, as the graph shows 1, 3, 5, 10, 30, 50, 100, and so on.