Your questions answered: Improving the design process for selective coordination applications

Selective coordination applications have a number of variables and challenges to consider during the design process. Here is additional information to supplement the webcast on “Improving the design process for selective coordination applications.”


Several questions were asked by the audience at the Nov. 18, 2015, webcast "Improving the design process for selective coordination applications." Speakers Dan Neeser and Erik Barnes from Eaton and Scott Koelzer from M.E. Group answer some of the most relevant ones here.

Dan Neeser is a Field Application Engineer with Eaton's Bussmann Division. Courtesy: EatonErik Barnes is a Marketing Manager with Eaton's Bussmann Division. Courtesy: EatonScott Koelzer is Chief Electrical Engineer with M.E. Group. Courtesy: M.E. Group

Question: When emergency generators or other power sources are interconnected via main-tie-main or transfer switches, can different scenarios (normal, emergency, etc.) be developed using Selective Coordinator Designer (SCD) v1.0?

Answer: SCD v1.0 provides access to a single circuit diagram. Solutions for systems with multiple circuit paths can be determined one circuit at a time, and results can be exported for documentation purposes when comparing other portions of the system.

Question: Can 80% vs. 100% circuit breakers affect selective coordination, and can they be mixed?

Answer: The continuous current rating does not have any affect on selective coordination since we are concerned with the operation of the circuit breaker under overcurrent conditions, not the continuous current rating of the circuit breaker. 80% and 100% rated circuit breakers can be mixed, often the 100% rated devices are used for mains or large feeders where the user wants to size the bus or conductors at 100% of the continuous current.

Question: Can SCD v1.0 be adapted to an existing distribution system? Can you plug-in existing protection devices?

Answer: SCD v1.0 allows the users to "pre-define" devices as needed. For example, if a certain upstream device had been specified as and RG frame breaker based on a solution from a different circuit path in the system, then the user can choose the RG frame breaker for that device when evaluating other portions of the system, and the results that appear in the solution list will only include those that involve the RG frame breaker at that device.

Question: Are you able to select copper vs. aluminum conductors?

Answer: Currently, SCD v1.0 estimates conductor size/quantity and validates device terminal conductor allowance based on copper conductors.

Question: Are there rules of thumb for molded case breaker ratio sizes that help achieve likely coordination, similar to what is available for fuse tables?

Answer: There are no ampacity ratio rules of thumbs for circuit beakers to achieve selective coordination. Even if it's only a level of coordination to 0.1 sec or above, it is very difficult to determine an ampacity rule of thumb for all applications.

Question: How do you get information on upstream short circuit capacity or do you just go with infinity for a worst case?

Answer: The estimation calculation for fault current assumes infinite power on the primary of the transformer. However, the fault current can be manually entered if known or calculated by other means.

Question: If we would like to coordinate a utility service, should we choose generator or transformer in the software? We may not know the details of the transformer serving the system.

Answer: The user has the option to enter a system with either a transformer or a generator, and provide the necessary information to estimate the fault current if needed. If the transformer size is unknown, then the user has the option to choose devices that will permit a reasonable fault current allowance by viewing the allowable fault current information for each device solution option. This is a main benefit of SCD v1.0, as users have visibility to potential options so they can better anticipate changes that may occur later.

Question: Please discuss fuse and relay coordination and transformer inrush for transformers fed by contactors. Contactors typically have very low interrupting capability and the fuse needs to clear any reasonable faults rather than a relay setting opening the contactor.

Answer: Primary transformer inrush occurs when initially energized and typically is 12X for 0.1 seconds or 25X for 0.01 seconds. As suggested, contactors typically have a low interrupting capability, 10X rated current, and three operations (per UL 508 for motor controller contactors). Because of this, the contactor should not be relied upon to open higher level currents-in those cases the fuse or circuit breaker needs to be properly selected to achieve a combination short-circuit current rating adequate for the available fault current-in which the upstream fuse or circuit breaker opens the circuit and protects the contactor.

Question: Is a minimum time interval for time-current characteristic (TCC) curves required to be considered coordinated?

Answer: There is not a minimum amount of time interval between TCC curves, provided the curves show the associated tolerance of operation. Provided there is "white-space" between the total clearing curve of the downstream device and the minimum trip/melt curve of the upstream device, selective coordination is achieved in the time current curves. However, remember for selective ion coordination, where higher overcurrents and for times below 0.01 seconds are required to be considered, separation of curves may not indicate selective coordination and as such manufacturer tested data must be considered in addition to the time current curves.

Question: What is the cost of SCD v1.0?

Answer: SCD v1.0 is a free tool, and is available at, or through the Eaton PowerEdge app for mobile devices.

Question: Does the program include Eaton MV relays?

Answer: Not at this time. SCD v1.0 is only available for low-voltage applications.

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