Selective Coordination Tools and Spreadsheets - Part One


I will focus in this study of selectivity Coordination on the following main points:

Part One: What is selective coordination?

Part Two: NEC code and Selective Coordination

Part Three: Method of performing a selectivity coordination study
  • First: By using Selective Coordination Time - Current Curves
  • second: By using Selective Coordination Charts/Tables.
  • By using Siemens calculators / spreadsheets.

Part OneWhat is selective coordination?


Selective coordination is a design requirement where only the overcurrent device immediately upstream from the fault will operate, minimizing the outage caused by the fault.

 

 In the diagram above, only device C would open under the fault. For selective coordination, the system would be designed so that for a fault of any magnitude (up to the available fault current at device C) device C is guaranteed to clear the fault with neither B nor A opening.

Part Two: NEC code and Selective Coordination

First: Before NEC 2005 Code:


1- Prior to 2005, Selective coordination was only required for some Elevator applications by article 620.62 Selective Coordination:
"Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices".


2- Until the 2005 NEC, a properly coordinated system was generally regarded as ensuring the timely operation of overcurrent devices to assure safety and protection of equipment (using damage curves) while minimizing nuisance trips. Designers have tried to minimize nuisance trips as much as possible by ensuring that there was no overlap in coverage in the thermal region of the time-current curves, and by adjusting the instantaneous range for as little overlap as possible. However, some overlap almost always existed and was accepted.



Second: in NEC 2005 & 2008 Codes:


1- 2005 NEC Article 100:
“Coordination (Selective)” definition: “Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.”


New requirements were added in the 2005 and 2008 National Electrical Code as follows:

2- Article 700.27 - Emergency Systems:
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.


3- Article 701.18 - Legally Required Standby Systems:
overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.


4- Article 708.54,
 NEC 2008  - Critical Operations Power Systems (COPS):
Citical operations power systems overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.


5- Article 517.26, 
NEC 2008 - Health Care Facilities:
This article requires compliance with Article 700 for emergency systems, unless amended by the requirements in Article 517.


6- Article 700.27, NEC 2008
a clarification on the scope of the required coordination– “This coordination must be carried through each level of distribution that supplies power to the emergency system.”


Third: NEC 2011 Code:


In NEC 2011, some changes were done for the selectivity coordination as follows:

1- Article 700.27 Coordination:
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.



2- Article 701.27 Coordination:
Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices. Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.

Paragraphs 700.27 and 701.18 modified the exceptions to address some questions about the required selective coordination. These exceptions don’t alter the impact of requiring selective coordination, but give technical assistance to the design engineer.



3- Article 517.17(C) Selectivity:
Ground-fault protection for operation of the service and feeder disconnecting means shall be fully selective such that the feeder device, but not the service device, shall open on ground faults on the load side of the feeder device. Separation of ground-fault protection time-current characteristics shall conform to manufacturer’s recommendations and shall consider all required tolerances and disconnect operating time to achieve 100 percent selectivity.

Paragraph 517.17(C) removed the requirement for six-cycle separation between the first and second level of ground fault protection. Instead, the new code allows manufacturer’s recommendations to be used to insure 100% selective coordination.


Part Three: How to perform a selectivity coordination study?


This can be done by using one of the following three ways/tools:

  • By using Selective Coordination Time - Current Curves.
  • By using Selective Coordination Charts/Tables.
  • By using Siemens calculators / spreadsheets. 

First: Selective Coordination Time - Current Curves:


Most manufacturers provide time-current curves for all their breakers which can be used for selectivity coordination study.

What is Time - Current Curves?


they are used to show how fast a breaker will trip at any magnitude of current.The following illustration shows how to read a time-current curve.The figures along the bottom (horizontal axis) represent multiples of the continuous current rating (In) for the breaker.The figures along the left side (vertical axis) represent time in seconds.

How to use these curves:


We always evaluate breakers in pairs for selective coordination, and only look at the curve to the left of the available fault current. If the curves on the left side of the available fault current do not overlap, the breakers selectively coordinate.

Example:  (selectivity Coordination for BQD20 & ED60 breakers at available fault current = 900A)
Selectivity Coordination for BQD20 & ED60 Breakers at 900A

The BQD20 breaker curve (red) and ED60 breaker curve (orange) clear each other in the entire thermal range (curved part), however the instantaneous ranges begin to overlap at just under 600A. So, the breaker pairs BQD20 & ED60 will not verify a selectivity coordination conditions at AFC=900A.

Selectivity Coordination for BQD20 & ED120 Breakers at 900A

Since we have 900A of available fault current in our example, we will need an upstream breaker that does not trip on instantaneous at that value – the first breaker with an instantaneous pickup (left edge of the vertical section) above 1000A is a ED125 (see green curve below). The 125A breaker will now selectively coordinate with the 20A branch at an available fault current value of 900A.


in the next Topic, I will continue explaining other methods for performing Selectivity Coordination Study. please, keep following.

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