What is behind Full Selectivity, Total Selectivity, and Enhanced Selectivity ?

[MercedesLopez]

I am confused between the different ways selectivity is defined.
Could you please clarify what is the difference between Full Selectivity,  Total Selectivity, and  Enhanced Selectivity ?

[MathieuGuillot · Accepted solution]

Hello,

Thanks for this relevant question. I understand it can be confusing indeed.

Selectivity (or discrimination) is achieved by overcurrent and earth fault protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected.

This is the main objective : minimize the part of installation switched off in case of fault such as overload, Line to Line short-circuit or Line to Earth fault.  The concern is to convert this general requirement in criteria for circuit-breakers selection.

Selectivity is achieved when the maximum short-circuit current at a point of installation (Isc_max) is below selectivity limit (Is) of the circuit breakers supplying this point of installation. That’s where Total / Full / Enhanced selectivity concepts come in the game… IEC 60364-5-53: 535 2019 is now providing some guidance.

If we consider two circuit-breakers, the following situations are possible:

1. Is < Icu or Icn : The selectivity limit (Is) between the two circuit-breakers is lower than the breaking capacity of the circuit-breaker on the load side (Icu or Icn). (In that case we talk about “Partial selectivity” between the two circuit-breakers.)

Example: ComPact NSX100F (36kA 400V) TMD 100A & iC60N (10kA 40V) C 32A Selectivity limit Is = 1kA

In a given electrical installation two cases are then possible:

• Is < Isc_max <= Icu or Icn : The maximum short-circuit current on the load side (Isc_max) is higher than this selectivity limit “Is”. (1kA in the example). In that case selectivity will not be achieved for all values of fault current. We call this situation “partial selectivity” in the installation and partial selectivity for circuit-breakers.

• Isc_max <= Is < Icu or Icn : The maximum short-circuit current on the load side is lower or equal to this selectivity limit Is. (1kA in the example). In that case we talk about “Full selectivity” in the installation even if it’s partial from circuit-breaker perspective.
  
  

1. Isc_max < Is = Icu or Icn The selectivity limit (Is) between the two circuit-breakers is equal to the breaking capacity of the circuit-breaker on the load side and the maximum short-circuit current on the load side is lower or equal to this breaking capacity. In that case we talk about “Total selectivity” in the installation and total selectivity for circuit-breakers.

Example: ComPact NSX100F (36kA 400V) Micrologic 2.2 100A & iC60N (10kA 40V) C 32A: Total selectivity

1. Icu<  Isc_max <= Icomb The maximum short-circuit current on the load side is higher than the breaking capacity of the circuit-breaker on the load side when using back up (or cascading) effect as permitted by IEC 60364-4-43 Ed 3 2008 § 434.5.1 or equivalent national standards such as BS7671, NFC15100. In that case the maximum short-circuit current shall be lower than the re-enforced breaking capacity (Icomb). The selectivity limit between the two circuit breakers in that specific case can also be enhanced or not according to manufacturer information. If selectivity limit is going above Icu of downstream circuit-breaker it ’is called “Enhanced selectivity” in the installation and enhanced selectivity for circuit-breakers.

Example: ComPact NSX100F (36kA 400V) Micrologic 100A & iC60N (10kA 40V) C 32A “20/20”

• Enhanced selectivity limit = 20kA
• Enhanced breaking capacity = 20kA

To wrap up :

Selectivity in a given installation according to circuit-breakers selectivity performance without back-up

Selectivity characteristics of two circuit-breakers	Short-circuit current	Selectivity consequence for the electrical installation
Partial (up to Is)	Is<= Isc max <= Icu or Icn	Partial up to Is
Isc max < Is<= Icu or Icn	Full
Total	Isc max <= Is=  Icu or Icn	Total

Selectivity in a given installation according to circuit-breakers selectivity performance when back-up  is used:

Selectivity characteristics of two Circuit-breakers	Short-circuit current	Selectivity consequence for  the electrical installation
Partial (up to Is)	Is < Icu < Isc max	Partial (up to Is)
Total	Is = Icu < Isc max	Partial (up to Icu)
Enhanced	Icu < Isc max <= Is	Enhanced selectivity (up to Is_enhanced)

From a designer perspective, when selectivity in an electrical installation is expected I recommend to specify the following : Either “Total Selectivity is required and back-up is forbidden” or “Total selectivity is required, if back-up is applied, enhanced selectivity up to maximum short-circuit current shall be checked.  

Mathieu Guillot

Schneider Electric

[MathieuGuillot · Accepted solution]

Sorry a "cell jam" makes my first table confusing. Read as follow:

Selectivity in a given installation according to circuit-breakers selectivity performance without back-up (cascading)

Selectivity characteristics of two Circuit-breakers	Short-circuit current	Selectivity consequence for the electrical installation
Partial (up to Is)	Is<= Isc max <= Icu or Icn	Partial up to Is
Partial (up to Is)	Isc max < Is<= Icu or Icn	Full
Total	Isc max <= Is=  Icu or Icn	Total

[Anonymous user]

nice too meet you all sir

[Anonymous user]

Thanks a lot for all your guys sharing, it helpful

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