Cascading and selectivity are two important concepts in designing power system protection and back-up. The principles of cascading and selectivity ensure systems’ reliability, protection, and cost-effectiveness while operating under a variety of fault conditions.
Selectivity (Discrimination)
Selectivity, or discrimination, is essential to ensure the continuation of power systems. The nature of selectivity lets fault isolation be left entirely with the nearest protective device without interference and continuing operation from the remaining parts of the system. This enhances reliability primarily to critical systems.
Definition:
Selectivity allows the upstream protective device to stay open, while downstream devices will interrupt faults.
Standards:
AS/NZS 3000 defines selectivity as the dependence on the operating characteristics of the combination of several devices. Discrimination study confirms such coordination by time-current curves and switchboard manufacturers data.
Methods
- Coordination Studies: Compare the time-current curves of a group of devices so that there is no overlap in the region of operation.
- Ratio: For smaller devices (C2 < 800A), the ratio of upstream devices (C1) to the downstream devices (C2) should be at least 1.5.
Cascading or Back-up Protection
Cascading ties together upstream and downstream devices to cope with fault currents that surpass the breaking capacity of the downstream device. It is normally applied to non-critical loads or when the cost factor becomes an important factor.
Definition:
A tested group of circuit breakers functions together, and the upstream circuit breaker assists the downstream one during high fault currents.
Standards:
It is accepted by AS60947-2 and AS61439. Cascading guarantees safe operation above the breaking capacity rated for the downstream device.
Applications:
- Applicable to non-essential load groups.
- Upstream device will most probably have tripping during fault scenarios.
Comparison of Cascading and Selectivity
Tools and Templates
- Cascade and Selectivity Tables: Standards like AS/NZS 60947 and AS/NZS IEC 60947.2:2015, offer verified combinations of the apparatus.
- Software Tools: Programs like TemCurve, PowerCAD, PowerTools are helpful in conducting the discrimination and coordination studies.
Important Points
- Discrimination may not be needed for the currents crossing the fault levels such as arcing faults, etc.
- Selectivity between utility service fuses and installation breakers is required to be sought with electricity distributors.
How Do I Ensure Selectivity
Selectivity is accomplished by a discrimination study. This study can be accomplished either by the use of ratios or by manufacturer’s data, based on the rating of C2.
Based on AS/NZS 3000:
“AS/NZS 3000:2018 – 2.5.7.2.3 General supply circuit discrimination (selectivity) In accordance with Clause 2.5.7.1, to minimize loss of supply, discrimination (selectivity) shall be arranged between protective devices for outgoing circuits and the upstream protective device. Discrimination is obtained by a discrimination study, the ratios in the following tables or manufacturer’s data and tables. Circuit-breakers with curves given in AS/NZS IEC 60947.2:2015 Figure K.1, current limiting and reflex tripping circuit-breakers may need special consideration.Discrimination need not apply above the arcing fault current Iarc which is assumed to be in the range of 30% to 60% of the prospective short-circuit current. Discrimination need not apply where protective devices are in series on the same circuit such as in UPS connected supplies.Downstream devices shall be selected to discriminate (provide selectivity) with upstream devices, using time-current curves, in accordance with the following:
- Circuit-breakers: Two circuit-breakers, connected such that C2 is the downstream device and C1 the upstream device, shall be selected:
- For ratings of C2 greater than or equal to 800 A, discrimination shall be provided by a coordination study using manufacturer’s data. NOTE: Curve references are found in AS/NZS IEC 60947.2:2015. Allowance for tolerances on settings may be required.
- For ratings of C2 greater than or equal to 250 A, and less than 800 A, discrimination shall be provided between overload curves. Discrimination is considered to be obtained if the overload setting of C1 ≥ 1.5 x C2, for example C1 1000 A with C2 630 A.
- For values of C2 below 250 A, discrimination is considered to be obtained if C1 ≥ 1.5 x C2, for example C1 MCB marked C63 with MCB C2 marked C40 (i.e. both C curves).
Exception: For ratings of C2 ≤ 80 A discrimination is not required.NOTES:
- ISD is not available on MCBs and only available on some MCCBs with electronic trip units.
- Where a circuit-breaker is installed for load limiting purposes, such as on submains, reliability of supply is not required.
For current-limiting breakers of applications, requirements exceeding more than the standards where selectivity is needed, in the instantaneous cross-over region of the 2 curves, reference to the manufacturer’s test data is needed to finish the study as shown in the following tables for documented combinations. For devices where C2 is less than 800 A, a ratio method of x1.5 can be used.
Regarding Safety Services, AS/NZS 3000:2018 – 2.5.7.2.2 Safety service circuit discrimination (selectivity) details the requirements.
With the understanding and application of cascading and selectivity, engineers can develop power systems that are reliable and yet cost-effective.
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