Table of Contents
Introduction
The objective of protection coordination is to use protective devices to minimise the damage and isolate a problem in an electrical system. A coordination study must be performed to properly coordinate all protective devices.
The role of protective devices in a coordinated system is to: (a) Reduce exposure and isolate the problem zone so other parts of the electrical system are not affected; (b) Limit the outage to only the problem area; and (c) Operate quickly to limit the damage to any equipment it is protecting.
There are several international Standards which cover requirements for protection coordination of low voltage electrical systems and this article provides a summary of those as a reference.
AS/NZS 3000:2018
Clause 2.5.7.2.2:
Discrimination is achieved using a discrimination study, the ratios shown below or manufacturer’s data and tables. Discrimination need not apply above the arcing fault current Iarc which is deemed 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.
Clause 2.5.7.2.3:
- Overload curves are those for times >0.01 s. Short-circuit data is based on the total I2t of F2 ≤ pre-arcing I2t of F1 .
- IPSC is the prospective short-circuit current. Iarc is the deemed maximum arcing fault current (= 60% IPSC). Ii is the instantaneous setting.
- Isd is the short delay setting. Isd is not available on MCBs and only available on some MCCBs with electronic trip units.
BS7671:2018
Clause 536.4.1.2
Selectivity under overload conditions between OCPDs (Overcurrent protective devices):
The design shall be verified either by:
- Desk study, taking into account the relevant product standards and manufacturer’s literature, or
- Appropriate software tools where information is provided by the manufacturer for this specific use, or
- Tests in accordance with the applicable product standard (in order to achieve the correct test performances and reproducibility), or
- Manufacturer’s declaration.
In the case of a desk study, when time/current characteristics are used to verify selectivity, account shall be taken of the reference ambient temperature for which the tripping curves are given and to load conditions before the overcurrent.
Clause 536.4.1.3
Selectivity under short-circuit conditions between OCPDs:
The design shall be verified either by:
- Desk study, taking into account the relevant product standards and manufacturer’s literature, or
- Appropriate software tools where information is provided by the manufacturer for this specific use, or
- Tests in accordance with the applicable product standard (in order to achieve the correct test performances and reproducibility), or
- Manufacturer’s declaration.
In the case of desk study, when energy limitation curves are used to verify selectivity, account shall be taken of the voltage for which the curves are given.
Clause 536.4.1.4
Selectivity between RCDs
The design shall be verified either by:
- Desk study, taking into account the relevant product standards and manufacturer’s literature, or
- Appropriate software tools where information is provided by the manufacturer for this specific use, or
- Tests in accordance with the applicable product standard (in order to achieve the correct test performances and reproducibility), or
- Manufacturer’s declaration.
Selectivity in case of residual current is given under the following conditions:
- The upstream RCD is of selective type (type S or time-delayed type with appropriate time delay setting), and
- The ratio of the rated residual operating current of the upstream RCD to that of the downstream RCD is at least 3:1.
Clause 536.4.1.5.1
Selectivity between RCD(s) and upstream OCPD
In the event of an earth fault, current may reach a high value that could exceed the instantaneous tripping current of the upstream OCPD. Therefore, when selectivity between RCD(s) and upstream OCPD is required, an RCBO or CBR shall be used and the requirements of selectivity according to Regulation 536.4.1.2 shall be applied.
Clause 536.4.1.6
Selectivity between RCD(s) and downstream OCPDs
IEC 60364-5-53:2019 + AMD1:2020
Clause 535.1
Selectivity between overcurrent protective devices
The OCPD on the load side (Q2) provides protection up to the level of overcurrent selectivity limit Is, without causing the upstream OCPD (Q1) to operate. To determine the selectivity limit current Is, reference shall be made to the instructions of the manufacturer of the downstream and upstream OCPDs. Where no information about this combination is available from the manufacturer, the selectivity limit current Is may be defined by comparison of operating time-current curves of the OCPDs.
The selectivity limit current Is shall be evaluated taking into account energy values, such as let-through energy for circuit-breaker and melting energy for fuses. See also relevant product standards.
Clause 535.3
Selectivity between residual current protective devices
To ensure selectivity between two residual current devices in series, these devices shall satisfy both the following conditions:
- The residual current protective device located on the supply side (upstream) shall be selected according to IEC 61008 (all parts), IEC 61009 (all parts), or IEC 62423 as type S or according to IEC 60947-2 as time delay type;
- The rated residual operating current of the device located on the supply side shall be at least three times greater than that of the residual current protection devices located on the load side.
Clause 535.4
Selectivity of RCD and OCPD
Under consideration.
AS/NZS IEC 60947.2-2015 Annex A, AS/NZS 60898.1-2004 Annex D
Clause Annex D.5 or A.5
Verification of discrimination/selectivity
Selectivity can normally be considered by desk study alone, i.e. by a comparison of the operating characteristics of C1 and the associated SCPD, for example, when the associated SCPD is a circuit-breaker (C2) provided with an intentional time-delay.
Selectivity may be partial or total up to the rated short-circuit breaking capacity Icu of C1. For total selectivity, the non-tripping characteristic of C2 or the pre-arcing characteristic of the fuse shall lie above the tripping characteristic of C1. (C1 is at downstream, C2 is at upstream)
Clause Annex D.6.2.2 or A.6.2.2
Verification of back-up protection by comparison of characteristics
In some practical cases and where the SCPD is a circuit-breaker, it may be possible to compare the operating characteristics of C1 and of the associated SCPD (short-circuit protective device).
The suitability of the association may be evaluated by considering the maximum operating I2t characteristics of the SCPD, over the range from the rated short-circuit capacity Icn of C1 up to the prospective short-circuit current of the application, but not exceeding the maximum let-through I2t of C1 at its rated short-circuit capacity or other lower limiting value stated by the manufacturer.
Note: Where the associated SCPD is a fuse, the validity of the desk study is limited to Icn of C1.
References:
- AS/NZS 3000: 2018 – Electrical Installations and Wiring Rules of Australia and New Zealand.
- BS 7671:2018 – Requirements for Electrical Installations by the British Standards Institution.
- IEC 60364-5-53:2019 – Low-voltage electrical installations – Part 5-53: Selection and erection of electrical equipment – Devices for protection for safety, isolation, switching, control, and monitoring.
- AS/NZS IEC 60947.2:2015 – Low-voltage switchgear and controlgear Circuit-breakers.
- AS/NZS 60898.1:2004 – Electrical accessories – Circuit-breakers for overcurrent protection for household and similar installations Circuit-breakers for a.c. operation (IEC 60898-1 Ed. 1.2 (2003) MOD)
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