Table of Contents
What is Protection Coordination
The objective of protection coordination is to use protective devices to minimise the damage and isolate a problem in an electrical system. A protection 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:
| Scenario | Requirement | How to achieve |
|---|---|---|
| Rating of C₂ ≥ 800 A | Discrimination is required between overload curves and instantaneous settings but need not apply above the arcing fault current (Iarc). | By manufacturer's data |
| Rating of 250 A ≤ C₂ < 800 A | 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 deemed to be achieved if the overload setting of C₁ ≥ 1.5 × C₂, e.g. C₁ 1000 A with C₂, 630 A. |
| Rating of C₂ < 250 A |
Discrimination is required between overload curves and is recommended up to the instantaneous setting (Ii) or short-time pickup (ISD) of C1 but need not apply above the arcing fault current (Iarc).
Exception: For ratings of C2 ≤ 80 A discrimination is not required. |
Discrimination is deemed to be achieved if C₁ ≥ 1.5 × C₂, e.g. C₁ MCB marked C63 with MCB C₂ marked C40 (i.e. both C curves). |
| Fuses | Provide discrimination (selectivity) on overload. |
Discrimination (selectivity) between HRC fuses is deemed to be achieved— 1. For overload when F₁ ≥ 1.6 × F₂, e.g. 16 A with 10 A; and 2. For short-circuit when F₁ ≥ 2 × F₂, e.g. 20 A with 10 A¹. |
| C₂ and F₁ | Provide discrimination (selectivity) between the overload curve and the instantaneous setting or short delay setting (ISD) of C₂ and the time-current curve of F₁. | - |
- 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 A53.3
Between circuit-breakers or circuit-breaker and CPS or circuit-breaker and overload relay or circuit-breaker and motor starter
Selectivity in case of overload is verified by the comparison of time/current characteristics of the devices involved. The maximum operating time of the device on the load side must be lower than the non-tripping time of the circuit-breaker for any overload current. Separation of the characteristics in both the time and current axes provides selective operation in this zone.
Clause A53.4 Between fuses
Selectivity under short-circuit conditions between OCPDs:
Selectivity in case of overload is verified by the comparison of time/current characteristics of the fuses involved. The total operating time of the fuse on the load side must be lower than the pre-arcing time of the fuse on the supply side. Fuses to BS EN 60269-2 of the same utilization category (e.g. type gG) with rated current of 16 A and above will provide total selectivity if the ratio of the rated currents 1.6:1 or greater. Separation of the characteristics in both the time and current axes provides selective operation in this zone.
Clause A53.5
Between circuit-breaker (upstream) and fuse (downstream)
Selectivity in case of overload is verified by the comparison of time/current characteristics, taking into account the trip setting of the circuit-breaker where applicable. When using published time-current characteristics, the maximum operating time curve be taken for the downstream device and the minimum operating time curve must be taken for the upstream device. Separation of the characteristics in both the time and current axes provides selective operation in this zone.
Clause A53.6
Between fuse (upstream) and circuit-breaker (downstream)
Selectivity in case of overload is verified by the comparison of time/current characteristics, taking into account the trip setting of the circuit-breaker where applicable. The maximum operating time of the circuit-breaker as given by the manufacturer must be lower than the minimum pre-arcing time of the fuse as given by the product standard. Separation of the characteristics in both the time and current axes provides selective operation in this zone.
Clause A53.7
Between circuit-breakers
Generally, reference should be made to the manufacturer’s documentation.
In practice, selectivity is assured up to the fault current level at which the peak current let-through of the downstream circuit-breaker is less than the peak value corresponding to the instantaneous tripping level of the upstream circuit-breaker. Where the upstream circuit-breaker has dedicated selective behaviour, the selectivity limit might be higher.
Clause A53.8
Between fuses
Clause A53.9
Between circuit-breaker (upstream) and fuse (downstream)
The peak let-through current of the fuse should be lower than the minimum instantaneous tripping current of the circuit-breaker.
Data for peak values of fuses should be taken from the relevant standard or the manufacturers’ documentation. If data is taken from the manufacturer, this should be stated in the documentation of the installation.
Clause A53.10
Between fuse (upstream) and circuit-breaker (downstream)
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.
| Type of selectivity | Definition |
|---|---|
| Partial selectivity |
The selectivity limit current Is is lower than the maximum prospective short-circuit current Isc_max at the installation point of the OCPD on the load side: Is < Isc_max |
| Full selectivity |
The selectivity limit current Is is equal to or higher than the maximum prospective short-circuit current Isc_max at the installation point of the OCPD on the load side and lower than its breaking capacity Icu or Icn: Isc_max ≤ Is < Icu or Icn |
| Total selectivity |
The selectivity limit current Is is equal to or higher than the maximum prospective short-circuit current Isc_max at the installation point of the OCPD on the load side and equal to its breaking capacity Icu or Icn: Isc_max ≤ Is = Icu or Icn |
| Enhanced selectivity |
The selectivity limit current Is is equal to or higher than the maximum prospective short-circuit current at the installation point of the OCPD on the load side (Q2) and lower than or equal to the combined current Icomb of this combination. The breaking capacity Icu or Icn on the load side (Q2) is lower than the maximum prospective short-circuit current at its installation point: Icu or Icn < Isc_max ≤ Is ≤ Icomb Enhanced selectivity can only be designed with respective information from the manufacturer of the devices. |
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)
