What Is Selectivity? Protection Coordination in Switchgear
How to make sure only the nearest device trips during a fault: types of selectivity and their effect on panel design.
ContentsSelectivity means that during a fault only the protective device closest to it opens, while the devices upstream stay closed. Well-built coordination prevents a short circuit on a single motor outgoing from blacking out an entire plant.
Why coordination matters
If a faulty socket in a factory trips the incomer, the problem is not the devices but the coordination. The loss is not only stopped production; restart time, cold inrush currents and process damage add to the cost. That is why coordination belongs at the very beginning of panel design.
Types of selectivity
Coordination can be achieved through three mechanisms, and most installations use them together.
| Type | Principle | Limitation |
|---|---|---|
| Current selectivity | A sufficient margin is left between downstream and upstream trip thresholds | Breaks down if the fault current falls between the two thresholds |
| Time selectivity | A short delay (e.g. 100–300 ms) is set upstream | The system carries fault current throughout the delay |
| Energy selectivity | The downstream device limits current so the upstream one does not trip | Must be verified against manufacturer tables |
Total and partial selectivity
Total selectivity means coordination holds up to the highest prospective short-circuit current. Partial selectivity applies only up to a stated current; above that value both devices open together. The technical file should state the limit up to which the assembly is selective. See short-circuit withstand for how these values are established.
What is zone selective interlocking (ZSI)?
The main drawback of time selectivity is that fault current persists during the upstream delay. With ZSI a communication link runs between breakers: when the downstream device sees the fault it sends a restraint signal upstream and holds it back. If no signal arrives, the upstream device trips without delay. Both selectivity and a short clearing time are therefore obtained together.
Do not confuse it with cascading
Cascading, or back-up protection, means that above the downstream device’s own breaking capacity the upstream device assists it. It is economical but not selective: both devices may open during a fault. Selectivity and cascading can deliberately coexist at different levels of the same assembly.
Practical rules in panel design
For coordination to hold in the field, the design should observe the following:
- Aim for at least a 1:2.5 rated current ratio between consecutive devices
- Prefer an electronically protected device with short-time settings (Isd/tsd) upstream
- Use the manufacturer’s selectivity tables rather than assumptions
- Use selective (type S) RCDs where residual current protection is graded
- Record the settings on the panel label and in the technical file
Documentation of settings is covered in the panel technical file.
Key takeaways: selectivity
- Why coordination matters — If a faulty socket in a factory trips the incomer, the problem is not the devices but the coordination.
- Types of selectivity — Coordination can be achieved through three mechanisms, and most installations use them together.
- Total and partial selectivity — Total selectivity means coordination holds up to the highest prospective short-circuit current.
- What is zone selective interlocking (ZSI) — The main drawback of time selectivity is that fault current persists during the upstream delay.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can thermal-magnetic breakers give total selectivity?
Usually not. Because their magnetic thresholds are fixed, coordination breaks down at high short-circuit currents. Using an electronically protected device at the main level gives far more reliable results.
Is selectivity calculated in software?
Manufacturers’ coordination software and selectivity tables are used. Plotting curves alone is not enough; energy selectivity can only be verified from tables.
Does adding a delay affect short-circuit withstand?
Yes. When the upstream device is delayed, the assembly and its busbars must carry the fault current for that time, so the Icw rating has to be chosen accordingly.
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- What is the IEC 61439 Standard? Type Testing and Conformity in Low Voltage Switchgear.
- What is a Low Voltage (LV) Switchgear? Structure, Components, and Applications.
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Devpan plans protection coordination together with device selection and documents the settings in the technical file. Share your single-line diagram and we will review the coordination with you.
