Selectivity: Positive discrimination for maximum safety

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This blog was updated in March 2020 to better reflect changes in the market.

“Selectivity”? “Discrimination”? Why do they mean the same thing?
The terms denote “selective” or “discriminative” tripping. In the event of a fault, one device only – immediately upstream of the fault –should trip. That’s because the system has been coordinated to discriminate between incoming and outgoing devices and prevent an outgoing fault current from tripping the whole system. But there’s more: there are two “degrees” of selectivity.

Degrees of selectivity

Selectivity may be “partial” or “total”. Let’s take partial first and imagine two circuit breakers in series. In the event of an overcurrent, the load side CB trips up to a given fault current limit – the “selectivity current limit” (“Is” in below diagram). Above that, both breakers open and there’s no more selectivity.

Then total, selectivity is ensured up to the short-circuit current of the installation. Only the downstream breaker trips, while the upstream one stays closed. This arrangement ensures maximum availability. It’s the right protective solution in critical environments like hospitals, data processing centers and production sides where continuity of service demands are very high. Finally, when it is not possible to achieve proper selectivity and it is essential for the correct operation, the installation of uninterruptible power supplies (UPS), generator units must be considered.

Current and time, the main types of selection

For protection against overcurrent, this generally concerns current selectivity and time selectivity. In current selectivity only the selected loadside CB will trip when a fault current falls within the selectivity current limit. The grounds for selectivity are the ratings of circuit breakers, with devices being assigned trip thresholds accordingly.
The other main type of selectivity is time-related. It operates by delaying the upstream device – to allow the downstream loadside device (nearest the fault) to trip first.

How do current and time selectivity work in practice?

Current

Imagine two circuit breakers – a 16A-rated loadside device and a 25A upstream CB. Both have the same time delay. In the event of an overload of, say, 40A, the downstream circuit breaker will trip between 10 and 20 seconds, when the upside time trip will trip between 100 and 200 seconds after it has picked up the fault current.

Time

To achieve time selectivity it is necessary to have an upstream circuit breaker with time-delay bands.
Let’s say, two CBs rated close nominal current. When detecting a fault current upstream circuit breaker also picks it up but takes longer to trip. By then the load-side breaker will already have cleared the fault.
If the fault is a short circuit, the tripping time lag is in tens of milliseconds.
In fact, protective devices are always staged in function of their time and current limitations in all selectivity techniques. What other kinds of selectivity are there?

Logic selectivity and energy discrimination

These are used to ensure selectivity to whatever level of short circuit between CBs. In logic selectivity, which also goes by the name of zone sequence interlocking (ZSI), CBs incorporate electronic trip units connected by wires in a cascade configuration.
When a trip unit on the CB nearest the load picks up a fault current that exceeds its trip limit, it transmits a logic instruction to the next one upstream telling it to wait. That trip unit does the same and so on. The only one not to get the order is the trip unit nearest the fault. It clears the fault immediately.
As for energy-based selectivity, well, let’s say it’s a method of discriminating between loadside faults and the loadside let-through currents. It’s pretty manufacturer-specific. This principle combines the exceptional limiting power of the upstream devices and reflex release, sensitive to the energy dissipated by the short circuit in the device.
When a short circuit is high, if it is seen by two devices, the downstream device limits it greatly and the energy dissipated in the upstream device is insufficient to cause it to trip.
Meanwhile, prescriptive specification engineers should check out manufacturers’ coordination tables and find tools for distinguishing between selectivity techniques.
And we will take a look at combined short-circuit coordination (cascading) and coordinated back-up protection.

I have written this blogpost in collaboration with Jean-François Rey.

> Related Blogpost: 

• Setting the standard for coordination
• Cascading and backup: Why coordination should be compulsory