Selectivity: Why your entire installation fails due to one small fault.

A short circuit in a fan heater should never cause your production hall's main switch to switch off. Yet in practice, this happens more often than it should. The result? Unnecessary downtime, high costs and safety risks.

Selectivity is the art of coordinating protections. The objective is simple: in the event of a fault, only the protective device directly above the fault should switch off. The rest of the installation must continue to operate undisturbed. Is your installation selective, or do you rely on luck? In this article, we explain how to get certainty through simulations and correct settings.

In brief: What you need to know about selectivity.

What is it: The coordination between protection components (such as circuit breakers and fuses) so that only the disturbed part of the installation is switched off.

The risk: A lack of selectivity causes an upstream distributor to switch off, unnecessarily de-energising a large part of the business.

The cause: Problems often arise from plant expansions, incorrect settings of protection devices or outdated designs that have not been recalculated.

The solution: A selectivity analysis(simulation) identifies bottlenecks. Based on this, settings of circuit breakers are changed or components are replaced.

Standardisation: NEN 1010 sets requirements for selectivity, especially in installations where operational reliability is crucial.

For whom is selectivity crucial?

This topic is crucial for professionals responsible for the continuity and safety of electrical installations:

Installation managers (IV): You are legally responsible for a safe and properly functioning installation. Selectivity is a core part of that safety.
Technical Managers & Maintenance Managers: You want to prevent unplanned downtime. A non-selective installation is a risk to your OEE (Overall Equipment Effectiveness).
Electrical Engineers: You design extensions or modifications. Without selectivity calculations, you gamble with the reliability of the design.
Facility Managers (in critical environments): In hospitals or data centres, selectivity is not an option, but a lifeline.

The technology behind selectivity: more than just power values.

Selectivity is achieved when, in a series connection of protection devices, the device closest to the fault shuts down the fault current, while the devices above remain closed. This sounds simple, but requires an understanding of the time-current characteristics (tripping curves) of your protection devices.

We distinguish various forms of selectivity:

Amperemetric selectivity (Current): This makes use of the difference in tripping current. The protection 'downstream' (closer to the consumer) speaks at a lower current than the protection 'upstream'. This works well for overloads and small short-circuit currents, but is often inadequate for high short-circuit currents near the distributor.
Chronometric selectivity (Time): This involves setting a time delay on the overhead protection. When a fault occurs, the main circuit breaker "waits" for a while to give the underlying circuit breaker a chance to clear the fault. This is very effective, but the installation must be able to tolerate the short-circuit current during that delay time (thermal and mechanical stress).
Logical selectivity: In modern, intelligent circuit breakers (Air Circuit Breakers or Molded Case Circuit Breakers), protective devices can communicate with each other. The circuit breaker that sees the fault sends a blocking signal to the upstream circuit breaker: "I see the fault and I am going to switch off, please wait." This combines speed with selectivity.
Energetic selectivity: specifically for current-limiting circuit breakers. This considers the pass-through energy. The 'downstream' automaton limits energy so fast that the upstream automaton does not receive enough energy to trip.

Nuance - Full vs. Partial selectivity In practice, full selectivity (up to the maximum short-circuit current) is not always economically or technically feasible without heavy intervention. An optimum is often sought where selectivity is guaranteed up to a certain short-circuit level. It is important to know where that limit lies in your installation.

The impact of poor selectivity on your operations.

Lack of selectivity is often an invisible problem, until the moment it goes wrong. The consequences are then immediately felt:

Unnecessarily large outages: A fault in one motor controller can bring down an entire production line or even an entire building if the main circuit breaker tripped before the group switch.
Difficult fault localisation: When a main distributor trips, it is unclear to the technical department where the fault is located. This increases search time and therefore downtime.
Safety risks: Failure of lighting or control systems can create unsafe situations for personnel.
Equipment damage: Frequent switching or switching off large loads can cause voltage spikes that damage sensitive electronics.
Compliance: NEN 1010 and insurers increasingly demand demonstrable selectivity, especially in installations with a safety function.

Signals that your installation is not selective.

No need to wait for a blackout to know if there are risks. Pay attention to these signals:

The "Mystery Trip": A circuit breaker in the main distribution board has tripped, but you find no obvious cause, and the terminal group circuit breakers are still just "in".
Replaced components: Heavier circuit breakers or fuses were installed in the past ("because it kept blowing out") without modifying the overhead protection.
New machines: large consumers have been added to existing distributors without recalculating short-circuit currents and settings.
Different brands/types: A mix of old fuses and modern circuit breakers makes coordination complex and often unreliable.

Case study: At a food manufacturer, the main power supply of the packaging line failed monthly. The technical department replaced the terminal group circuit breaker (25A) several times, thinking it was faulty. After measurement and simulation by HyTEPS, it turned out that the motor's inrush peak was just within the terminal group's curve, but triggered the magnetic threshold of the (too tightly set) overhead busbar box circuit breaker (63A). A simple adjustment of the setting of the 63A circuit breaker permanently solved the problem.

From diagnosis to solution: The way to a selective installation.

Restoring or ensuring selectivity does not start with the screwdriver, but with data.

Step 1: Inventory and Measurement: We need to know what is present in the installation. Which circuit breakers, which cable lengths (essential for impedance and therefore short-circuit current) and which loads? Measurements validate the theoretical models.

Step 2: Simulation (The key to success): You cannot test selectivity in practice without risk. That is why we use advanced simulation software (such as Vision). We digitally recreate your installation. In this model, we simulate short-circuits at every level.

We immediately see which protection appeals first.
We see whether cables are thermally protected.
We visualise time-current curves on top of each other.

Step 3: Optimisation: Often, expensive hardware modifications are not necessary. In many cases, we can restore selectivity by:

Changing settings: Adjusting the Ir (thermal), Isd (short time delayed) or Ii (instantaneous) values of adjustable circuit breakers.
Selective patterns: Applying specifically matched fuse patterns.

Structural measures (If necessary): Sometimes the installation is not physically sound. Then we advise:

Replacing a "fast" circuit breaker with a selective type.
Weighting cabling to influence short-circuit current.
Installation of UPS systems for critical control.

Pitfalls in security coordination

Blind reliance on factory settings: Many circuit breakers are set by default to their minimum or just maximum values, not to what your installation needs.
The "two-step" rule: Thinking a 16A fuse is always selective behind a 32A fuse. This depends on the type (gG, aM) and the manufacturer. With short circuits, this is far from always true.
Forgetting cable lengths: Long cables dampen short-circuit current. If the current is too low, the magnetic protection may not trip fast enough, causing the cable to burn before the circuit breaker tripped.
Focus on rated current only: Selectivity is mainly about behaviour at fault currents (kAs), not just the rated operating current.
Failure to update drawings: Making adjustments without updating the file makes future analyses impossible.

Roadmap for selectivity testing.

Want to know if your installation is safe and selective? Follow these steps:

Collect documentation: Ensure up-to-date single-wire diagrams and cable lists.
Inventory protections: Note brands, types and current settings of all main and sub-distributors.
Determine short-circuit capacities: obtain short-circuit data from the grid operator (at the takeover point).
Run a simulation: Have an engineer calculate the installation in a simulation model. Manual calculation is error-prone in complex grids.
Analyse curves: Check that the curves of protection devices connected in series do not cross each other in the relevant area.
Implement changes: Adjust settings and document them directly on the physical component (sticker/label) and in the schematic.

When do you call in a specialist?

For a single group in a residential home, an installer's basic knowledge is sufficient. But in more complex situations, specialist knowledge is required. Contact our engineers when:

You work with large power ratings (>250A) and adjustable circuit breakers.
Operating reliability is critical (hospitals, data centres, continuous production processes).
There are regular, unexplained outages.
The installation is obsolete and documentation is missing or questionable.
You are going to expand (e.g. PV installations, EV charging plazas or heat pumps) and want to know the impact on current protection.

HyTEPS combines on-site measurements with in-depth simulations. We do not guess, we calculate. So you get a concrete setup report that ensures selectivity.

Want to know more about Power Quality?

Deepen your knowledge with these related topics:

Simulations (Loadflow & Short circuit)

Short-circuit current and impedance

Harmonics

Power Quality Monitoring

Certainty about your security?

In doubt as to whether your protections are set correctly? Don't wait for production to shut down. Our engineers can use a targeted simulation to expose and optimise weaknesses in your installation.

Call: 0492 37 12 12

Mail: office@hyteps.nl

HyTEPS

Beemdstraat 3

5653 MA Eindhoven