Grip the enormous forces in your electrical installation

A short circuit is one of the most destructive phenomena within an electrical installation. In milliseconds, forces are released that can destroy components, jeopardise operational safety and - in the worst case - threaten the safety of persons. For the engineer and installation manager, short-circuit current is therefore not an abstract concept, but a crucial parameter in the design and management of the installation.

Can your installation withstand the maximum possible short-circuit current? And do your protective devices switch off in time, but only where necessary? Understanding short-circuit currents according to the IEC 60909 standard is the basis for a safe and reliable power supply.

In brief: What exactly is a short-circuit current?

Basically, a short circuit is a connection between two or more points with different potentials, where the impedance (resistance) is close to zero. As the current chooses the path of least resistance, the current increases to a multiple of the rated current in a fraction of a second. We then refer to this as the short-circuit current.

For a technical manager or engineer, however, "the" short-circuit current is not a singular number. To design an installation safely, we look at different values in the time domain, as defined in IEC 60909:

The initial symmetrical short-circuit current ( $cap I sub k double prime$ ): The effective value of the current at the time the closure occurs. This is the basic value for sizing circuit breakers.
Impact short-circuit current (_ip): The absolute, asymmetric peak value. It determines the mechanical forces (electrodynamic forces) acting on busbar systems and cables.
Thermal short-circuit current (_ith): The measure of the heat generation that conductors and components must be able to withstand during fault time.

Who is short-circuit current relevant to?

Installation managers (IV): Who must ensure that the installation complies with safety standards (such as NEN 3140 and NEN 1010).
Electrical Engineers: who are responsible for dimensioning and protection settings.
Maintenance Managers: who want to minimise risks of downtime and damage.

Why understanding short-circuit currents is essential for reliability

Blindly relying on data from deliveries a decade ago is a risk. Installations change: transformers are added, cable lengths change and motors are replaced by frequency-controlled drives. Each change affects the impedance of the grid and therefore the short-circuit current.

Consequences of incorrect estimation:

Explosion hazard and Arc Flash (Arc Flash): If a switch has to switch off a short-circuit current higher than its breaking capacity, the switch can explode. This creates life-threatening situations for personnel.
Wrongful failure (Selectivity): In the event of a fault in a sub-distributor, only that specific group should switch off. If the protections are not properly matched to the actual short-circuit levels, the main circuit breaker can fly out. Result: the entire production is shut down due to a local problem.
Mechanical damage: Impact short-circuit current(_ip) causes enormous magnetic forces. Bus bars can bend and support insulators can break down if they are not designed to withstand the surge currents.
Thermal ageing: Even if a protection device trips, cable insulation that is too thin can melt or degrade due to the enormous heat (I²t), laying the foundation for the following failure.

Case study

Situation: An industrial bakery expands with a new production line and installs a heavier transformer to supply power.

Problem: The existing main distributor was calculated at the time for a lower short-circuit capacity ( $cap S sub k double prime$ ). Due to the lower impedance of the new transformer, the potential short-circuit current rises above the limit of the existing circuit breakers.

Risk: In the event of a shutdown, switches fail, resulting in a devastating arc and weeks of downtime.

Solution: An advance simulation had shown that switches needed to be replaced either or current-limiting coils.

Causes: What causes a short circuit?

Although we simulate short circuits in calculations as static events, in practice the cause is often dynamic or human.

Insulation failure: Cable ageing (due to harmonic contamination and heat), mechanical damage during excavation work or action of moisture and chemicals.
Human error: Tools left behind in a switchboard after maintenance, or misconnecting phases.
Component failure: Internal closure in motors, transformers or capacitor banks due to overloading or Power Quality phenomena such as voltage spikes (transients).
Animals: In rural areas or semi-open installations, pests (mice, rats, martens) regularly cause closure between rail systems.

Nuance: The effect of motors. It is often forgotten that running motors (induction) briefly act as generators during a short-circuit. They feed the fault. In a plant with many large motors, this can make the total short-circuit current significantly higher than just the contribution from the mains (the transformer). This is explicitly included in the IEC 60909 calculation.

What can you do? From calculation to protection

You cannot "solve" a short-circuit current once it is there; you have to design the installation so that it can safely shut off the current. This is a process of prevention by calculation.

Step 1: Short-circuit current calculation (Simulation): Measuring is knowing, but with short-circuit currents, measuring is not an option (unless you want to blow up the installation). We use advanced simulation software (such as Vision or NEPLAN) to digitally recreate the network.

We enter data of cables (length, diameter, material), transformers and motors.
We calculate the maximum short-circuit current to test whether switchgear is strong enough.
We calculate the minimum short-circuit current at the end of long cables. This is crucial: if the current is too low due to the high cable resistance, the protection may not "see" the closure and will not switch off (in time).

Step 2: Selectivity analysis: After the calculation, coordination follows. We set the protection relays and circuit breakers so that their time-current characteristics perfectly match. The protection closest to the fault should trip first.

Step 3: Hardware measures: Does the analysis show that the short-circuit current is too high for your current installation? Then there are engineering solutions:

Installation of short-circuit current limiting coils.
Increasing the short-circuit voltage of transformers (note that this can exacerbate voltage dips).
Application of faster protection systems (e.g. light arc detection).
Bus bars weighing down or reinforcing support points.

Checklist: Is your installation short-circuit-proof?

Use these steps to identify risk:

Inventory: are all single-wire diagrams up to date (as-built)?
Data verification: Is the grid operator's power statement still correct?
Simulation: Is there a recent short-circuit calculation (maximum 3-5 years old) in accordance with IEC 60909?
Component check: Are the circuit breaker power values of the circuit breakers higher than the calculated $cap I sub k double prime$ .
Selectivity check: Has it been verified that the protections do not interfere with each other?

When will you talk to an engineer from HyTEPS?

In complex situations, standard calculations are not enough. Engage us when:

You doubt the selectivity of your protections (e.g. after an unexplained total failure).
You will be expanding with large capacities (new transformers, CHP, solar farms).
There are 'floating grid' or complex earthing systems.
You want assurance on safety (Arc Flash studies).

More about Power Quality and simulations

Delve further into the technology behind a stable installation:

Selectivity analysis: ensure that only the right circuit breaker trips when a fault occurs.

Harmonic analysis: the impact of pollution on your capacity.

Power Quality Measurements: The basis for every simulation.

Voltage dips: Result of short circuits elsewhere in the grid.

Certainty about your installation?

Do you doubt whether your protective devices are still calculated for the current situation? Don't take a risk with safety and reliability. Our engineers analyse your network configuration and provide a conclusive security plan.

Call: 0492 37 12 12

Mail: office@hyteps.nl

HyTEPS

Beemdstraat 3

5653 MA Eindhoven