Selecting a circuit breaker

Author: Viacheslav Yurdyk, Quality Engineer at LK Energy Group.
Acceptance of equipment at the plant (quality control department) and technical supervision on electrical installation sites.

Short answer

A circuit breaker is chosen not by load rating but after the cable cross-section and the short-circuit currents have been calculated. The PUE (PUE — the Ukrainian electrical installation code) says so directly: protective devices and their characteristics are selected after the cross-sections of wires and cable cores have been determined and the short-circuit currents calculated [ПУЕ, п. 3.1.9].

And the key figure that commercial offers keep quiet about: the breaking capacity of the device must be not less than the short-circuit current at its point of installation [п. 3.1.12]. Two breakers both marked «250 A» can differ eightfold in breaking capacity — and the one whose capacity is below the short-circuit current at your site will not clear the fault but become part of it.

Three requirements a breaker must meet at once

The Rules state the protection task as follows: the network must be arranged so that simultaneously three conditions are met [п. 3.1.6]:

the network carries the full design load current for an unlimited time;
the network carries the anticipated short-term overload current — motor starting, energising transformers and heating devices — for a time during which live parts and insulation do not heat above permissible temperatures;
the network or part of it is disconnected within a defined time if it poses a threat to property or to the health of people and livestock.

🔑 The second condition is the one that makes «we took it with a margin» fail. A breaker chosen «so that it definitely does not trip» during motor starting may fail, in normal operation, to see an overload that is slowly burning the insulation. The Rules require the device to clear an overload current before the heating becomes dangerous for the insulation, joints and terminals [п. 3.1.14]. Short-circuit protection must act before the current creates a hazard through thermal and mechanical effects on conductors and their joints [там само].

Order of work: why the breaker is chosen last

The rule that reverses the usual sequence [п. 3.1.9]:

The selection of protective devices and their characteristics must be carried out after the cross-sections of wires and cable cores have been determined and the short-circuit currents calculated.

And then comes the part that is rarely done: if the requirements cannot be met with the selected devices, the network characteristics must be changed and the device selection repeated [там само]. In other words, the Rules accept that the right answer may be «a different cable», not «a different breaker».

What exactly is calculated [п. 3.1.10]: two-phase and three-phase short-circuit currents for supply and group circuits regardless of the system type (TN, IT, TT). The three-phase fault is determined at the point where the protective device is installed, and the two-phase fault at the end of the protected circuit. These are two different figures for two different questions: the first — will the device withstand it, the second — will it operate at the far end of the line.

Symbols you will see in the calculation [п. 3.1.4]:

Symbol	What it means
I_B	the highest load current of the circuit
I_Z	the permissible continuous current of the cable (wire)
I_n	the rated current of the protective device (for adjustable ones — the current of the chosen setting)
I_2	the current that ensures reliable operation of the device

Breaking capacity (Icu): the most expensive saving

The requirement [пп. 3.1.12 і 3.1.24]: - the breaking capacity of the protective device must be not less than the short-circuit current at the point where the device is installed; - devices must, by their breaking capacity, correspond to the maximum short-circuit current at the start of the network section being protected.

🔴 Why this is exactly where savings are made unnoticed. The rated current is printed large on the breaker case — 63, 160, 250 A. The breaking capacity is printed small and in kiloamperes. For devices with the same rating it can differ severalfold: 85, 50, 10 kA. The price differs in the same way — and that is exactly what «we found it cheaper» is built on.

The difference between them shows exactly once — at the moment of a short circuit. A device with a margin clears the current and remains a device. A device without a margin may fail to break the arc — and then the damage is taken not only by it but by the whole assembly.

When the Rules allow a device «weaker» than the short-circuit current [п. 3.1.25] — they do allow it, but only under three conditions at once: - the device protecting it, or the nearest device towards the supply source, provides instantaneous clearing of the short-circuit current; - the settings of the instantaneous releases of those devices are below the breaking capacity of each device in the group of non-withstanding devices; - non-selective tripping of the whole group does not threaten an accident, damage to expensive equipment and materials, or disruption of a complex process.

🔑 Read the last condition again. Such protection works «at the neighbours' expense»: a short circuit will trip not only the faulty circuit but the entire group. For a warehouse that is acceptable; for a line with a continuous process it is not. So «we have a powerful breaker upstream, that is enough» is a decision made by the designer for a specific facility, not a supplier's argument.

Selectivity: so that not everything goes dark

The requirement [п. 3.1.17]: short-circuit protective devices are selected so that a fault current in any circuit of the network is cleared by the device in the circuit where the fault occurred, without any effect on other circuits. The fault clearing time must at the same time be as short as possible.

Two requirements pulling in opposite directions: clear as fast as possible — and clear only what is needed. That is why selectivity is not a property of an individual breaker but a property of the entire chain from the incoming device to the final one. It cannot be bought together with a device; it is either designed in or absent.

What clears what [пп. 3.1.19 і 3.1.17]:

What is cleared	By what
current overload	thermal releases of breakers, fuses, thermal or electronic relays with contactors
current short circuit	electromagnetic (thermal-magnetic, electronic) releases of breakers or fuses

What may be used to protect a network against overcurrents [п. 3.1.8]: circuit breakers; fuses; combinations of switching devices with thermal relays and fuses; specialised electronic devices.

Service life: a breaker is not forever

A device has a switching and mechanical endurance — the number of switching operations after which the characteristics are no longer guaranteed. This is not a defect and not grounds for a claim: the endurance is used up in the same way as a car's mileage.

The practical consequence: a breaker that has tripped frequently is a candidate for replacement, even if it looks intact. A device that has cleared several short circuits has already worked at the limit of its breaking capacity.

🔴 That is why under the PTEES devices are not «inspected» but tested. The scope and frequency of checks of devices and secondary circuits are set by annex 1 to the PTEES; for indoor switchgear assemblies a major check is carried out at least once every 8 years, and it includes measuring contact resistance and checking mechanical operation.

More on the frequency — on the PTEES page.

Two requirements visible on site

A label on the device is a requirement of the Rules, not the installer's tidiness [п. 3.1.27]: every protective device must carry an explanatory label with the device's rated current, the release settings or the rated current of the fuse link. The duties are split: - on the device the label is applied by the manufacturer; - on the diagram next to the place of installation — by the company responsible for installation (or operation).

Connection of the supply conductor [п. 3.1.26]: with single-end supply the incoming cable or wire is connected to the fixed contacts of the device. For screw-in fuses and breakers — so that when the plug is unscrewed the screw shell is de-energised.

Both requirements can be checked by a one-minute inspection — and both are regularly violated.

Protection against loss of voltage

A requirement rarely remembered, yet critical in industry [п. 3.1.15]:

if voltage dips followed by restoration may create a hazard for people, animals or property, protection is provided that acts to disconnect the installation;
if the equipment tolerates a short interruption of supply, such protection may be applied with a time delay;
🔴 if re-energising may cause a hazard — it must not be automatic.

The last item is about machine tools and conveyors that restart by themselves once voltage returns, while a person is standing next to them.

Frequently asked questions

How do you choose the rating of a circuit breaker?

Not by the load rating. First the cross-section of wires and cable cores is determined and the short-circuit currents are calculated — and only after that the device and its characteristics are selected [п. 3.1.9]. The device rating must be matched to the permissible continuous current of the cable, otherwise there is nothing to protect: the breaker will carry a current the cable can no longer withstand.

What is Icu and why do two 250 A breakers cost differently?

It is the breaking capacity — the highest short-circuit current the device can clear. For devices with the same rating it can differ severalfold (for example 85, 50 and 10 kA), and that is what the price difference is built on. The Rules require the breaking capacity to be not less than the short-circuit current at the point of installation [п. 3.1.12].

What happens if a breaker with a lower breaking capacity is installed?

In normal operation — nothing, you will see no difference. The difference appears at the moment of a short circuit: a device without a margin may fail to break the arc, and the damage will be taken not only by it but by the assembly around it. This violates clause 3.1.12, and it has no outward signs until the fault occurs.

Can a «weaker» breaker be installed if there is a powerful one upstream?

The Rules allow it, but only under three conditions at once: the upstream device provides instantaneous clearing of the short circuit; the settings of its instantaneous releases are below the breaking capacity of each of the «weak» devices; and non-selective tripping of the whole group does not threaten an accident, damage to expensive equipment or disruption of the process [п. 3.1.25]. So it is a designer's decision for a specific facility, not a universal argument.

What is selectivity in plain words?

It is when a fault trips only the circuit where it occurred and the rest of the facility keeps running. The Rules put it as a requirement for device selection: the fault current must be cleared by the device in the faulty circuit without any effect on other circuits [п. 3.1.17].

Can you buy a «selective breaker»?

No. Selectivity is a property of the chain of devices from the incomer to the final one, not of an individual product. It is either designed in or absent. So the question «are your breakers selective?» should properly be put to the designer, not the supplier.

What protects against overload and what against short circuit?

Against overload — thermal releases of breakers, fuses, thermal or electronic relays with contactors. Against short circuit — electromagnetic (thermal-magnetic, electronic) releases of breakers or fuses [пп. 3.1.17 і 3.1.19].

Why is a breaker «with a margin» not always better?

Because the Rules require three conditions to be met at once: carry the working current indefinitely, withstand a short-term overload during motor starting or transformer energising, and disconnect within a defined time in case of hazard [п. 3.1.6]. A device chosen only for the starting current may fail to see an overload dangerous to the insulation.

Which short-circuit currents must be calculated?

Two-phase and three-phase, for supply and group circuits, regardless of the system type (TN, IT, TT). The three-phase fault is determined at the point where the protective device is installed, the two-phase fault at the end of the protected circuit [п. 3.1.10].

What if no device meets the requirements?

Change the network characteristics and repeat the device selection [п. 3.1.9]. That is, the correct answer may be a different cable rather than a different breaker — and the Rules provide for this explicitly.

Must anything be written on the breaker?

Yes. Every protective device must carry an explanatory label with the device's rated current and the release settings (or the rated current of the fuse link). The label on the device itself is applied by the manufacturer, and the one on the diagram next to the place of installation by the company responsible for installation or operation [п. 3.1.27].

Is the service life of a circuit breaker used up?

Yes, a device has switching and mechanical endurance. A breaker that has repeatedly cleared short circuits has worked at the limit of its breaking capacity — and is a candidate for replacement even if it looks intact. That is why under the PTEES devices are not inspected but tested at a prescribed frequency.

Is protection against loss of voltage required?

If a voltage dip followed by restoration may create a hazard for people, animals or property — yes, and it acts to disconnect the installation. If re-energising is dangerous, it must not be automatic [п. 3.1.15]. This is precisely about machine tools and conveyors that restart by themselves after supply is restored.

We will select the devices for your diagram

The breaking capacity of a device is chosen by the short-circuit current at a specific point, not from a price list. Send a single-line diagram or a completed questionnaire and we will work out the board configuration for your currents.

This material is provided for information only and is neither an offer nor legal advice. We do not publish the full text of the Rules — only quotations with clause numbers; obtain the official text from the source indicated above.

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Selecting a circuit breaker: rating, Icu, selectivity