1. The Philosophy of Selectivity and Coordination of Protective Devices
A protective device must disconnect only the faulty section and leave the remaining loads energised – this is selectivity. In RUSP and SchM panels the same few things break it: identical ratings on the incoming and outgoing breakers, RCDs with no grading by residual current, and a type B breaker on a circuit with inrush currents. The result is either the incomer tripping and the whole site going dark, or the protection not tripping at all.
The engineer must understand the physical processes that occur during overloads and short circuits, and must be able to correctly match the time-current characteristics (TCC) of devices connected in series. Reviewing typical mistakes makes it possible to eliminate accidents at the design stage.
2. Mistake 1: Mismatch Between the Breaker Rating and the Protected Cable Cross-Section
This is the most common and dangerous mistake, made by amateurs and inattentive installers alike. The core technical misconception goes like this: "The breaker rating should be selected based on the power of the connected appliance." This is fundamentally wrong. A circuit breaker is designed to protect the cable line from overheating and fire, not the appliance itself!
If a flexible copper cable with a 1.5 mm² cross-section (whose continuous permissible current per PUE is 19 A in conduit) is used to connect a power tool, and a 32 A circuit breaker is installed for that line in the RUSP board, the protection will not work. At a load current of 28 A the cable will start to heat up intensively, the PVC insulation will melt, and a fire will begin, but the 32 A breaker will not trip, because for it this current is within the normal range. The coordination rule is strictly governed by the PUE formula:
Where: I_calc is the calculated load current; I_n is the rated current of the breaker's release; I_perm is the continuous permissible current of the cable for the given installation conditions. To correctly determine the breaker rating: for a 1.5 mm² cable the maximum breaker rating is 16 A, for a 2.5 mm² cable it is 25 A (under ideal conditions) or 16 A (for harsh operating conditions in bundles with reduced heat dissipation).
3. Mistake 2: Ignoring Time-Current Characteristics (Types B, C, D)
The second critical mistake lies in misunderstanding the purpose of the letter indices (B, C, D) preceding the numerical rating of the breaker on its body. These letters indicate the tripping multiple of the electromagnetic (instantaneous) release relative to the rated current. The characteristics of modular circuit breakers are specified by GOST IEC 60898-1-2020.
- Type B – trips in the range of 3 to 5 times the rated current (used for long lines and lighting, where short-circuit currents may be low due to wire resistance).
- Type C – trips in the range of 5 to 10 times the rated current (a universal type for standard loads).
- Type D – trips in the range of 10 to 20 times the rated current (designed for loads with high inrush currents).
If, in a mechanization board, a 16 A type-B breaker is installed for a line feeding a powerful compressor or plastering station with an 80 A inrush current, the breaker will trip instantly on every motor start. For characteristic B the instantaneous tripping zone lies between 3 and 5 times the rating, which for a B16 is 48 to 80 A. An inrush current of 80 A falls within this zone, so tripping is likely; the release is guaranteed not to operate only up to 3 times the rating, that is up to 48 A. Installing a type D16 breaker completely solves the problem: it will withstand a short-term inrush pulse of up to 160-320 A while still providing reliable protection against a real short circuit. Knowing how to select breakers by TCC type prevents technological downtime of equipment.
| Protection Characteristic Type (GOST IEC 60898-1-2020) | Tripping Current Multiple of the Instantaneous Release | Typical Application in RUSP / SchM | Consequences of an Incorrect Choice |
|---|---|---|---|
| Type B | 3 · I_n - 5 · I_n | Extended lighting networks, site cabins, remote points | Nuisance tripping when hand tools are switched on |
| Type C | 5 · I_n - 10 · I_n | Standard hand power tools (drills, rotary hammers, angle grinders) | Baseline standard; a compromise solution |
| Type D | 10 · I_n - 20 · I_n | Concrete mixers, compressors, pumps, hoist cranes | If installed on thin lines – will not trip on a remote, weak short circuit |
4. Mistake 3: Incorrect Selection of Breaking Capacity (Classes 4.5 kA, 6 kA, 10 kA)
The breaking capacity (BC) parameter indicates the maximum short-circuit current that a circuit breaker is able to interrupt while maintaining its mechanical integrity and preventing the electric arc from spilling out. Household breakers most often have a 4.5 kA class.
In construction mechanization boards (SchM) located in the immediate vicinity of main transformer substations (KTP), the total impedance of the phase-neutral loop is extremely low, and real short-circuit currents can reach values of 7-9 kA. Installing a 4.5 kA or even a 6 kA class breaker in such a board will result in the breaker's contacts welding together and the housing exploding at the very first hard short circuit, triggering a large-scale fire inside the board. Per the PUE and the requirements for industrial installations, main mechanization distribution boards must use protective devices with a breaking capacity of no less than 10 kA (GOST R 50030.2-2010).
5. Mistake 4: Absence or Incorrect Selection of Residual Current Protection (RCD)
A circuit breaker protects only the cable from overcurrents. It is completely "blind" to leakage currents that arise when insulation inside a tool is damaged and the phase potential is carried over to the metal body of a drill or concrete mixer. A person touching such a body becomes a conductor of current to earth. A current of 50-100 mA through the human body is lethal, but a 16 A breaker will never trip such a load.
It is a mistake to completely ignore RCDs (residual current devices) in RUSP boards or to install a single common RCD with a 300 mA trip setting for the protection of people (300 mA is a fire-protection RCD that protects the building from ignition; for a human this current is lethal). Protection of a board to which portable tools are connected is built on an RCD with a trip setting of no more than 30 mA: the value is set by clause 7.1.79 of the PUE, while clause 7.1.82 makes it mandatory for socket circuits outdoors and in premises of particular danger or increased danger. The 10 mA setting for wet zones is a manufacturers' recommendation and is not present in Chapter 7.1 of the PUE. It is also a mistake to choose a type-AC RCD (which responds only to alternating sinusoidal leakage current) for lines that use modern inverter welding machines or speed-controlled tools – there the leakage currents have a pulsating DC component, and a type-A RCD is required.
6. Protection Coordination: Joule Integral and Arc Energy Calculation (I²t)
Selecting circuit breakers solely by rated current and time-current characteristic (types B, C, D) is insufficient for building cascaded (multi-level) protection of complex facilities. The pinnacle of engineering design for RUSP and SchM is achieving full selectivity. Selectivity means that when a short circuit occurs on a specific power tool, only the breaker feeding that particular socket (for example, 16 A) should trip, while the board's incoming breaker (for example, 100 A) should remain closed, maintaining power to the other loads. A breakdown of selectivity leads to the complete de-energization of the entire construction site due to a fault in a single angle grinder.
At high short-circuit currents (hundreds and thousands of amperes), the time-current curves of the breakers merge. Both breakers (the 16 A and the 100 A) enter the zone of instantaneous electromagnetic tripping and attempt to trip simultaneously (in less than 0.01 seconds). In such modes, selectivity is ensured by the method of energy coordination, based on limiting the let-through energy (the Joule integral, I²t). Step-by-step calculation and verification of energy selectivity:
Step 1: Determine the short-circuit current. Assume the calculated short-circuit current in the circuit is 3000 A (3 kA).
Step 2: Analyze the characteristics of the downstream (outgoing) breaker. The let-through energy at a given short-circuit current is taken from the I²t curves in the catalogue for the specific device: there is no single value for a 16 A rating. GOST IEC 60898-1-2020 defines the I²t characteristic (clause 3.5.13) and permits classification by it (clause 4.7), but sets no numerical class boundaries and does not require the class to be marked on the device: the manufacturer supplies the I²t characteristic on request. The division into classes 1, 2 and 3 is therefore not always comparable between brands.
Step 3: Analyze the characteristics of the upstream (incoming) breaker (CB2). The 100 A incoming breaker has its own non-tripping energy (the energy required to melt its bimetallic strip or activate its powerful electromagnet). The energy limit that does not cause the incoming device to trip is likewise taken from its catalogue data.
Step 4: Mathematical verification of selectivity. The condition of energy selectivity is met if the let-through energy of the downstream device is strictly less than the non-tripping energy of the upstream device. The let-through energy of the outgoing device is compared with the non-tripping energy of the incomer using the catalogue curves of both. If the former is smaller, during a short circuit the outgoing 16 A breaker will take on the arc and interrupt the circuit, while the 100 A incoming breaker will not even have time to react, since the released energy will be insufficient to activate it.
This same parameter (I²t) is used to verify the thermal withstand of outgoing cables. A 2.5 mm² copper cable with PVC insulation withstands a thermal impulse of I²t = (K × S)² = (115 × 2.5)² = (287.5)² = 82,656 A²·s. The let-through energy of the breaker taken from the catalogue must be less than these 82,656 A²·s – that is what protects the insulation against ignition. In cheap breakers the arc burns longer (letting through energy of up to 100,000 A²·s), which inevitably leads to the shutdown of the entire substation and the charring of cables.
View in catalog: Modular Circuit Breaker (MCB), Residual Current Circuit Breaker (RCCB), Residual Current Device (RCD), Modular Residual Current Devices (RCD).