Walk through any industrial facility or commercial building and chances are the low-voltage distribution board was sized by someone working fast under deadline pressure. The result? Breakers that trip without warning, cables that overheat in silence, and arc-flash events waiting to happen. Most of these problems trace back to a handful of recurring specification errors—not exotic failure modes, just mistakes made every day on real projects.

This article unpacks six of those mistakes in plain terms. If you are an electrical engineer, a panel builder, or a facility manager who has inherited someone else’s design, you will likely recognise at least one of them.

Mistake 1: Selecting by Rated Current Alone and Ignoring Short-Circuit Rating

Of all the sizing errors on this list, this one causes the most catastrophic failures. A circuit breaker’s rated current tells you how much continuous load it can carry; its short-circuit breaking capacity (Icu or Ics) tells you whether it can interrupt a fault without destroying itself. Confusing the two is like choosing a fire extinguisher based on weight while ignoring what class of fire it covers.

When the prospective short-circuit current (PSCC) at the installation point exceeds the breaker’s rated breaking capacity, the breaker cannot extinguish the arc. The resulting arc-flash can rupture the enclosure, start fires, and cause severe injuries. The failure happens in milliseconds and there is no second chance.

What to do instead

• Calculate the PSCC at every distribution point using the actual source impedance and cable lengths.

• Select a breaker whose Icu rating is at least 20 % above the calculated PSCC—not exactly equal to it.

• Remember that PSCC changes if the upstream transformer is ever replaced with a lower-impedance unit. Document your calculations so future engineers can re-verify.

Mistake 2: Over-Sizing the Rated Current “Just to Be Safe”

The instinct to specify a larger breaker than the load requires sounds prudent. It is not. Overcurrent protection only works when the breaker trips before the cable it protects reaches its thermal limit. A 63 A breaker on a circuit that was designed for a maximum 25 A cable will sit there happily while the cable overloads and its insulation degrades — it will not trip until the current reaches a level the cable simply cannot survive.

The upstream breaker is there to protect the cable, not the load. Size the breaker rating at 1.1 to 1.2 times the design current, never more than twice the cable’s continuous current rating.

A practical check

Before finalising a rating, confirm that the breaker’s overload trip threshold at 1.45×In falls below the cable’s safe current-carrying capacity at the installation temperature. If it does not, you need a smaller breaker or a larger cable.

Mistake 3: Mismatching Trip Curves to Load Types

Standard trip curve categories exist precisely because loads behave differently at start-up versus steady state. A Type B curve trips at 3–5×In, a Type C at 5–10×In, and a Type D at 10–20×In. Choosing the wrong one causes nuisance tripping during normal inrush, or — worse — delayed tripping during a genuine fault.

Common mismatch scenarios:

• Type C on motor circuits: Motors draw 5–8× rated current at start-up. A Type C breaker may trip during every start, especially for large motors on weak supplies.

• Type D on lighting circuits: The magnetic trip threshold is so high that a fault current capable of starting a fire might not trigger the instantaneous element at all.

For motor feeders, use Type D or a breaker with an adjustable magnetic threshold and a motor start-up delay. For resistive and lighting loads, Type B keeps fault clearing fast. For general power sockets with mixed loads, Type C is usually the right balance.

Mistake 4: Choosing the Wrong RCD Type for the Load

Residual current devices (RCDs) are not interchangeable. The IEC 62423 and EN 61008 families define several types based on the waveform of leakage current they can detect: Type AC responds to sinusoidal leakage only; Type A also covers pulsating DC; Type B covers smooth DC and high-frequency leakage.

Two mismatches come up repeatedly on modern installations:

• VFDs, EV chargers, and PV inverters behind a Type AC or Type A device: These loads generate high-frequency and DC-component leakage currents that a Type AC or even Type A RCD may not detect consistently. The result is either nuisance trips (device reacts to normal operating leakage) or complete non-detection. Both outcomes are safety failures.

• Trip-type RCDs on fire-safety circuits: If a fire fault triggers an RCD protecting the fire suppression panel or emergency lighting, those systems lose power exactly when they are needed most. Use an RCD with an alarm output instead, or place fire-safety loads on a separate supply upstream of the RCD.

Specify Type B RCDs for circuits feeding VFDs, chargers, UPS systems, and PV strings. For circuits where unwanted disconnection is dangerous, use a residual current monitoring device (RCMD) with an alarm relay rather than an auto-trip RCD.

Mistake 5: Skipping Selectivity (Discrimination) Between Upstream and Downstream Breakers

Selectivity means that only the breaker closest to a fault opens, leaving the rest of the distribution network energised. Without it, a fault on one branch circuit trips the main incomer and blacks out the whole floor—or the whole building.

The most common failure is specifying purely instantaneous (non-delayed) breakers throughout the entire hierarchy. When a fault occurs, every breaker from the branch all the way to the incomer sees the same high current simultaneously. The one that happens to be physically closest to the fault should trip first, but with no intentional time stagger, it is a race condition you cannot control.

How to build selectivity into your design

• Current selectivity: Set at least a 1.6:1 ratio between the instantaneous-trip thresholds of adjacent upstream and downstream breakers. This gives the downstream device a credible chance to clear before the upstream device sees enough current to trip instantaneously.

• Time selectivity: Fit the incomer and main distribution breakers with short-time-delay (STD) trip units. A 100–150 ms delay on the incomer versus an instantaneous response on branch breakers creates a clean discrimination window without compromising fault damage.

• Energy selectivity (zone-selective interlocking): For critical LV main switchboards with multiple tiers, use ZSI wiring so the incomer only applies its delay when a downstream breaker is already reacting.

Mistake 6: Not Applying Derating Factors for High Ambient Temperature

A circuit breaker’s rated current is defined at 40°C (or sometimes 25°C) ambient. In foundries, heat-treatment lines, cement plants, and any outdoor installation in a tropical climate, ambient temperatures routinely exceed those reference conditions. Every extra degree reduces the thermal headroom inside the breaker’s bimetal element, which means the overload trip threshold drifts downward even though the dial still shows the original rating.

The practical effect: a 100 A breaker set to 100 A in a 55°C enclosure may trip persistently at 75–80 A under steady load, because the bimetal has been heated partly by the ambient before the current even flows.

Design practices that prevent this

• Check the manufacturer’s derating curve for the specific breaker model at the expected ambient temperature.

• Either reduce the set-point to compensate, or select a breaker with a higher continuous current rating so the derated value still covers the load.

• For sustained high-temperature environments, choose products explicitly rated for elevated ambient operation (many manufacturers offer 50°C or 55°C variants).

• Improve enclosure ventilation or air-conditioning before accepting persistent unexplained tripping as a “normal” condition.

Putting It All Together

None of these six mistakes is obscure. They appear in project after project because specifications are copied from similar jobs, reviewed under time pressure, or left to whoever is pricing the panel. The fix is not more sophisticated software—it is a short checklist applied consistently at the design stage:

• Verify PSCC at each distribution point and apply a 20 % safety margin on breaking capacity.

• Size rated current to protect the cable, not just to handle the load.

• Match trip curve category (B / C / D) to the actual inrush characteristics of each load.

• Specify RCD type (AC / A / B) based on the leakage waveform the load actually generates.

• Deliberately design current and time selectivity into each tier of the distribution hierarchy.

• Apply manufacturer derating curves whenever the installation ambient exceeds the reference temperature.

A few extra hours at the specification stage routinely prevents years of nuisance tripping, unexplained failures, and — in the worst cases — fires. That is a trade-off any design team should be comfortable making.