Distribution Boards & Panels

A modern distribution system is a chain: utility / genset → main switchgear → distribution boards → final circuits. Each link has its own protection device, its own short-circuit rating, and its own coordination with the device upstream and downstream. Get the chain right and a fault clears in 50 ms with one breaker; get it wrong and a fault takes the building dark.

Cable sizing is the most common source of trouble. BS 7671 Tables 4 specify ampacity by installation method (clipped, conduit, tray, ground, derated for grouping and ambient). The rule is conductor must carry full-load current × 1.25 continuous, plus voltage drop ≤ 3% to the MDB and ≤ 5% to the final socket. We see 4 mm² runs of 35 m on 32 A circuits weekly — that is a fire risk no breaker will catch.

Earthing strategy precedes everything. TT, TN-S, TN-C-S and IT systems each have specific earth-fault behaviours. East-African urban supplies are typically TN-C-S; medical wards must be IT (isolated) for life-protection circuits. Mixing strategies inside one building is a recipe for circulating currents and tripping nuisance.

Protection coordination uses two graphs side-by-side: the upstream device curve and the downstream device curve. Discrimination means the downstream curve sits entirely below the upstream curve up to the let-through current. Modern Schneider, ABB, Siemens, and Eaton catalogues publish let-through tables that make this almost mechanical — but only if the engineer reads them.

Short-circuit current at the busbar must be calculated, not guessed. The transformer impedance, source impedance, and cable run determine prospective fault current. Type-2 coordinated motor starters require this number to specify the contactor ratings; an MCB rated 6 kA on a 25 kA bus will explode rather than trip.

Harmonics from rectifiers, VSDs, UPS systems, and LED lighting now dominate the harmonic profile of commercial buildings. The classic over-rated neutral conductor and K-rated transformers are still the right answer; ignoring them produces neutral overheating fires that are mis-attributed to "overloaded" sockets.

Switchgear types: ACBs (air circuit breakers) for incoming mains, MCCBs (moulded case) for distribution feeders, MCBs / RCBOs for final circuits. RCDs come in Type AC, A, F, and B — Type B mandatory anywhere DC components exist (EV chargers, VSDs, solar inverters). A Type AC RCD downstream of a VSD can become "blind" to earth faults — silent and dangerous.

Capacitor-bank power-factor correction must be detuned (typically 7% reactor) on harmonic-rich sites or it amplifies the 5th and 7th harmonics into resonance. We have rebuilt three Nairobi industrial PF banks in 2025 that exploded for exactly this reason; the original installer specified untuned capacitors.

Documentation is part of the install, not an afterthought. The single-line, switchboard schedule, earth-loop test results, insulation-resistance log, and thermography baseline form the commissioning pack. Without them there is no defensible answer when an inspector or an insurance adjuster asks "what is the prospective fault current at this point in 2026?"

Maintenance follows two timelines. Annually: thermography of every joint at ≥ 50% load (a hotspot of > 20 °C above ambient is alert; > 40 °C is shutdown). Five-yearly: secondary injection on every protective relay to verify pickup and time-delay. Skipping the second test means relying on hope.