An assembly is a frame, a busbar and a set of devices working together. Knowing what each device does makes a device list readable, and a device list is where most of the cost of a board actually sits.
Protective devices
Air circuit breakers (ACB)
Used as incoming devices on larger boards. They carry the full current, interrupt high fault currents, and are usually equipped with an electronic trip unit that provides protection settings and, increasingly, metering and communication.
Moulded case circuit breakers (MCCB)
The workhorse for outgoing feeders in industrial distribution, typically from around 16 A to 1600 A. Fixed or adjustable thermal-magnetic, or electronic for larger ratings.
Miniature circuit breakers (MCB)
Final circuit protection in distribution boards. Small, modular and cheap per way, with defined breaking capacities that must be checked against the fault level at that point.
Fuses and switch-fuse units
Still common for transformer protection and for duties where a very high breaking capacity is needed in a small space.
Switching and control
Contactors
For repeated switching of motor and heating loads. Selected by utilisation category (AC-1 for resistive loads, AC-3 for motors), which is why the same nominal current can be two different products.
Thermal overload relays
Protect the motor against sustained overload. Set to the motor's service factor and coordinated with the short-circuit protection.
Soft starters and variable speed drives
Where starting current or process control requires it. Drives bring their own requirements: heat dissipation, harmonics and cable screening.
Measurement and protection
Current transformers
Step the current down to a level the meter or relay can read. Ratio, burden and accuracy class all matter, and a mismatched CT is a common cause of unreliable readings.
Protection relays
Overcurrent, earth fault and, on larger installations, more elaborate schemes. Modern relays also record events, which is often more useful than the trip itself.
Meters and power monitors
From a single incoming meter to per-feeder monitoring with communication. This is one of the three items that most affects the final price.
The current-carrying parts
Busbar
Copper, sized by current and by short-time withstand. The cross-section, the support spacing and the insulation all contribute to the rating. A busbar is a system, not a bar.
Insulators and supports
Hold the busbar and withstand the electrodynamic forces during a short circuit. Under-specified supports are a hidden weakness in cheap assemblies.
Terminals and cable lugs
Where the installation meets the panel. Terminal size, torque and identification decide whether commissioning is quick or painful.
The rest of the panel
- Surge protection devices: increasingly specified at the incoming, particularly for exposed sites.
- Capacitor switching equipment: contactors and protection rated for capacitive duty.
- Pilot devices: indication lamps, pushbuttons, selector switches and emergency stop.
- Anti-condensation heaters: small, cheap, and the difference between a reliable board and a corroded one.
- Door interlocks and mechanical safety: the parts that protect the person opening the door.
Which three drive the price
In most assemblies, the incoming breaker, the outgoing device family and the metering package account for the majority of the material cost. The enclosure and the busbar matter, but they vary less between suppliers.
This is why a price comparison built on the board rating alone is unreliable. Two 1600 A boards can differ by a factor of two in device cost while carrying the same label.