The path power takes
Electricity enters a facility at a service point, often at a higher voltage than the machines use. From there it is divided, protected and stepped down through a series of stages. The names differ by site, but the pattern is similar nearly everywhere.
- Service entrance and metering. The utility connection, where supply is measured and the facility’s responsibility begins.
- Main switchgear or main distribution board. Receives the incoming supply and splits it into feeders. Larger sites may have medium-voltage equipment first, as described in the basics of switchgear.
- Transformers. Reduce the voltage to levels used by equipment. Industrial transformers are often placed near the loads they serve to limit long runs at low voltage.
- Distribution boards and panels. Divide power further into branch circuits, each with its own protection. Two common forms are covered in other articles on this site.
- Motor control centers and local disconnects. Group starters, drives and protection for motors, as described under motor control centers.
- Branch circuits and equipment. The final cables and conduits to the machines, lighting and sockets.
Why it is organized in layers
The layered design has two goals. The first is protection: each stage has devices, such as circuit breakers and fuses, that open if current exceeds safe limits. The second is isolation: when a fault happens, ideally only the smallest section of the system loses power and the rest keeps running. Engineers study this under the heading of coordination, and it affects which devices and settings are chosen.
Planning before installation
Good installation starts on paper, not on site. Typical planning items include:
- A load list showing every significant load, its power, voltage and duty.
- Allowance for future growth. Many facilities add equipment, and spare capacity and physical space in panels are cheaper to provide at the start.
- Calculation of fault levels so that equipment is rated to interrupt the highest fault current it might see.
- Voltage drop checks on long cable runs.
- Consideration of power factor and harmonic loads from drives, which relate to power quality.
- Single-line diagrams and layout drawings that match what is actually built.
- Decisions on critical loads that need backup, such as uninterruptible power for controls or standby generators for essential services.
All of this is the job of a qualified electrical designer working to the local code, because requirements for conductors, protection and clearances vary by location.
General principles of safe installation
This article gives general principles only. It is not installation instructions.
- Qualified people only. Distribution equipment carries lethal voltages and can release large amounts of energy in a fault. Installation, modification and testing should be done by trained, licensed or certified personnel as local law requires.
- Follow the codes and manuals. National and local electrical codes, plus manufacturer instructions, set the rules for equipment, spacing and labeling.
- De-energize and verify. Standard practice is to isolate, lock out and tag out equipment and verify that it is dead before work, using a documented procedure. Live work is avoided wherever possible.
- Correct equipment ratings. Voltage, current and fault ratings must suit the location. Equipment in dusty, wet, corrosive or hazardous areas needs suitable enclosures.
- Sound grounding and bonding. Good grounding systems help protective devices operate and reduce shock risk.
- Clear working space and access. Panels must have unobstructed space in front of them and clear exits, as local rules specify.
- Labeling. Each panel, breaker and disconnect should be identified, and drawings kept current.
- Commissioning. Inspection and testing before energizing, as in commissioning checklists, catches wiring errors and loose terminations.
After installation
Distribution systems are not set and forget. Terminations loosen, dust builds up, breakers age and loads change. Regular inspection, thermal scanning and testing by qualified staff are part of distribution maintenance, together with updating drawings whenever the system is altered. Electrical safety training and the right electrical safety equipment support everyone who works near these systems.