What Information Is Required Before Designing an LT Electrical Panel?

Single-Line Diagram

The single-line diagram (SLD) is the primary document for panel design. It shows the incoming supply, the busbar arrangement, the outgoing feeders, the protection devices, the metering points and the interconnections between panels. The SLD must be approved by the project electrical engineer before panel design begins. Changes to the SLD after panel design has started can result in significant rework.

System Voltage and Frequency

The system voltage — 415 V three-phase, 230 V single-phase or other — and the frequency must be confirmed. The panel components — circuit breakers, contactors, relays, meters — must be rated for the system voltage and frequency. For export projects, the local supply voltage and frequency must be verified.

Connected and Demand Load

The connected load is the sum of the ratings of all equipment connected to the panel. The demand load is the maximum load that the panel is expected to carry at any one time, taking into account the diversity of the connected equipment. The busbar and incoming circuit breaker must be rated for the demand load, not just the connected load.

Short-Circuit Level

The prospective short-circuit current at the panel incoming terminals must be confirmed by a fault-level study. The panel components — circuit breakers, busbars, cable terminations — must be rated to withstand this fault current for the duration required to clear the fault. Using components with an insufficient fault rating is a safety hazard and a non-compliance with the applicable standard.

Incoming-Source Details

The incoming supply source — transformer, generator, UPS or another panel — must be identified. For transformer-fed panels, the transformer rating, impedance and vector group affect the fault level and the protection coordination. For generator-fed panels, the generator rating and transient characteristics affect the starting of large motors and the protection settings.

Feeder Schedule

The feeder schedule lists every outgoing circuit from the panel, including the load description, the rated current, the cable size, the circuit breaker rating, the type of starter (for motor feeders) and any special requirements. A complete and accurate feeder schedule is essential for panel design. Incomplete feeder schedules result in panels that cannot be completed without additional information.

Protection Philosophy

The protection philosophy defines how faults are detected and cleared, and how the protection devices are coordinated to minimise the extent of the outage. Overcurrent, earth fault, differential and other protection functions must be specified. The protection settings must be coordinated with the upstream and downstream protection devices to achieve selective tripping.

Metering Requirements

The metering requirements — energy meters, power factor meters, current transformers, voltage transformers, communication interfaces — must be specified. For panels subject to utility metering, the meter type and accuracy class must comply with the utility requirements. For internal energy management, the data points and communication protocol must be agreed with the BMS or SCADA engineer.

Ingress Protection and Installation Environment

The ingress protection (IP) rating of the panel enclosure must be appropriate for the installation environment. Indoor panels in clean, dry conditions may require IP31 or IP41. Panels in dusty, wet or corrosive environments require a higher IP rating. Outdoor panels require IP54 or higher. The ambient temperature, humidity and altitude at the installation location affect the current-carrying capacity of the panel components.

Cable Entry and Termination

The cable entry arrangement — top entry, bottom entry or both — and the cable gland sizes must be specified. The terminal block sizes and types must be appropriate for the cable cross-sections. For large cables, the bending radius and the space required for termination must be considered in the panel layout.

Communication and Automation

If the panel is to be integrated with a SCADA, BMS or PLC system, the communication protocol, data points, cable type and termination requirements must be specified. The control philosophy — local/remote selection, interlocking, permissive signals — must be defined in the control philosophy document.

Applicable Standards and Approved Makes

The applicable standards — IS/IEC 61439, IS 8623, project-specific standards — must be identified. The approved makes list for circuit breakers, contactors, relays, meters and other components must be provided. Using non-approved makes can result in rejection of the panel at the inspection stage.

Testing and Documentation

The testing requirements — factory acceptance test (FAT), site acceptance test (SAT), routine tests, type tests — must be specified. The documentation requirements — general arrangement drawings, schematic diagrams, bill of materials, test reports, operation and maintenance manual — must be agreed before panel design begins.