Before starting any works on any electrical system, the owners shall obtain an Electrical Permit from the Office of the Building Official (OBO). In securing the electrical permit, the services of a licensed electrical practitioner are required under the Republic Act 7920 or national electrical engineering law. Surprisingly, an Electrical Engineer from the Office of the Building Official (OBO) required a Design Analysis Report for a two (2) storey house installation as a pre-requirement for obtaining a building permit. The owner sought my services to prepare the Design Analysis Report.
Many electrical plans for domestic installations particularly those prepared by Architects as part of their design package only include the load schedule. The load schedule alone is not sufficient to support the selection of cables and the overcurrent protective devices. Let alone the selecting the correct interrupting ratings of overcurrent protection as provided in the Philippine Electrical Code Section 10.1.1.9.
Interrupting Rating
Equipment (fuses or circuit breakers), intended to interrupt current at fault levels, shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
PEC Section 1.10.1.9 requires that all fuses and circuit breakers intended to interrupt a circuit at fault levels have an adequate interrupting rating wherever they are used in the electrical system. Fuses or circuit breakers that do not have adequate interrupting ratings could rupture while attempting to clear a short circuit. The interrupting rating of overcurrent protective devices is determined under standard test conditions that should match the actual installation needs.
Interrupting ratings should not be confused with short-circuit current ratings.
Overcurrent Protective Devices
The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. The fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s) permitted in PEC Section 2.50.6.9. The calculation of these parameters is an integral part of the Design Analysis Report.
The Design Analysis Report will be the basis that overcurrent protective devices (such as fuses and circuit breakers) are selected to ensure that the shortcircuit current rating of the system components is not exceeded should a short circuit or high-level ground fault occur.
Available Short-circuit Levels
Utility companies usually determine and provide information on available short-circuit current levels at the service equipment. Literature on how to calculate short-circuit currents at each point in any distribution generally can be obtained by contacting the manufacturers of overcurrent protective devices or by referring to IEEE 141-1993 (R1999), IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book). Adequate short-circuit protection can be provided by fuses, molded-case circuit breakers, and low-voltage power circuit breakers, depending on specific circuit and installation requirements.
Design Analysis Report
A Design Analysis Report shall contain the following as a minimum
- Purpose of the document
- Location Plan of the property
- Single Line Diagram
- Load Schedule
- Cable Sizing Calculations
- Cable Sizing Criteria
- Maximum Admissible Conductor Temperature
- Short-ciruit calculations
- Selection of Overcurrent protective devices