ALLPCB
Overview
In power systems, secondary circuits refer to secondary equipment used to monitor, control, regulate, and protect primary devices. Diagrams that depict these secondary circuits are called secondary circuit diagrams. Because these diagrams are highly logical and drawn according to certain conventions, understanding those conventions makes them easier to read.
Basic composition
Generally, secondary circuits include control circuits, signal circuits, measurement circuits, and automatic device circuits. When reading a secondary circuit diagram, follow these four rules:
- Read AC before DC: Start with the AC circuits in the secondary wiring diagram. After understanding the AC circuits and how their electrical quantities change during faults, infer the DC logic circuits and then review the DC circuits. AC circuits are generally simpler and easier to interpret.
- For AC circuits, find the power source; for DC circuits, find the coils: For AC circuits, start from the power supply. Identify which set of current transformers or voltage transformers provides the AC source.
- Track contacts rigorously: After locating a relay coil, find and verify all associated contacts one by one.
- Top-to-bottom, left-to-right; do not omit equipment outside the panel: When reviewing terminal block diagrams, use the expanded wiring views for cross reference and ensure devices located outside the panel are all accounted for.
