Secondary design engineers are not primary design engineers: How to tell the difference
The mix-up that costs months
We get a version of this call about once a month: "We hired a Design Engineer six months ago. They are technically strong, well-liked, and completely unable to do what we need. We think we hired the wrong profile."
The pattern is almost always the same. The organisation needed a Secondary Design Engineer, someone to own protection, control, and automation from philosophy to SCADA. What they hired was a Primary Design Engineer, whose expertise is HV equipment selection, single-line diagrams, and civil coordination. Both are genuinely skilled. Neither is more valuable in the abstract. But they are not interchangeable, and hiring one when you need the other is an expensive mistake that takes four to six months to become undeniable.
Primary design
Primary design covers the high-voltage systems, the equipment you would physically see on a substation site.
Typical primary scope
- Single-line diagram development
- HV equipment specification: transformers, switchgear (AIS or GIS), busbars, cables, earthing
- Civil and structural coordination: cable routes, foundations, building interfaces
- Insulation coordination and fault level analysis
- Equipment ratings, short-circuit withstand, thermal ratings
- Interface with DNO/TSO standards and connection agreements
Primary engineers think in plant, equipment, and electrical layout. Their tools are load flow and fault analysis software, IEC and BS equipment standards, and the client’s standard drawings.
Secondary design
Secondary design is the low-voltage control, protection, and automation that monitors and operates the primary plant: the substation’s nervous system.
Typical secondary scope
- Protection philosophy development and relay coordination
- Relay specification and settings (distance, differential, overcurrent, earth fault)
- Control and interlocking logic
- SCADA and RTU configuration
- Marshalling cubicle design and wiring schedules
- FAT and SAT documentation and execution
- IEC 61850 data model and logical node configuration
A useful shorthand: primary is what you see on site. Secondary is what makes the primary work reliably and safely.
Where they overlap, and where they diverge
Both need strong power systems fundamentals and fluent drawing literacy, and each should understand the other’s interface. The overlap is real but bounded; neither can usually replace the other on a complex project.
The clearest diagnostic for a primary engineer: "Walk me through specifying a transformer for a 33/11kV substation." They should cover rating, impedance, vector group, tap range, cooling, and connection without hesitation. For a secondary engineer: "Walk me through developing a protection philosophy for a 33kV busbar." They should discuss busbar protection options, relay types, discrimination, and the interaction with upstream and downstream protection. If either gives a generic answer that could apply to both questions, you have your answer.
Ten questions to distinguish the profiles
For Primary Design Engineers
- AIS versus GIS design: the key layout and engineering trade-offs?
- Walk me through earthing design for a new 132kV substation.
- How do you approach insulation coordination for a mixed-voltage substation?
- How do you manage conflicts between client standards and project requirements?
- Describe a complex primary design challenge you resolved.
For Secondary Design Engineers
- Describe developing a protection philosophy for a meshed 33kV network.
- Which relay platforms have you used, and how do you approach distance protection settings?
- Walk me through your IEC 61850 experience and its common complications.
- How do you manage the interface between secondary design and SCADA?
- Describe your FAT and SAT experience and how you resolve test failures.
A note on hybrid profiles
A small cohort has genuine depth in both, usually from smaller consultancies where the distinction was less rigidly maintained. They are valuable, rare, and command a premium. If you think you need a hybrid, be specific about which discipline should dominate, because even a dual-qualified engineer has a stronger foot on one side.