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IEC 60338: Telecontrol Equipment — Communication Standards for Power System Remote Monitoring
The “Four Remotes” Behind Substation Automation: Telemetry, Telesignal, Telecontrol, Teleadjustment
IEC 60338:1970 specifies telecontrol equipment for power systems. While modern substations have migrated to IEC 61850 digital communications, the analog telemetry standards (4–20 mA, 0–10 V) and pulse telesignal interfaces established by IEC 60338 remain in widespread use in legacy equipment.
| Signal Type | Standard Range | Application |
|---|---|---|
| Analog Telemetry (TM) | 4–20mA, 0–10V | P, Q, V, I, temperature |
| Pulse Telesignal (TS) | Dry contact / 24V DC | Breaker position, isolator status |
| Telecontrol (TC) | Relay output, pulse command | Open/close, tap change, capacitor switching |
Why is 4–20 mA the industry standard? Because it inherently provides open-circuit detection — 0 mA means sensor loop open (fault), 4 mA corresponds to zero-scale, 20 mA to full-scale. With 0–20 mA, 0 mA is both “zero reading” and “open circuit” — impossible to distinguish.
TNLab — 4–20 mA was not chosen arbitrarily. Its “live zero” design is one of the most elegant engineering insights in industrial sensing.
