{"id":3104,"date":"2026-03-04T09:05:54","date_gmt":"2026-03-04T09:05:54","guid":{"rendered":"https:\/\/xbrele.com\/?p=3104"},"modified":"2026-04-07T14:49:29","modified_gmt":"2026-04-07T14:49:29","slug":"interposing-relays-plc-scada-mv-coil-control","status":"publish","type":"post","link":"https:\/\/xbrele.com\/fr\/interposing-relays-plc-scada-mv-coil-control\/","title":{"rendered":"nterposition de relais et interface PLC\/SCADA : Mod\u00e8les logiques de contr\u00f4le fiables pour les bobines MT"},"content":{"rendered":"\n

A PLC digital output module costs $300\u2013500. The closing coil on a 12 kV vacuum circuit breaker draws 6 A steady-state at 220 VDC, with inrush peaks hitting 12\u201315 A during the first 20 milliseconds. Connect them directly, and you\u2019ll replace that output module\u2014once you understand why, you never make the mistake again.<\/p>\n\n\n\n

Interposing relays form the essential bridge between programmable logic controllers and medium-voltage switchgear coils. They translate low-power digital signals into robust commands capable of actuating MV circuit breaker mechanisms while providing the galvanic isolation that protects sensitive automation electronics from the electromagnetic brutality of power apparatus switching.<\/p>\n\n\n\n

Why MV Coils Require Interposing Relays Between PLC Outputs<\/h2>\n\n\n\n

The fundamental incompatibility between PLC outputs and MV operating coils creates three immediate failure paths when directly connected. Understanding this mismatch explains why interposing relays remain non-negotiable in every properly designed control system.<\/p>\n\n\n\n

Standard PLC digital outputs deliver 24 VDC at 0.5\u20132 A maximum. MV circuit breaker coils demand something entirely different:<\/p>\n\n\n\n

Parameter<\/th>PLC Transistor Output<\/th>VCB Closing Coil<\/th>VCB Trip Coil<\/th><\/tr><\/thead>
Operating voltage<\/td>24 VDC<\/td>110\u2013220 VDC<\/td>110\u2013220 VDC<\/td><\/tr>
Steady-state current<\/td>0.5\u20132 A max<\/td>3\u20138 A<\/td>2\u20135 A<\/td><\/tr>
Inrush current<\/td>Not applicable<\/td>10\u201315 A (20 ms)<\/td>8\u201312 A (10 ms)<\/td><\/tr>
Back-EMF on de-energization<\/td>Negligible<\/td>400\u2013600 V spike<\/td>300\u2013500 V spike<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The transistor output fails through overcurrent saturation during coil energization, voltage stress from back-EMF transients punching through the semiconductor\u2019s rating, and conducted noise corrupting the PLC communication bus.<\/p>\n\n\n\n

When current flows through an MV operating coil, energy stores in the magnetic field\u2014typically 5\u201315 joules for a vacuum circuit breaker<\/a> closing coil. The moment the control contact opens, this stored energy seeks release. A transistor rated for 30 VDC withstands perhaps 60 V absolute maximum. A 450 V transient destroys it in microseconds.<\/p>\n\n\n\n

According to IEC 61131-2 (Programmable Controllers \u2013 Equipment Requirements), PLC digital outputs must maintain electrical isolation of \u22651500 Vrms<\/sub>\u00a0between field circuits and internal logic. The interposing relay provides an additional isolation barrier, typically rated at 2500 Vrms<\/sub>\u00a0per IEC 61810-1 (Electromechanical Elementary Relays), creating a combined isolation architecture that protects sensitive control electronics from transient voltages common in MV switching environments.<\/p>\n\n\n\n

Substation environments compound these challenges. During bus faults, ground potential rise at the switchgear location can exceed 1 kV relative to the control room. Capacitive coupling from flashovers induces kilovolt-level transients on control wiring. Interposing relays provide 2\u20134 kV isolation between coil and contact circuits, physically separating the automation domain from the power apparatus domain.<\/p>\n\n\n\n

\"Signal
Figure 1. Signal flow from PLC digital output (24 VDC, 0.5 A) through interposing relay to MV closing coil (220 VDC, 12 A inrush), with 2.5 kV galvanic isolation barrier.<\/figcaption><\/figure>\n\n\n\n

How to Select Interposing Relays for MV Control Circuits<\/h2>\n\n\n\n

Relay selection determines whether your control interface operates reliably for a decade or fails within two years. The specifications that matter most aren\u2019t always the ones prominently displayed on data sheets.<\/p>\n\n\n\n

Contact Rating and Material<\/strong><\/p>\n\n\n\n

The interposing relay must handle actual coil current, not catalog nominal values. For a closing coil drawing 6 A steady-state with 12 A inrush, calculate minimum contact rating at 150% of inrush\u201418 A in this case. Then apply a 40% derating factor for DC inductive loads. You need contacts rated for at least 30 A resistive equivalent.<\/p>\n\n\n\n

Contact material selection directly impacts service life. Silver cadmium oxide (AgCdO) offers excellent arc resistance for DC coil switching. Silver tin oxide (AgSnO\u2082) provides a cadmium-free alternative with comparable performance. In mining applications with frequent switching cycles exceeding 20 operations daily, field testing demonstrated 40% longer contact life with tungsten-faced contacts versus silver alloy alternatives.<\/p>\n\n\n\n

Coil Voltage Matching<\/strong><\/p>\n\n\n\n

Match the interposing relay coil to available PLC output capability:<\/p>\n\n\n\n

PLC Output Type<\/th>Relay Coil Voltage<\/th>Coil Power Range<\/th><\/tr><\/thead>
Transistor (NPN\/PNP)<\/td>24 VDC<\/td>0.5\u20131 W<\/td><\/tr>
Relay output<\/td>24 VDC\/VAC<\/td>1\u20132 W<\/td><\/tr>
Triac output<\/td>24\u2013120 VAC<\/td>0.5\u20131.5 W<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Low-power relay coils under 0.5 W suit direct transistor drive. Higher-power types may require an intermediate pre-interposing relay\u2014creating a two-stage isolation chain for critical applications.<\/p>\n\n\n\n

Response Time Budget<\/strong><\/p>\n\n\n\n

Every interposing relay adds delay. For protection applications, this delay must fit within coordination time margins:<\/p>\n\n\n\n