Feature: Connectors, cables and enclosures


Designing a reliable interface layer By Darren Hodson, Managing Director, WAGO UK & Ireland


and firmware rarely cause the late- night call-out. The trouble sits at the boundary, where a cable terminates, a signal crosses between two circuits with different ground references, or a 24V DC rail is required to cope with a short circuit fifty metres down the line. That boundary is the interface layer.


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In a typical control cabinet it contains the switch mode power supply (SMPS) and its secondary protection, the signal conditioners and isolation amplifiers that clean up analogue measurements, the relays and optocouplers that separate control and load circuits, and the connectors and system cables that carry data between controller and field. Each of these elements is a connection problem as much as an electronics problem, and each needs careful specification.


sk any commissioning engineer where faults are found and the answers are pretty much the same. Processors


Start with the DC rail Most industrial electronics run on 24V DC derived from an SMPS. Modern units are highly efficient, with WAGO’s Eco 2 family reaching up to 95%, which reduces heat inside the enclosure and extends component life. Efficiency is only part of the specification, however. Engineers should also consider whether the application needs redundancy modules to parallel two supplies, whether communication and configuration features are needed for remote diagnostics, and how the output will be protected. That last point is where many designs


fall short. A switch mode supply limits its output current very quickly when overloaded. This protects it, but it creates a problem for the fuses and circuit breakers that engineers habitually fit on the secondary side. A miniature circuit breaker relies on a large overcurrent to trip magnetically. For a B-characteristic device, the threshold is three to five times nominal current and, once safety factors are applied, the worst case can


36 November 2026 www.electronicsworld.co.uk


require 7.5 times nominal current. If the power supply can’t deliver that current, protection falls back on thermal tripping, which is slow at low overcurrents. A fuse facing a twofold overload, for example, can take more than 100 seconds to clear it.


Cable resistance makes matters worse.


Consider a sensor circuit fed through 50m of 0.75mm² copper cable. With a resistivity of about 0.0175Ω mm²/m, the 100m loop presents roughly 2.3Ω, which limits a dead short at the far end to about 10A, even before the supply’s own current limit is considered. A 6A B-curve breaker may need up to 45A for instantaneous magnetic tripping, so that fault can persist for seconds or minutes while the conductor heats. EN 60204-1 requires hazardous earth faults in control circuits to be disconnected within five seconds, and thermal tripping alone can’t guarantee this. Electronic circuit breakers (ECBs)


address the problem directly. The ECB continuously measures output current on each channel. When current exceeds


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