Industrial


Selecting a power relay for safety in industrial applications


By Mark Patrick, Mouser Electronics C


hoosing relays for use in industrial applications requires considerable thought. Standard selection criteria include reviewing the voltage and


currents involved and the maximum space available for the relay, taking note of any specific component separation distances. A more complex consideration is whether any particular safety standards might apply to the application. This article investigates the requirements design engineers face when developing industrial and commercial applications and selecting suitable relays. There are two technical aspects engineers need to carefully review during the relay selection process. This article will highlight how inrush current peaks can significantly damage relay contacts to the extent they can no longer open. This failure can highlight the need for compliance to safety standards for equipment such as elevators, industrial cutting machines, and railway signalling. The second topic involves space-constrained environments such as an industrial control cabinet and finding sufficient space to install the required number of relays.


Selecting relays for electrically demanding environments In the last ten years, the industrial manufacturing landscape has experienced rapid transformation. With a focus on increasing overall operating effectiveness, trends such as Industry 4.0 and the Industrial Internet of Things (IIoT) have driven the growth of industrial automation equipment and systems.


Industrial systems need an efficient way of controlling high power rails to equipment from a low voltage interface. Electromagnetic relays provide an effective method of achieving such actions and typically can switch multiple sets of contacts on or off from one signal. While the humble relay can offer a straightforward and low-cost solution, there may be situations where the equipment controlled is subject to stringent functional safety requirements. For example, an operator using a hydraulic press must be 100


14 April 2022 Components in Electronics www.cieonline.co.uk


per cent certain that it operates only when instructed. Functional safety standards, such as IEC 61508 and its industrial equipment derivatives, stipulate the safety integrity levels (SIL) required to protect equipment users and prevent equipment damage. One safety standard, IEC 61810-3, previously known as EN 50205, applies explicitly to relays used for safety and self-monitoring control applications.


Dealing with inrush currents Inrush currents can be potentially damaging to any item of electrical switchgear such as switches, contactors, and relays. Inrush currents occur when the supply voltage arriving at the load causes excessive currents to flow. Inductive loads such as motors and transformers, and capacitive loads such as energy storage banks, are particularly prone to experiencing high inrush currents. The magnitude of inrush currents might be up to forty times the average steady-state current. In a relay, the sudden current surge may result in an instantaneous arcing between the relay contacts, resulting in them becoming welded together. The relay


Figure 1 - Close-up image of FGR contacts from TE Connectivity


may appear to operate as expected, but its ability to switch the power to the load off has been compromised.


There are several ways to reduce inrush currents. These include slowly increasing the voltage applied to the load or using a zero-crossover switch (ZCS) such as a solid-state relay (SSR). A ZCS works on an AC supply voltage by detecting when the voltage becomes zero, and hence the current is zero, between the positive and negative peaks - the zero-crossing point. Once the zero-crossing occurs, the relay can switch the load on or off.


Figure 2 - The TE Connectivity SCHRACK force guided relay portfolio (source: TE Connectivity SCHRACK)


Unfortunately, the nature of highly inductive loads is that the point of maximum current may lag the voltage peak considerably, also displacing the timing of zero voltage and current. Consequentially, the zero crossing and solid-state relay approach may result in switching high currents. Also, ZCS/SSRs are well-known as a potential source of disruptive and unwanted electromagnetic interference (EMI) from the switching methods used. Achieving functional safety compliance with a ZCS/SSR requires complex monitoring of the semiconductor circuitry.


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