Increased reliance on automation will be pivotal in helping manufacturing facilities to raise productivity levels and reduce wastage. Upholding the ongoing operation of machinery and robotic systems and avoiding downtime due to the replacement or repair of constituent components, must be the highest priorities – or else the profitability of the facility will be severely impacted upon. Likewise, rapidly responsive safety mechanisms need to be put in place to protect valuable equipment from damage and factory staff from the prospect of injury.

With this in mind, it is clear that the relays incorporated into such machinery/systems should be chosen wisely. As well as attaining strong performance benchmarks, they must exhibit exceptional robustness and prolonged working lifespans. The following article will look at the key aspects that relays —both electromechanical and semiconductor based— need to possess if they are to be successfully utilized in a modern industrial context.   

Understanding relays’ operation and their usage

Electromechanical relays have been employed in electronic/electrical hardware for many decades, providing both system switching and safety related tasks. How they function is simple enough to grasp. The applying of a small current will result in the generation of a magnetic field within the relay’s coil. This will in turn activate a switching mechanism via which a circuit running at a higher current can be controlled while maintaining electrical isolation between the two circuits.

Solid-state relays are likewise able to provide isolated switching, but here another approach is taken for delivering the isolation (usually based on an optoelectronic-oriented arrangement) instead. As we will see later, there are times when use of electromechanical relays will prove to be most apt, and others where solid-state relays will be the best option.

Relays in either of these two forms will be deployed extensively throughout industrial sites. They can be found in control panels, industrial drives, and programmable logic controllers (PLC). Increasingly, they are being used for managing the movement and ensuring the safe operation of robotic systems, in particular for ‘light curtain’ application – where they are used to stop a robot arm from moving if a human comes too close to it.

Relay reliability is a very high priority when it is going to be integrated into some form of industrial equipment. If a malfunction event occurs, then it is likely to result in very serious consequences. Should the output of a facility’s production lines end up being halted, then the company’s revenue will suffer during the downtime that ensues. Exposure to extreme temperatures and heavy vibrations will, of course, potentially need to be taken into account as these will regularly be present in factories and processing plants.

The risk of failure must be mitigated as much as possible, with only minimal ongoing maintenance being required. This is especially important in situations where it will be difficult to access the equipment in which the relay is contained after installation.   

Operational costs can be a concern too, with power consumption becoming increasingly critical. Therefore, it is advisable to pick devices that will help to minimize factory utility bills. There may also be space constraints to contend with, and in such cases a compact sized relay solution will need to be specified.

Thought needs to be given to the electrical properties of the relay. Compliance with the IEC-61010 insulation standard is likely to be expected, so that lives of factory operatives are not put in danger. Next a decision will need to be made on what type of casing will be appropriate. The conducting of IEC-60335-1 glow wire testing will confirm that plastic material from which the relay casing is made is not flammable and is therefore acceptable to use in industrial systems. As industrial drives and motors are inductive loads, they can be prone to high inrush currents. Rugged relay solutions are thus essential here, otherwise the contacts may melt together.

Another pivotal trait the relays must have if they are to be deployed in challenging industrial settings is the ability to deal with liquid ingress. Devices that have wash-tight sealed housings may need to be specified, in accordance with the IEC-61810 standard. They may also need to be flux-proof so that they have adequate protection during the soldering process. If a relay specification mentions RTIII then this signifies that it is wash-tight, while RTII confirms that it is flux-proof and there will be no problem when it is being soldered to the PCB. Wash-tight will only be used if absolutely necessary, with flux-proof being preferred, as outgassing from the housing can lead to a damaging of the contacts. The soldering of PCB-mount relays onto a board will require a second separate soldering process. Through-hole relays are thus preferred as a conventional reflow process can be followed, which will be cheaper.

The number of poles (which determines how many contacts there are and how many circuits can be controlled) is a further consideration. The more poles that can be accommodated into a relay device, the greater the space saving derived will be. There are additional upshots from this – in terms of a lower overall bill-of-material cost, a smaller power budget, etc.

Given that industrial systems can potentially stay in service for many years, there is the issue of legacy equipment still needing to be kept running. The sourcing of drop-in replacement relays for outdated models (that have the same form factor with equivalent operational parameters) can hence be an issue that needs to be attended to.   

When deciding on whether to use an electromechanical or a solid-state relay, the first question that ought to be asked is how many switch cycles the selected relay will need to complete over the course of an average day. The higher cycle durability of solid-state relays may be the deciding attribute here. If cycling is very high (above 100,000 cycles during the relay’s working life) then the reliability of an electromechanical one might not be enough. Where solid-state relays have another significant edge is in relation to ongoing vibrational forces. They perform much better in such scenarios, as they do not have any moving parts. Low noise is another advantage to be aware of. In comparison to their electromechanical counterparts, solid-state relays are markedly more expensive. Nevertheless, this may be justified if their use avoids replacements needing to be made, with equipment thereby running for longer.

There are plenty of circumstances where electromechanical relays show themselves to be the best route to take though. Solid-state relays will get hotter when there is intense switching activity, so accompanying heatsinks will be mandated, whereas electromechanical relays have a lower contact resistance and so this will not be necessary here. The increasing expense associated with heatsink inclusion, plus the extra space that will be taken up, can preclude solid-state relays from certain applications. Another issue which should be mentioned is that solid-state relays have greater vulnerability to electrostatic discharge (ESD) strikes —and this is something that is likely to be commonplace in industrial locations.   

Drawing on all the different factors outlined, here are some of the relays that vendors currently offer, and the reasons why they will be applicable in particular use cases.  

When relays are required to withstand high inrush currents then Omron’s G5RL-HR or TE Connectivity’s RTS will be worth investigating. If insulation is of prime importance, relays like the Panasonic PA-N should be looked into. This through-hole mounted device complies with the IEC-61010 standard, withstanding surge voltages of up to 6kV. It features a monostable coil with 3V-24V coil voltage and draws only 110mW of power. It has dimensions of just 5mm x 20mm x 12.5mm.

The SRL7 6A-rated 7-pole force-guided safety relay from TE Connectivity has 2 poles normally closed (NC) and 5 normally open (NO), with reinforced insulation between the contact circuits. It has IEC-61810 and RTII compliance, plus a 10×106 mechanical operation endurance (thanks to its silver alloy contacts). The working temperature range of this relay spans from -40°C to 85°C. Its low coil power of 700mW means that very little heat is generated when it is activated. Another characteristic is its low profile (with a height of just 10.8mm). There is a 4.000Vrms dielectric strength between the NO and NC contacts.

The SFM safety relays from Panasonic also conform to IEC61810-3 guidelines and features reflow soldering. Requiring just 100mW of holding power and with voltage ratings from 3V reaching up to 24V, they can be used in PLCs, machine tools and robotic systems. These relays have been tested to withstand 20g shocks and high vibrations. They are RTII compliant as standard, with RTIII compliant versions also available.

Part of Sensata Crydom’s NOVA22 series of solid-state relays, DR22 solid-state relays are AC output DIN rail mounted units that each have a 35A output rating. They are often used in extrusion moulding and food/beverage production processes, where there is continuous cycling at elevated heat levels. They have built-in aluminium heat sinks, which removes the need for conducting complex thermal calculations. An over-voltage protection function is also included.

Under circumstances where faster switching is going to be called for, or there is demand for small space on board, or switching component lifetime is reducing efficiency of manufacturing process, MOSFET relays will probably be the most applicable. OMRON G3VM product series, in particular high-current and high-voltage models in SOP or DIP packages, can meet requirements of Factory Automation industry. There are also general purpose types like G3VM-xxG or G3VM-xxVY, popular in such applications.

Omron’s G3VM-63G devices are single-pole, single-throw normally closed (SPST-NC) PCB-mount MOSFET relays which are highly optimized for industrial deployment, and regularly feature in the servos of robotic arms, etc. Supplied in 4-pin SOP packages, they can handle peak load voltages of up to 60V and have an operational temperature range spanning from 40°C to +105°C, plus a 1500Vrms I/O dielectric strength.

Based on the company’s proprietary OptoMOS optocoupler architecture and intended for use in industrial controls or data acquisition systems, the CPC1018N series from Littelfuse IXYS is comprised of single-pole, NO-contact MOSFET relays that can deliver accelerated switching speeds. They have a 1500Vrms isolation voltage and can handle 600mArms continuous loads. Their resilience to electromagnetic interference (EMI) is another plus point. These relays are supplied in a highly compact package format (with body dimensions of 3.81mm x 4.08mm x 0.48mm), meaning they can be installed in space-constrained environments.

Within the industrial sector, there is a need for heavy duty relay solutions with high operational integrity. These must deliver reliable and safe operational capabilities, alongside power efficiency and cycle durability. Small form factors, ease of integration, vibrational resilience, attractive price points, can be other points of consideration —with all of these influencing the selection process. TTI has a portfolio of relays, stocking many models of solid-state and electromechanical types. When replacement relays need to be found for use in older legacy equipment, the TTI team can find new alternatives that will be suitable.

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Jan Erhan is technical development manager at TTI Europe.

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