There is a long-standing debate as to which connection type is better. Is the screw connection terminal better than the push-in spring connection? As a case study consider the Phoenix Contact RIFLINE relay sockets shown in Figure 1 including the 2900930 (screw) and the 2900931 (push-in spring).
Figure 1: Many relays such as these Phoenix Contacts examples are available in conventional (screw) as well as spring (push-in) terminals.
Product ratings
As pictured, both Figure 1 relay sockets feature the 2981192 DPDT relay. The continuous rating for this relay is plug-in 8A. According to the datasheet, the relay sockets are rated with a “depends on application / assembly” of 15.5 A (screw) and 13 A (push-in). The results are inconclusive as the relay, not the socket, is the limiting factor.
An informal analysis suggests that the Phoenix Contact devices are representative of a wide variety of industrial control components including terminal blocks and even Programmable Logic Controllers (PLC). In general, there is a slight advantage to the conventional screw terminal connections. However, since we typically don’t run equipment at the design maximum values, the decision is not wholly decided on the current ratings of the device.
Determining factors
Since the decision is not necessarily limited to the current rating of the connection, we must consider other factors:
Ferrule connections
Continuing with our representative RIFLINE relays, we consult the data sheet to determine that both relays will accept two wires per contact. The push-in contact has two slots while the traditional relay has one. The range of possible connection is shown in Figure 2:
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Left: Conventional (screw): Terminal 11 with twin ferrule and terminal 21 with two bare wires.
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Right: Push-in (spring): Terminal 11 pair with two single ferruled wires and the terminal 21 pair with two bare wires.
While the number of connecting wires is the same, there is a vast difference in terms of the feel of assembly.
Single wire ferrule
There may be an advantage to using single wire especially for large projects. Automated machines may be used to strip and install ferrules. Generally, this automation favors single wire ferrules suggesting an advantage for the spring type connections.
Twin wire ferrule
There is more to panel construction that just placing the ferrules on a wire. There may be advantages to using twin ferrules when it comes to prints and labeling the wires. Having a single twin-wire connection could save the added expense and effort of adding labels.
Tech Tip: Some of the twin-ferrule argument is mitigated by using jumpers. For example, most relay families include jumpers to connect a relay’s common terminal (the common between a N.C. and a N.O pair). Jumpers are also available to connect a chain of relays together. One such jumper is Phoenix Contact RIFLINE 3030336. Leveraging these seemingly simple accessories can go a long way to reducing the clutter in your control panel.
Figure 2: The conventions (left) and spring (right) both accept two wires per terminal.
Ideal vs real-world experience
Earlier we implied that the conventional screw connection may have a slight advantage. When properly installed the contact resistance may be lower. Here the term “properly” implies the proper torque was applied to the screw, applicable ferrules were installed with the proper crimp tool, the wire was stripped and sized according to specification. There is also an expectation that these ideal practices hold over the decade long lifetime of the equipment.
In my opinion, this is a tall order.
Perhaps you too share my perspective that grew out of one to many 2 AM troubleshooting sessions. Things are not ideal in the mental fog of sleepiness with pressure from above to restore the equipment operation. In this stressful environment, there is more to consider than the ideal on-paper specifications.
Screw torque
For optimal performance, the screw in the conventional connector must be properly torqued. There are any number of reasons this may not occur. Vibration is certainly a concern. In fact, when I served on ships, it was common practice for the contractor to walk though the entire control panel making sure each and every screw was properly torqued. Another potential concern is the foggy 2 AM technician who may not remember to properly tighten a screw. This would include being too loose or in some cases a tired and overzealous technician may overtighten and strip the threads. In either case, the contact resistance is not as low as it could be.
Spring tension
While there is less room for error, spring terminals are not without their problems. An improper ferrule to wire, ferrule to terminal, or ferrule to crimp tool can result in a high resistance connection. Finally, it’s possible that repeated wire insertion and removal could weaken the spring.
In all cases we see that problems can creep into the system. This is especially true when we consider the lifecycle of equipment designed to operate for many decades. Many hands are involved in the process or maintaining and modifying such equipment.
Visual inspection of the connection
As a rule, wires enter the conventional terminal block from the side while the spring connections are from the top. This can be advantageous for the spring terminal, as a visual inspection of the wire / ferrule is possible. On a related note, the “tug test” is a bit easier with the top mount spring terminals.
Too many holes
One potential downside for the spring push-in terminals is the number of holes. For example, consider the 4PDT relay shown in Figure 3. At 2 AM, it is very easy to misplace a wire. The real danger is that the wire may be associated with an auxiliary function that is not readily apparent unless the full machine commissioning procedure is performed.
Figure 3: An assembler or technician could easily misplace a wire in the matrix of spring terminal connections on this 4PDT relay. The advantage does to the conventional terminal relay.
Conclusion
Looking back, there is no clear answer to the conventional screw vs the push-on spring terminal connection. The very fact that we can easily find example of each type often in same family provides support that both are valid. As a student, it is essential that you recognize and master the proper electrical connections to both types.
Please help expand this conversation by providing a few comments in the space below. Do you have a preferred type? If so, why?
Best Wishes,
APDahlen
About the author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey
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