We have customers with varying preferences for the method of terminating field connections outside the panel. We even have our own internal disagreements. We'd like to simplify, and we wonder if anybody, whether it's from vendors or independent test centers, etc., has compiled meaningful data, meaning some mean time between failure (MTBF) comparisons about termination failures, which include M8/M12 as well as screw, spring and IDC. It would be helpful to argue a preference based on data rather than mostly subjective or anecdotal viewpoints.
—from June'14 Control Design
[The following are some of the responses we received when we posted the question on LinkedIn's Industrial Automation and Process Controls Network Group.]
Sooner or Later
I haven't compiled any meaningful data, but I would express reservations about the usefulness of MTBF calculations with connectors. I guess you meant mean time to failure (MTTF), as connectors are generally replaced rather than repaired. MTBF is more appropriate for electronic and electromechanical devices where there is some chemical or fatigue based wear-out phenomena. Connector life (or lack thereof) is generally determined by the initial installation, and after that by subsequent insults from mechanical or environmental conditions.
Wear-out phenomena reduce life deterministically according to the Arrhenius relation or similar. For example, insulation in most connectors contains antioxidants, with typically enough antioxidant to get to 20 to 30 years at maximum operating temperature (say 90 ºC), then it goes chalky and brittle. The lifetime doubles for every 10 ºC decrease in temperature. The temperature increases with (square of) current, and typically a pin-socket connector is matched fairly well to the wire gauge, so a 1.6-mm pin might be used on a 1.5-mm2 (or 16 gauge) wire and be “rated” at 16 A. So if you run at 10 A, it should last four times longer (= 100 years), but overload to 20 A, and it will last only 4.5 months.
The point here is that most field wiring has a large safety margin, so it will fail because of something other than old age. My experience is that non-overloaded connectors will last forever until they get wet or until bumped.
My personal experience with field connections is that the M12 connector, when fitted to the rear end of a proximity switch is incredibly unreliable (i.e., about six failures in 80 connectors in three years). The proximity switches are mounted in awkward places inside machinery and usually need to be adjusted by non-instrument-technician workers. The right-angle connector especially is used as a handle to stop the prox switch rotating when the stop nuts are tightened, and usually cracks internally or damages the prox, but it might take weeks before it starts failing intermittently.
The other big killer for connectors is lack of strain relief, and this can be made worse by poor choice of wire (e.g., 26-gauge wire with 600-V insulation is all sheath and no wire).
Finally, any pin-socket connectors that might get wet (i.e., not mounted in the computer room) should have silicone grease applied to the contacts.
Another problem connector is the rectangular one fitted to solenoid valves. Some of these fall apart in months (the ones with clear plastic, so the LED is visible, suffer environmental stress, cracking due to solvents in the oil/coolant/degreaser), and they're usually held on with a really flimsy screw.
Salbay Engineering Pty
Dial M for Good Connections
I recommend fast connections such as plugs (M18/12/8, etc.) with molded ends. Our wiring problems are close to 0%. The additional material costs compared to the labor and troubleshooting is minimal. Most of the time the job has to be done during shutdowns or weekends. Time is money.
Spring connection (cage clamp) should be used because they never get loose, even from high vibrations. Stay away from putting wire furls on. It doesn't make the connection better and is unnecessary. We use, if the customer allows us, only cage clamp terminals, such as those from Wago, Harting Plugs, Phoenix Contact, even with relays, power contactors, motor controllers, power supplies, etc.
If the customer understands this, it's not necessary any more to tie up all connections. When someone doesn't understand the advantage of those connections, then troubleshooting must be fun for him. I don't know one case in our company when we ever had a problem with the cage-clamp technique.
We also go with over-molded connectors for connections outside the panel or for connecting multiple panels together. It simplifies wiring and, other than a rare bad cable, it's pretty much trouble-free.
automated systems engineer
[We received these responses when we posted the question to LinkedIn's Automation Engineers Group.]
It's Technique, Not Type
I look forward to seeing empirical evidence. I have heard a lot of arguments from suppliers, consultants and end users. My experience is that the terminations that fail are the ones that are not correctly installed (i.e., screws not tightened, crimps not crimped properly, etc.).