Assemble a cable with NEMA L5 connectors. Reliable? - Page 1
Forum section: electronics / beginners

author: aurmer
author_rank: Regular Contributor
timestamp: May 03, 2018, 04:07:58 am
content: I am working with a stage lighting vendor. Today we had issues with losing power to a group of lights. Turns out the power cable leading to the lights was poorly terminated with a store bought plug. By the time we got to it, the male and female connectors were melted together.I spoke with the vendor who had done the install, and he laughed it off saying, "That happens all the time with these connectors".So, I came here to check in with some experienced folks before I give in to impulse and find another vendor. My gut tells me not to trust him with any more installations.Would you say that a skilled individual canconsistentlyassemble reliable cables?Or is it your experience that no matter how skilled you are, some of these will come loose?

author: Bratster
author_rank: Frequent Contributor
timestamp: May 03, 2018, 04:45:21 am
content: If the connector is rated for the current that you were pushing through it, then that vendors an idiot saying this happens all the time and I would stop using him.It is not hard at all to properly install a power connector on a cable. As long as you're not overloading it there should never be problems.Now if this is a cable that is constantly plugged/ unplugged then the female side can wear out and not provide the same force on the contacts which will lead to heating and a bad time.Sent from my Moto x4 using Tapatalk

author: ikrase
author_rank: Regular Contributor
timestamp: May 03, 2018, 04:53:36 am
content: IMO, kinda need some info on the particular failure mode. What melted?

author: aurmer
author_rank: Regular Contributor
timestamp: May 03, 2018, 01:07:27 pm
content: Here are some details on this installation.There are 4 theater lights on one breaker. (spec sheet says 750W maximum... 4.8A typical @ 120V)spec sheet linkThis circuit is on a 20A breaker.3wire 12AWG cable.There are two cables along this run. The cables are joined by a L5-20 plug and receptacle.It was this L5-20 which failed. The failure appears to be a poor connection between a wire and the connector. Images attached.This connection is in the ceiling. It isn't touched, pulled, or stressed unless we go up in the lift to modify the installation.EDIT: The spec sheet does say that typical half-cycle inrush current is 10.3A. They are all turned on at the same time, so that is a potential of 41A inrush. I don't know if this could be an issue on our circuit. It has been fine for about a year until this failure.

author: mleyden
author_rank: Contributor
timestamp: May 03, 2018, 02:47:54 pm
content: Worth a read:http://www.pbase.com/mainecruising/installing_a_smart_plug

author: dmills
author_rank: Super Contributor
timestamp: May 03, 2018, 03:44:04 pm
content: Not actually that uncommon, the screws loosen with time and temperature, the connection resistance rises and the thing cooks itself.More spectacular when it happens on a 125A, three phase part or on any form of powerlock, but anyone who has been around the industry a bit will have seen it on one connector or another.The cure is good preventative maintenance, taking everything apart once a year to inspect and tighten the screws, together with correct installation (Do NOT tin the wires that are going to be used in a screw termination).Particularly in the US there is not really a completely satisfactory connector for power in the entertainment industry, twist locks do not really take the inevitable abuse well, and stage pin tends to come apart at the wrong times, CEE never seems to have caught on.Regards, Dan.

author: aurmer
author_rank: Regular Contributor
timestamp: May 03, 2018, 04:39:10 pm
content: Thank you, this helps me to shift my expectation and focus towards the need for regular maintenance.Until we buy a lift, rental required to access these connectors costs $750. Top priority on minimizing yearly occurrence.Here is a crazy idea... What if I bought a thermal imager to check these connectors regularly from the floor? I am not sure what temperature / time graph looks like for a failing connector, but if it is gradual enough, we can see it with the imager and correct it before it does any damage. Thoughts?

author: rstofer
author_rank: Super Contributor
timestamp: May 03, 2018, 06:51:38 pm
content: Of course the connectors are reliable within their rating. The semiconductor industry uses them extensively for the locking feature. There will be a problem if the wire clamp gets loose as has happened here. I would put it down to workmanship. It just doesn't happen all that often and never just out of the blue.I can't read the brand of the connector. Hubbell is ALWAYS my first choice and, if I write the spec, will be the only acceptable device. GE and Bryant also make good connectors. Won Hung Lo? Not so much.It is quite common in the semiconductor industry to do periodic thermal inspection of all electrical panels. We didn't get down to the cord connector level but we could have.I know there are different standards for difference industries but the National Electric Code does say that extension cords are not to be used as a substitute for fixed building wiring. There may be an exception somewhere for the entertainment industry. There is another one for Christmas season.If you want the ultimate in reliability, look for 'pin and sleeve' connectors. We used those on all the heavy electrical cords in the aerospace industry (welders, ground power, etc) and IBM used them for all of their mainframe cables. Russelstoll brand were used extensively. I see they are now part of Thomas & Betts. There are other manufacturers including Hubbellhttp://www.tnb.com/pub/en/node/199https://www.hubbell.com/wiringdevice-kellems/en/industrial-solutionsIn my version of a perfect world where cables are inaccessible and out of sight, I think I would be moving to pin and sleeve.

author: Unknown
author_rank: Unknown
timestamp: May 03, 2018, 08:16:32 pm
content: From the photo, it's a Pass & Seymour brand, which is what home depot stocks. I noticed hd switched to this brand a couple years ago (my area). Out of curiosity, what's the operating voltage?

author: aurmer
author_rank: Regular Contributor
timestamp: May 03, 2018, 09:49:09 pm
content: All of our equipment runs on 125V.

author: Gregg
author_rank: Super Contributor
timestamp: May 03, 2018, 10:00:08 pm
content: I’ve had a lot of experience with L5-20 plugs and receptacles in data centers where we used thousands of them. In 15 years I only experienced one failure like you have shown and it was because the screw to a conductor wasn’t tight. That seems to be the case with what you have pictured.On the other hand you are drawing more power than you should for the application if there are 4 of the fixtures on a single 20 amp circuit considering the elevated temperatures near the ceiling of a stage and you should de-rate the load by 20% for continuous use. Either reducing the load to three fixtures per circuit or going to a 30 amp circuit with 10 ga wire would pay off in the long run.What I found about tightening stranded wire in those type connectors is to tighten them and then wiggle / tug on the wire and tighten again to cause the strands to seat against each other properly.Hubbell used to make the premium connectors followed closely by Leviton; Pass and Seymour used to be just OK.

author: dmills
author_rank: Super Contributor
timestamp: May 04, 2018, 09:00:11 am
content: Loose screw was clearly the proximate cause, but datacentre is a very benign environment in a lot of ways.You see this occasionally in the entertainment industry, thread lock helps, but the cables tend to have a really hard life, PM is the answer.I am slightly surprised to see it happening in a cable that is just left in situ, it is far more common in stuff that gets thrown in and out of flight cases all the time. The thing about this failure is that a slightly loose screw tends to cascade into this fairly quickly due to the increased I^2R losses adding more heat.Thermal imaging could work, but it is a small target from the floor and probably right next to some very hot lights, you would need a better then average camera.

author: richard.cs
author_rank: Super Contributor
timestamp: May 05, 2018, 01:48:01 pm
content: The melting on the neutral but not the line is a clear indication of a high resistance connection. The currents on the two sides are equal yet only one has gotten hot. It's hard to tell much more from the photos but as other posters have said loose screws would be the most common cause. If it happens "all the time" to cables they assemble definitely get somewhere else to do it.Being from the UK I am not familiar with that connector but judging by your photos and a quick googling it does look pretty flimsy. We have Ceeform / IEC 60309 for this kind of use* which is rather robust though could still be damaged by a poorly terminated cable. People routinely use the 16 A size for welders and similar at 20-30 A (when they should use the 32 A size) but it's very rare for anything bad to happen. They normally have tunnel terminals like these:https://i.ytimg.com/vi/RMEfYaICLj4/maxresdefault.jpgwith the better quality ones having two screws per pin. Do your connectors have something like that or are they a "wrap wire around screw" design? The latter tends to give much poorer connections and reliability.Accepted best-practice here is to crimp a ferrule onto the stripped end of the flexible cores and then fit a good-quality connector. If I were to tell a supplier here I wanted crimped ferrules, a connector with double-screw tunnel terminals and everything done up to a controlled torque they wouldn't question it or think it unusual (but the very cheapest places might not have the tooling for either or just be too lazy).*Though a lot of (most?) stage lighting here uses 15 A roundpin BS546 for historical reasons.

author: Monkeh
author_rank: Super Contributor
timestamp: May 05, 2018, 02:44:18 pm
content: Quote from: richard.cs on May 05, 2018, 01:48:01 pmAs I recall this style is generally a clamp plate arrangement, like barrier strip. Not generally a problem and often fine with bare stranded wire, too. But idiots can still make a mess of it.Rest of the arrangement's as shoddy as ever, though. Long overdue a modern replacement.

author: dmills
author_rank: Super Contributor
timestamp: May 05, 2018, 03:51:57 pm
content: Most UK theatres (Touring and gig lighting are a bit different) use the old 15A standard for dimmed power, and (often) 16A 60309 for non dim, this is nice as plugging a discharge mover into a dimmer can make bad things happen to both mover and dimmer.A properly crimped ferrule is best practise but you don't actually see it that often.I have seen plenty of 60309 in the larger sizes suffer exactly this failure, more common at 32A and up however, nothing quite like getting THAT smell from the 125A Three phase connector on the generator just as the first dance of the wedding starts.....Regards, Dan.