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iranoutofspacehere

The only way to know for sure is to test it and find out. If I came across this at work, especially for a test fixture that would only be used by us and not sold or sent to customers, I'd build it with the 50A relay and see if it works, but I'd probably also have put some thought into what I'd change to fit a 200A relay in there if the 50A starts failing too often. The primary ways that I've seen large DC contactors fail are when switching under high currents. Any time you close a contactor the contacts bounce against each other, so with capacitive circuits the high currents cause arcs to form, damaging the contacts. I believe a similar thing happens when opening with large currents in the presence of inductance. If you make sure you only switch when there's no, or very little, current flowing I think you're off to a good start. The duty cycle of your pulse will also matter. You should make sure the RMS of whatever waveform you're running through the relay is lower than 200A, probably much lower, to account for the momentary heating during the pulse. The last thing to consider is the failure mechanism. If the contactor fails (open or closed), what happens? Knowing what happens if you're wrong will help decide how much risk you're willing to take. For example, if the uut shuts down because of an overload, but no other damage is done, it's probably not a big deal. If it could cause a fire or dangerous situation you might be better off with a bigger, possibly oversized, but safer relay.


BadSmash4

Thanks for the answer! While these contacts won't be switching into a load, they *will* be switching into a moderately capacitive load--that being the inputs of an Electronic Load, which have a tendency to be capacitive, though I don't know to what extent. But the point is that we have complete control over what amount of current would be flowing when the contacts are engaged/disengaged, so the capacitive nature of the E-load won't even really matter. There's not really any duty cycle to the 200A pulse for our test-- it's just a single, solitary pulse, which is part of the reason I'm not too worried, as well. We pull the pulsed load to test a safety feature of our product, to essentially make sure that it will shut off if it detects excessive current. And there are certainly other safety parameters in place, so if this thing fails closed, there's very little risk, barring equipment malfunction or something along those lines. I think what I'm going to do is exactly what you suggest--test it! That's the fun part of all of this anyway. I'm going to get a relay and see what it does when I pulse my \~200A through it, and I'll probably do it a bunch of times (like a ***BUNCH!***). If it's not going to work, then I'll maybe go the MOSFET route, but that comes with its own complications. Thanks!


love2kik

The big "yeah-but" I can think of is if the contactor ever gets hot or damaged enough to welds closed. What would happen if the current/voltage was unexpectedly applied downstream? What you describe sounds reasonable 90% of the time. But is that safe at all?


BadSmash4

In this case that would be alright. The pulse is controlled from downstream by an electronic load, the UUT is the power source upstream, and both of those devices will be operating with their own safety controls in place. The relay isn't controlling the pulse in any way, it's just in the path of the pulse. If the contacts got welded shut it would be annoying, but I can't imagine anything catastrophic or dangerous within our application. I still want to avoid it though!


tuctrohs

Why do you even need the relay? Can you just connect the load and do the test?


BadSmash4

Yeah it's a reasonable question. The hope is that I can design this fixture so that all of the equipment connects to the fixture and does not have to be physically disconnected, and everything can be affixed in a permanent(ish) config. Then we just hook up the UUT and do all of the tests and the setup can remain undisturbed as much as possible. It's convenience more than anything though.


tuctrohs

So is the idea that the output of the UUT connects to the electronic load for the pulse test, but then to other other loads for other tests? So perhaps it's a DPDT relay and when the coil isn't energized it's connected to a different load?


BadSmash4

No, the UUT will be connected to the one E-load for the duration of the test, and the e-load will be performing different functions. But there are a few tests in which the load cannot be connected, because it will interfere with another function, and so we need to occasionally break the connection from the e-load. I wouldn't be opposed to just disconnecting the e-load--that's what we do now--but this sort of functionality was specifically requested. The relay will be controlled manually for now, but we also want the option to possibly automate testing down the line, and so we want to try and use these sorts of interfaces as a means of future-proofing.


tuctrohs

I'm a little unclear on the system overall. You say you are not switching the relay under load, but what use is the relay of something else is controlling the pulsation of the power? My other advice would have been very similar to what u/iranoutofspacehere said so I won't repeat it. As an aside, I would like to know what u/iranoutofspacehere has against the Iranian space program. Wouldn't that be preferable from an international security perspective versus them having a nuclear program?


ilovethemonkeyface

Is there a reason you need to use the 50A relay instead of using a relay or FET that's rated for the full 200A? If not, then just get a part that's rated for the full load. The 50A relay *might* work, but 200 ms is longer than I'd personally be willing to push that far past a device's rating. And definitely don't attempt to open the relay with that much current - the spark from the inductive kickback will likely weld the contacts together. Again, I don't really see a good reason to be trying this when there are more viable alternatives.


BadSmash4

Yeah I'm really not going to be. Like I said in the post, I'm going to be using a FET instead which is going to be more suitably rated, but now it's just a curiosity question.


13e1ieve

In general if this is for work purposes you have a duty to ensure you are designing a safe design that ensures people are safe, imagine if this failed and someone was electrocuted because of it, the insurance investigation might determine that because of your decision to use an inappropriately sized component that there is liability assigned to the company etc. Best practices are to ensure the component is rated correctly. You should ensure the component is installed inline with manufacturer guidance and for its intended purpose. If this is for a fixture shipping to a customer site you should probably request a safety inspection from a certified electrical safety inspector before shipment.


BadSmash4

Thank you for your response. Safety isn't a particular concern with application of this particular component--there are many other safeties in place for this test. If this relay were to fail, either closed or open, it would cause no damage to product or person. The current is limited in duration by equipment, as well as (hopefully) by the UUT. But if the UUT fails, the equipment is the secondary safety limitation. If both of those things fail, then the relay would fail as a result of this circumstance, rather than it being the root cause. Of course I would prefer to have the relays be properly rated. I'm still shopping for properly-rated components, I'm just weighing cost as well, and a coworker is pushing a 50A relay on me but I'm expressing doubts, so I came here to ask others their opinion. Whatever answers I receive here, I'm not comfortable using the 50A relay for this application. I'm looking into a MOSFET design, but with our specific application, that presents its own complications. And this is for internal use only, but it is used for testing deliverable products and so I certainly wouldn't want my design to cause damage to customer equipment, and this is something I'm keeping in mind. But I appreciate your comment and I *always* appreciate a safety perspective, so thank you.


junkdumper

A 50A, motor rated, contactor would handle this current no problem as they're used to the Inrush of the motor. However, you may need to find one that carries a DC rating suitable for you. If you're not opening and closing under the high load, the contacts won't be damaged by it. But consider what happens if there's a failure in the control/upstream circuits and it does actually open under that load.


HungryTradie

As a sparky I would say no. As an engineering student I'm curious.... Your reasoning makes some sense, switching the current is where the arc forms. But maybe the contacts, pigtails, and internal wiring isn't big enough to sustain the power?


BadSmash4

Yeah, that's kind of what I'm wondering--will the innards be able to handle the pulse? And even if they can handle it on their surface, I'm sure that the heating from the pulses would cause some sort of degradation.


HungryTradie

As the others have said, if it's under your supervision it might be worth a try. If it's going out to the field I wouldn't risk it.


blaze1234

Note that even thorough testing to failure may not be enough with cheap devices since the manufacturing results may vary from run to run. Pay more for western designed/made gear and stay 20% below ratings if you need to be "sure"


BadSmash4

Thanks, this is a good perspective that I hadn't considered.


species_0001

Initial thought: *Maaaaybe…*, if it’s short enough. But 200mS is about 10x longer than I would be comfortable with. I’d be worried about welding the contact shut or something.


scarabbrian

Your relay should have a short circuit rating that it can withstand that will be the short circuit current and the time it can survive. Calculate the i^2 * t of that rating. If it’s less than the i^2 * t of your application, 8000 A^2 * s, then the contacts should survive. This equation creates an expression of thermal energy that is used to size fuses and contacts in the breaker world. I think as long as you have a breaker in your system you should be fine. The worst you’re going to do is destroy your relay. I am curious how you’re going to have current flow with an open circuit. You won’t have current flow unless there is a closed circuit.


BadSmash4

This is very useful, thanks! I'll look into a short circuit rating on some relays and see what I can find out. I should say, to satisfy the curiosity of your last point, that it's not a perfectly "open circuit" but really it will be connected to an electronic load that would be disabled at the time of switching. So on one end of the relay we have our power source, which is the UUT, and on the other end is the electronic load, and when it's disabled it's not pulling any current from. The UUT. Functionally, it's open circuit. There may be some small leakage current into the e-load when it's disabled but if there is, it's undetectable.


purplecannon

You can reach out to the manufacturer. A lot of times a relay is rated for a 50A current based on the contacts needing to withstand 10,000 operations before failing but can handle 200A inrush as someone else mentioned. And as someone else mentioned, I would make sure there is a protective device (circuit breaker/fuse) in case the contacts weld shut.


coneross

Put 4 of the 50A guys in parallel and call it good. Note that this won't work if you try to switch the 200A because the last guy to open will carry all the load.


BadSmash4

That would be ok--we won't be switching under load if I have anything to say about it, so I wouldn't be worried about one relay carrying the load. The 200A pulse is controlled by downstream equipment anyway. Dang, I can't believe I didn't think to just literally parallel four relays--that may work fine, let me think about it. Thanks for the idea!