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So, I'm fiddling with a design for a widget, and one of the "fun" parts is to ensure that power is reliably cut without human intervention. Power is normally controlled by relays, which have at least a theoretical potential to experience contact welding and therefore fail to open when the coil is de-energized.

It occurred to me that it ought to at least be theoretically possible to, say, stick a fuse in the whole thing and rig something to create a deliberate short (thus melting the fuse and interrupting power) if the relay fails to open.

Is there any existing art for such a setup?

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    \$\begingroup\$ Crowbar circuit. Or another relay which is NC which you exclusively use to disconnect in such cases. \$\endgroup\$
    – Wesley Lee
    Jan 25, 2023 at 21:18
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    \$\begingroup\$ If you're worried about contact welding, you might also want to look into whether a snubber circuit at the contacts is appropriate for your application. \$\endgroup\$
    – vir
    Jan 25, 2023 at 21:34
  • \$\begingroup\$ Use quality relays within their specification? The manufacturer has already figured all this out. \$\endgroup\$ Jan 26, 2023 at 22:47
  • \$\begingroup\$ @Harper-ReinstateMonica, I considered including "...or am I being overly paranoid?" in the question. OTOH, manufacturers also encourage the use of fail-safes when failure to perform could be hazardous. After all, components do sometimes fail to perform as expected. \$\endgroup\$
    – Matthew
    Jan 27, 2023 at 17:52
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    \$\begingroup\$ @winny, I'm switching US mains (120VAC). I have 5V and 12V available for coil control, though I can use mains for that also if necessary. \$\endgroup\$
    – Matthew
    Jan 31, 2023 at 21:53

5 Answers 5

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It's called a "crowbar circuit" because it's like throwing a crowbar across the terminals of the power supply.

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    \$\begingroup\$ Hmm. This is partly useful, at least inasmuch as validating the idea of deliberately blowing a fuse. It doesn't seem to apply to relay contact welding, however. For one, as best I can determine, that would ideally include some sort of timing circuit. \$\endgroup\$
    – Matthew
    Jan 26, 2023 at 15:08
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Is enough energy guaranteed to be available to operate the fuse?
It may not. Typically these circuits are last in line of a number of protection circuits. Eg: relays, circuit breakers and trip coils.

Since when this circuit or device does fail, it will ignite and cause a fire.

Things that do exist for this purpose are pyrotechnic fuses. The fuse is coupled with a small explosive that you can trigger. They are highly specialized and not for your average widget.

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  • \$\begingroup\$ So, notionally the failure condition here isn't "too much power". Although, that is what's most likely to cause contact welding, and your question whether I can even draw enough current to break the fuse is valid. (Indeed, doing so "safely" resulted in this follow-up.) Alas, pyrotechnic fuses are, as you say, exotic. I'm probably going to go with creating a deliberate ground fault instead and hoping the breaker will do what the relays didn't. \$\endgroup\$
    – Matthew
    Jan 31, 2023 at 16:31
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Microwave oven door switches are typically set up such that there are two contacts operated sequentially, one in series with the MOT such that it is ONLY closed if the door is shut, and the other wired directly across the transformer primary to eat the fuse if the first contact fails to open.

It is a very well conceived piece of very cheap safety interlocking.

For relay use, you may wish to investigate the 'Force guided contact' relays from the likes of Pansonic that can be used as part of a solution. These are actually made for the solar inverter market, but are mains rated and have the desired safety characteristics. For industrial safety relays, most of the factory automation vendors have product in this space.

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  • \$\begingroup\$ I'm actually planning on Elesta SIP 512s. (I've had pretty abysmal luck finding anything rated for more than 10A. The Eleasta's are rated for 12A/16A and I plan to gang three of the poles. Not all four, because for some reason the fourth has less electrical isolation.) \$\endgroup\$
    – Matthew
    Jan 31, 2023 at 16:42
  • \$\begingroup\$ Note - Relays more than an amp or two of rated capacity are sometimes (usually) called power contactors. This terminology may depend of the industry. \$\endgroup\$
    – SteveSh
    Jan 31, 2023 at 22:30
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It should be obvious that you cannot guarantee 100% correct operation (that is, the current flow being interrupted when commanded). You may be able, with different design techniques be able to come up with a 99%, 99.9%, 99.999% solution, but you'll never be able to reach 100%.

That being the case, I think you need to take a more structured approach, bringing some of the tools from reliability engineering to bear on the problem.

Here's a rough outline of what I think needs to be done.

First you need to define what you mean by success. How often does the current need to be interrupted? Once a minute? Once an hour? Once a day? And over how long a period of time. One year, 10 years?

This gives you an estimate of how many times your current interrupter needs to operate. Lets say you need to do this 3,000 times over a ten year period, just to put some number down on paper.

Then you need to determine your probability of success Ps over ten years. Remember, it can never be 100%. Let's make that 99.9%.

Now you start laying out implementation options. Start with a single properly rated relay. What is it's reliability in your application, or it's FIT (Failures in Time) rate. From this you can calculated the expected Ps. Is it sufficient?

If not, you can look more reliable parts. Or look at architecture changes, such as putting two such relays in series. This improves your Ps significantly. Let's say a given relay is determined to be 99% reliable in the above scenario, reliable means being able to interrupt the current. That means there's a 1% chance that it won't perform as needed. If you put two such relays in series, the interrupter's Ps is now 99.99%.

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Try a 2nd relay in series, and monitor if any of them fail. Possibly use a different make/type of relay, maybe mosfet.

Or you could use the 2nd relay to cause a short circuit and blow the fuse, although it's hard to do auto test it.

But you are right everything will fail eventually, the question does it cause harm/damage or not.

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