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designing to meet regs - "utilization category"

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Siwastaja:

--- Quote from: 2N3055 on January 20, 2023, 10:27:35 am ---These are disconnect devices. They are supposed to break in emergency. They don't weld because they are not used as ON/OFF switch.

--- End quote ---

And emergency cannot happen when current is flowing? Worse, emergency cannot happen when short circuit current is flowing, and there is inductance before the short circuit?

These are the cases when semiconductor switches fry short, and mechanical contacts weld. Proper design is needed in either case and safety proved, although arguably it's easier to design the mechanical version. That doesn't mean it's forbidden to use a solid state design.

tom66:

--- Quote from: Simon on January 20, 2023, 10:36:42 am ---It's a winch type application but not lifting:

Death no
Injury if you do not follow instructions and do something stupid yes
Property damage, not really, maybe you materials.

Ultimately a fuse blows, this disconnect device is to cut power at maximum operating conditions, the physical connector cannot interrupt this current so something else must.

--- End quote ---

So the way I think about problems like this is imagine that something has gone wrong and I have to defend it to ordinary, technically experienced people.  A good example might be a regulator or, hopefully not, a judge.  Would your implementation be seen as a reasonable compromise or are you just shaving pennies and having a greater risk? What additional steps could you take to improve safety and reduce failure modes - for instance, why could you not use a better connector?

This is one reason I'm never keen to be involved in safety-critical systems.

2N3055:

--- Quote from: Siwastaja on January 20, 2023, 12:56:28 pm ---
--- Quote from: 2N3055 on January 20, 2023, 10:27:35 am ---These are disconnect devices. They are supposed to break in emergency. They don't weld because they are not used as ON/OFF switch.

--- End quote ---

And emergency cannot happen when current is flowing? Worse, emergency cannot happen when short circuit current is flowing, and there is inductance before the short circuit?

These are the cases when semiconductor switches fry short, and mechanical contacts weld. Proper design is needed in either case and safety proved, although arguably it's easier to design the mechanical version. That doesn't mean it's forbidden to use a solid state design.

--- End quote ---

Contacts weld predominately on switch on. There is arc and force pushing them together. On disconnect, force is working to tear them apart and arc starts when they already start moving out.. There will be arc event of course, but a properly dimensioned disconnect device will extinguish it.
Contacts weld more likely on ON/OFF device because of wear and arc pitting. That creates larger and rougher contact surface that will stick better and more likely have more arcing (geometry and oxides and whatnot).
When you are trying to disconnect relay and you realize it is stuck, it is most likely it got welded on previous connecting event...
Then you tap on it with screwdriver handle and it releases alright. As soon as you overcome bond force, regardless if current is still flowing.That is for small relays that have small disconnect forces. Industrial contactors have significant force and welding is even less likely on disconnect.

Simon:

--- Quote from: tom66 on January 20, 2023, 01:20:08 pm ---
--- Quote from: Simon on January 20, 2023, 10:36:42 am ---It's a winch type application but not lifting:

Death no
Injury if you do not follow instructions and do something stupid yes
Property damage, not really, maybe you materials.

Ultimately a fuse blows, this disconnect device is to cut power at maximum operating conditions, the physical connector cannot interrupt this current so something else must.

--- End quote ---

So the way I think about problems like this is imagine that something has gone wrong and I have to defend it to ordinary, technically experienced people.  A good example might be a regulator or, hopefully not, a judge.  Would your implementation be seen as a reasonable compromise or are you just shaving pennies and having a greater risk? What additional steps could you take to improve safety and reduce failure modes - for instance, why could you not use a better connector?

This is one reason I'm never keen to be involved in safety-critical systems.

--- End quote ---

This is not a case of safety critical, it's a case of any machine can pose a threat if not designed properly and sensible precautions are not taken to prevent incorrect use so there are laws and directives and standards that need adhering to and if you want to sell it you have to take it to a company that does the required approvals.

There are two aspects here, I need to provide a method of disconnecting power, this may be a switch or a connector. We don't really fit switches to these machines as they are portable and the power is plugged in at each use so I have to put a plug on anyway. Any plug I use will not have the breaking capacity, there is a reason your UK 13A plug has terminals 10x the size of the contacts I have used in the past that will carry 13A but not break 13A. So I for myself already envisioned a controlled/interlocked breaker to prevent damage to the connector.

Now I find that this requirement and my very method of interlocking a breaker with the connector are a requirement, but the standard demands a utilization category. In my case DC-13 at 50A and in the future 120A

I also have to provide an emergency stop and not wanting to have more than one high current switch will make this the same. This makes it more important for me to look at something with proper ratings rather than cobble my own and argue about it.

But, no one is using 48V at 120A or even 50A. I cannot find anything that is rated to DC-13 utilization category with a 50A capacity. I can find contactors like the ones from albright and these also came as a contactor and emergency stop combined. It therefore makes sense I think to use one of these so that we kill several birds with one stone.

What I was hoping was to put the switch device in the battery so that we did this work once and it cost once and then no machine would require a chuncky contactor and several machines can run off the same battery.

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