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designing to meet regs - "utilization category"

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tom66:

--- Quote from: Simon on January 20, 2023, 10:14:23 am ---Yes so my question is, if I do my own, does my own testing justify my choice of own brew component or will it always need an official category certification under the machinery directive. If that is the case maybe I should just buy one.

--- End quote ---

If something goes wrong with your device, is it likely to lead to death, serious injury or substantial property damage?

jonpaul:
OP kindly details? post  is too vague.

Environment industrial,consumer, vehicle, seaborne?

Mains V, cfg 1, 3 ph wye, delta ?
Rated KW/KVA/Irms ?
different answer for a 120 V fractional HP lathe vs. a 480V 3 ph 50 HP crâne hoist.

If safety, shock, fire  is a concern consider your  liability.
The excellent electricians forums may be a better resource, eg Mike Holt.

Bon courage

Jon

2N3055:

--- Quote from: Siwastaja on January 20, 2023, 09:44:40 am ---
--- Quote from: tom66 on January 20, 2023, 09:32:41 am ---It's possible to use MOSFETs in safety critical systems, for instance TI make a number of eFUSE devices which are rated to replace traditional fuses in automotive applications.  These devices are extensively tested to ensure they do not fail short in any expected operating condition.  I am not sure if they would be approved for your application - it took a lot of effort and development to get eFUSEs in cars.

--- End quote ---

This, plus mechanical switches and relays have exactly same failure mode - they fail short, too, due to contacts welding.

Sure, one could claim that on average, maybe, given some engineering budget or level of experience, mechanical switch has higher chances of sustaining larger fault currents and voltages than solid state, but that gets pretty handwavy.


--- Quote from: 2N3055 on January 20, 2023, 09:07:53 am ---No solid state devices because they can fail short..

--- End quote ---


I'm 99% sure standards do not prohibit use of semiconductor based switches. Standards may require things not to ever fail short in given conditions*, and then it's up to you to prove the reliability, and you can't escape that requirement by saying "mechanical switches never fail short", because that's simply not true.


*) all switches will fail short if you apply, say, 10000000000000V with 10000000000000A available; you have just a huge fireball of conducting plasma

--- End quote ---

These are disconnect devices. They are supposed to break in emergency. They don't weld because they are not used as ON/OFF switch. Also they have special construction.
But even if you use standard relays, one that is used as ON/OFF and another that is always on and don't get connect/disconnect cycles, they won't both weld at the same time.
That is the logic behind it. Also, on single phase systems, if you use separate contactors to break live and neutral you can pass some safety standards.

As I said, it is very specific, based on application..

Simon:

--- Quote from: tom66 on January 20, 2023, 10:18:12 am ---
--- Quote from: Simon on January 20, 2023, 10:14:23 am ---Yes so my question is, if I do my own, does my own testing justify my choice of own brew component or will it always need an official category certification under the machinery directive. If that is the case maybe I should just buy one.

--- End quote ---

If something goes wrong with your device, is it likely to lead to death, serious injury or substantial property damage?

--- End quote ---

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.

Simon:

--- 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. Also they have special construction.
But even if you use standard relays, one that is used as ON/OFF and another that is always on and don't get connect/disconnect cycles, they won't both weld at the same time.
That is the logic behind it. Also, on single phase systems, if you use separate contactors to break live and neutral you can pass some safety standards.

As I said, it is very specific, based on application..

--- End quote ---

Primarily this is to completely remove power from a machine. The standard covers all machinery and heavily focusses on fixed installs where you wire the machine in so put a bit switch in to turn the power off so that you can work on it. We are doing portable machinery so there are connectors, these are acceptable as disconnects but as the connector is not designed to be hot plugged it must interlock with a switch device of appropriate rating. As it happens I would put the estop into this interlock as well so that the one power on/off is controlled by the connector or the estop button.

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