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How to design the following system as safe as possible? The components are:

  • mains 230VAC

  • enclosed control box, grounded

    • custom connector: the user plugs/unplugs the heater cable
    • pushbutton: the user starts the heater with the pushbutton
    • solid state relay (SSR) activated by the pushbutton (so that the load does not flow through the pushbutton)
  • PTC heater, enclosure grounded. I have the choice among:

    • 24VDC - inrush current: 16A, stable current: 0.4A
    • 48VDC - inrush current: 8A, stable current: 0.2A
    • 230VAC - inrush current: 28A, stable current: 0.2A

Questions:

  1. If there is a short, for example in the custom connector or in the heater connections, then what option would be the less dangerous for the user: 24VDC, 48VDC, 230VAC? I believe that it is not good to let a user handle a custom cable/connector assembly that transmits mains AC (even if everything still well dimensioned and well assembled)?

  2. Should I add a circuit breaker? Do I need the three RCD, ELCB, GFCI? I believe that I would have to use a time-delay circuit breaker due to the inrush current? What rating would be good?

  3. With 48VDC for example, do I need a power supply rated for the max inrush current (8A) or can it be lower?

All of this is to let me sleep at night knowing that my sold device is absolutely safe for the user and that I will easily pass CE (LVD directive) and UL and/or FCC for european and US markets.

Thanks for any feedback

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    \$\begingroup\$ Most of your questions would be answered by the relevant standards. I’d suggest that since this is for a commercial product that you engage suitable qualified people to design the system and have it tested to ensure it complies. \$\endgroup\$
    – Kartman
    Commented Dec 3, 2021 at 4:37
  • \$\begingroup\$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. \$\endgroup\$
    – Community Bot
    Commented Dec 3, 2021 at 5:15
  • \$\begingroup\$ If you're handling AC, you need to comply with a draconian list of rules for doing so. Also, your diagrams show "Ground" for AC mains. Mains doesn't work that way, you don't use Ground for anything except shielding, current is forbidden on it. Mains requires a separate return wire called Neutral. That rule relates to mains power, not to its AC-ness. If you had 20VAC you could use "GND" in the normal electronics way. \$\endgroup\$ Commented Dec 4, 2021 at 2:49
  • \$\begingroup\$ Thanks @Harper-ReinstateMonica. Agree, I wont use AC then. Agree with your comment on the diagram too. \$\endgroup\$
    – thomast
    Commented Dec 4, 2021 at 21:30

2 Answers 2

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Q1 = 24vdc

Q2 = you should have all safety devices that are required by code and how the product will be used. Typically, when connecting a device to a person, you would use a medical-rated power supply. Not much more expensive but well worth it. Also, talk to a company that sells product liability insurance. If your product is declared unsafe and needs to be recalled - insurance is very helpful. Or if it actually injures/burns or kills someone, or lights their house or bed on fire, you will suddenly realize how cheap the insurance is. The insurance company will have some requirements on circuit breakers/fusing to minimize risk.

Q3 = not necessarily. You can add a capacitor and PWM to slowly ramp up the power to max of your (for example 3 amp power supply). The heater will heat slowly until you approach your limit temp. Unfortunately, you may not get to your target temp if the room is too cold to start with and a lower current power supply cannot provide enough power.

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  • \$\begingroup\$ Thanks. It is not a medical device, rather a sort of 3D printer (I reduced the scope to ask a simple question). My power supply actually specifies an inrush current (40 typ). I believe that if the heater inrush current stays below the PSU inrush current spec then it is OK? I should a separate question on that. \$\endgroup\$
    – thomast
    Commented Dec 3, 2021 at 16:53
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If this is a commercial product then you need to find out what regulations are applicable to the market you will sell it in.

Typically this means following IEC61010 and getting the product certified at and NRTL (nationally recognized testing lab). At the end of the day, it would be best to find a consultant to make sure your product qualifies or someone who has gone through the process. I'll answer the question, but I cannot guarantee that every answer will be correct, it will only give you an idea of how to design.

If there is a short, for example in the custom connector or in the heater connections, then what option would be the less dangerous for the user: 24VDC, 48VDC, 230VAC? I believe that it is not good to let a user handle a custom cable/connector assembly that transmits mains AC (even if everything still well dimensioned and well assembled)?

Anything over 30Vrms or 60VDC is considered dangerous (ideally a user should never come into contact with any voltage). The chassis should be grounded to AC mains so if a fault occurs it blows a fuse or trips a breaker. The current limiting safety device should allow for normal operation but cut current during a fault. Ha

Should I add a circuit breaker? Do I need the three RCD, ELCB, GFCI? I believe that I would have to use a time-delay circuit breaker due to the inrush current? What rating would be good?

It would be more safe but more costly. If you think a user could risk shock I would probably go with a ground fault current interrupter (RCD is same thing different name). They measure the ground fault current, and will trip if they detect more than a few mA leaving the device. They need to be able to handle the max current.

With 48VDC for example, do I need a power supply rated for the max inrush current (8A) or can it be lower?

The supply needs to handle the max current. There are probably ways to mitigate the current so the supply doesn't see the full 8A inrush, like using a resistivity heater without inrush current and using a different form of control.

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  • \$\begingroup\$ Thanks a lot (cannot upvote due to rep.). I will add a GFCI like this one. Is my reasoning correct: it specifically detects ground faults and protects users from shock if there is an insulation failure somewhere in the circuit (all casings are grounded but I read at least one more safety measure is needed). It covers my biggest user safety concern. I could also add a time-delay current-limiting breaker. If I choose 24VDC with 16A inrush currents, then the breaker rating could be 20A. \$\endgroup\$
    – thomast
    Commented Dec 3, 2021 at 16:47

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