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For a mains-supplied circuit I want to use an aluminium case, so the case can act as a heat sink. However, I want it to be safe, also when there's no earth connection in the socket where the circuit is plugged into (this is still quite common in Europe).

What is the common way to make the circuit safe to use in an aluminium case?

I know that normally you would connect the earth-wire to the case, so in case one of the wires internally touches the case, the circuit breaker will turn off the power, protecting people from touching the enclosure while there is 230V on it (see also this question). But of course this does not work when the socket has no ground connection...

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    \$\begingroup\$ What is this circuit? I can't envisage the diode thing, a circuit diagram would help. \$\endgroup\$ – pjc50 Jul 20 '16 at 10:52
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    \$\begingroup\$ So - you're proposing to deliberately energize the case with every second half-cycle of the mains and thinking this might somehow make your device safer .... ? \$\endgroup\$ – brhans Jul 20 '16 at 11:25
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    \$\begingroup\$ Just to make this perfectly clear - using diodes like this is very dangerous and should not be done. \$\endgroup\$ – Steve G Jul 20 '16 at 11:30
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    \$\begingroup\$ Note that consumer electronics that use metal case (for example, a PC) don't make the assumption that earth can be left unconnected. The AC-DC supply is within the metal case, and is not doubly insulated. So basically, here, you want to be safer than the current safety standards. Maybe you're overthinking this. \$\endgroup\$ – dim Jul 20 '16 at 11:51
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    \$\begingroup\$ @Douwe66 In this case, you'd better upgrade your electrical installation ASAP. Because you'll have the problem with any other consumer product with a metal case. So even if you manage to design your specific device safely in this regard, you're still risking electrocution every day, here. \$\endgroup\$ – dim Jul 20 '16 at 14:42
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Under IEC rules a mains applicance that does not have a ground (earth) connection is a class II appliance, a.k.a. 'double insulated'. This means that any mains conductors must be protected by either two layers of suitable insulation, known as 'basic' and 'supplementary' insulation, or by 'reinforced' insulation.

If you can construct your appliance using a power supply unit that qualifies by itself as class II equipment - i.e. it would be safe and legal to use outside the metal enclosure - then I believe (though IANAL) you can also install that PSU inside a metal enclosure to make a piece of equipment that also counts as class II. Of course you can't interfere with the mains wiring to that PSU in any way, so you would have to either route the intact mains lead through a cutout in the case, or have a cutout allowing a mains lead to be plugged in to the PSU's mains inlet. Also of course, you need to follow good practices for strain relief of cables, protecting them from sharp edges etc, and be aware that heat dissipation inside a case will not be as good as in free air.

If you are OK with having a class II power supply outside your equipment, this is also a safe option and I think means your equipment then classifies as class III, although I haven't seen this symbol on common devices that do use class II external power supplies so it's possible it only applies when the extra-low-voltage power supply is a fixed installation.

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    \$\begingroup\$ Thank you for your answer! What I don't understand from these rules, what if you use a ground connection in your appliance and plug it into a socket without a ground. Suddenly the appliance lost its safety. \$\endgroup\$ – Douwe66 Jul 20 '16 at 14:13
  • \$\begingroup\$ Pretty much, running a Class 1 appliance without an earth puts you a "single fault" away from low impedance live mains on the case, even in the absense of faults capacitive coupling can lead to small shocks. \$\endgroup\$ – Peter Green Jul 20 '16 at 16:19
  • \$\begingroup\$ Yes. That is why most americans think the european "tradition" of not grounding sockets in dry areas of residential space is stupid and dangerous. In the US and Canada, it is not possible to plug a device with a ground pin into an ungrounded socket, unlike e.g., the shuko style plugs. In any case, there is one other thing you can do to improve safety, which is to use a ground-fault interruptor that will shut off the power if there is a ground fault (after it shocks you, but hopefully before it kills you). \$\endgroup\$ – Evan Jul 20 '16 at 16:19
  • \$\begingroup\$ It's a tricky one, making everything class 2 is not really practical and has it's own problems. Forcing homeowners to have their houses rewired is politically very difficult, especially as despite the theoretical danger very few people actually get killed in practice. \$\endgroup\$ – Peter Green Jul 20 '16 at 16:28
  • \$\begingroup\$ @PeterGreen It should be less politically difficult to force all new houses (or any house subject to significant renovation work) to be wired properly though. Nobody forces everyone to have their house rewired in the US - generally if they have an appliance requiring a three-prong plug they'll use a "cheater adapter" (which has a separate lead for a ground connection, which they will attach to a screw into the junction box, which they will hope is grounded but probably isn't) and plug it in at their own risk. \$\endgroup\$ – Random832 Jul 20 '16 at 18:45
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The common way to make the circuit safe to use in a conductive case is to make the gadget DOUBLE INSULATED. EVERY part of the circuit MUST be abundantly ("double") insulated from the outside conductive enclosure. There must be NOTHING that the user can touch that is connected to the circuit in any way.

Of course, you have revealed nothing about what this circuit IS, so we don't know whether this is practical. If the circuit has input and/or output connections then this is impossible. You have also not disclosed WHY you are unable to use a proper grounded mains connection? Or whether you can use a GFCI/RCD as alternative safety measure.

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  • \$\begingroup\$ +1 Four layers of insulation is used in some SMPS transformers (3 on primary, 1 on the secondary). \$\endgroup\$ – Spehro Pefhany Jul 20 '16 at 11:46
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    \$\begingroup\$ DC adapters such as phone chargers have an output connection but do not require a ground connection, so the second paragraph is not correct. I assume that these appliances qualify as double insulated because they meet adequate standards for isolation of the low voltage side from the mains side. \$\endgroup\$ – nekomatic Jul 20 '16 at 13:06
  • \$\begingroup\$ DC adapters such as phone chargers are BOTH double-insulated AND have non-conductive enclosures (i.e. plastic). Different situation than using a conductive (aluminum) enclosure. And we still don't know what kind of gadget this is, so we are all shooting in the dark. \$\endgroup\$ – Richard Crowley Jul 20 '16 at 16:39
  • \$\begingroup\$ My point was that it can be OK for a double-insulated device to have a touchable output connection, whether or not it has a metal case. Another example would be a mains-powered radio with a headphone socket. More detail from the OP would help, yes. \$\endgroup\$ – nekomatic Jul 21 '16 at 9:52
  • \$\begingroup\$ It was meant as a generic question, but in my particular case I want to measure the current trough a mains wire and use it to enable/disable a slave device. My main concern are the mains leads going to the transformer. The current sensor can be made without exposing the wire... \$\endgroup\$ – Douwe66 Jul 22 '16 at 7:56
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I don't think you can, legally or safely. The normal way round this is to move the mains-to-DC conversion into a power brick.

The other approach is "double insulation", which would require an insulated sub-enclosure inside the aluminium case.

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