Safety concerns using metal switches near 300V (low current) lines?

Question

I am building a nixie tube clock whose power supply in the best (or worst) case scenario will put out ~300V. This is obviously quite hazardous if not handled correctly. I need to use at least 1 spdt switch to configure the clock/switch it on off. Shopping around for switches, I see pretty much all the small flipping type switches are metal.

I managed to get a generic one. When I check for continuity using a multimeter, I can see that the terminals are isolated from the "switch" part which could touch the user, but how can I find out by how much? More importantly, why isn't this sort of thing more defined? Why isn't using metal switches such a big concern? For me it seems like a very big concern. What happens if some wire inside accidentally makes contact with the metal switch case? Why would you take the risk? On the switch it says 250Vac 2A, 120Vac 5A, but I'm thinking this is more to do with the resistance of the switch itself? I.e. how much can pass through it before it can get too hot, cause arcing between the terminals etc? (Correct me if I'm wrong).

I am only planning to switch ~5V for a uC digital signal, and I am planning on integrating a relay to switch the power supply on. Since I want to give this to someone who has minimal electronics knowledge, the last thing I would want to happen is for it to become a hazard. So I want it to make it as safe as possible.

So, my question is quite a general one, what sort of design safety steps do you guys take when you design something like this? Any tips? Why are metal switches so ubiquitous? Because they are more robust? Why isn't the maximum handling voltages clearly stated? How does it deal with sudden voltage spikes?

Answer 1

I have on my desk some of these ordinary toggle switches.

The datasheet reports "Insulation resistance: 1000M ohm min" and "Dielectric strength: 1000V RMS min @ sea level". The latter is the figure of merit, telling you that it's guaranteed to contain voltages up to 1kV without breaking through the insulation.

However, if you look around you, you won't see very many products with metal toggle switches on. This is because the normal technique for avoiding this problem is double insulation : http://en.wikipedia.org/wiki/Appliance_classes ; you'll see plastic toggle switches, or plastic buttons, or (in one case I saw) a switch internal to the case operated by a plastic lever from the outside switch. (The IEC standards are probably the "long answer" to all questions about electrical safety engineering).

Capacitative sensing has also become more popular, as it can operate through a thin plastic insulated case and avoid all this hassle.

Answer 2

I think you are worrying excessively about this (although it is good to worry a bit about it).

The wire carrying 300 volts should have insulation rated at 300 volts or more, so it can lie against a grounded chassis with no problems, so there should be no problem if it contacts the metal case of the switch. You should make sure that the metal parts of the switch are grounded by mounting the switch in a grounded metal panel, or running a wire from the metal parts to the circuit ground.

Good construction practice recommends that high voltage wiring should be routed away from low voltage digital wiring, and that all wiring should be secured with wire ties or cable clamps. This should eliminate the possiblility of your high voltage wiring from contacting the metal parts of the switch (or anything else the user might contact).

All switches I've used have had the user-accesible parts well insulated from the actual switch contacts.