Arc-safe relay - lightning protection! - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: June 09, 2011, 09:35:43 pm
content: In the summer, there's a lot of thunder when it's got in the summer. It has already taken out my Apple Airport Extreme router which made me a bit angry so I'm thinking of building some kind of remote controlled device for "unplugging" expensive equipment. So I need some kind of relay which is arc safe for lightning induced voltage spikes. Is this even possible?I'm thinking I could maybe battery power the device itself so that it's protected from lightning.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2011, 09:53:02 pm
content: Why not just use a standard change over relay so the live and neutral are both connected to earth when it's in the off state?

author: shadewind
author_rank: Frequent Contributor
timestamp: June 09, 2011, 10:00:55 pm
content: Live and neutral connected to earth? Wouldn't that simply mean short circuit, i.e. blown fuse?

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2011, 10:02:33 pm
content: No because they'll be disconnected from the mains, that's the whole point of using a relay. Just ensure the relay is break before make but don't worry too much standard relays are.

author: shadewind
author_rank: Frequent Contributor
timestamp: June 09, 2011, 11:26:22 pm
content: So what's the idea here? Why will this protect it?

author: tekfan
author_rank: Frequent Contributor
timestamp: June 10, 2011, 12:20:25 am
content: Quote from: Hero999 on June 09, 2011, 09:53:02 pmThat's a very elegant solution. I hope you were thinking of the same circuit. When the relay is energised the load will receive the supply but when de-energised it will short the load to ground. So even if there is an arc from the input to the load it will go directly to ground thus protecting it. Of course this would only work withproperly grounded outlets!The moving armature should also have sufficient clearance between the stationary contacts because you can get a nice arcover if supplying highly inductive loads.

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2011, 02:45:46 am
content: Yes that's the circuit I was talking about.Anyway, connecting to earth will help to an extent but it's not a perfect solution and still won't protect against electric shock. During a lightning strike, such a high current can flow up the earth conductor, it will be at a different potential to you and different conductive objects so it's possible to get a shock but it shouldn't be too serious and by then your house will probably catch fire so it'll be the least of your worries.

author: Trigger
author_rank: Regular Contributor
timestamp: June 10, 2011, 06:49:54 am
content: Quote from: shadewind on June 09, 2011, 09:35:43 pmWhy not just make something that physically unplugs it? Yes you'd have to manually plug it back in but it'll protect your equipment and you don't need to worry about arcing. Yes you can debate grounding, relays, etc, all day but what's proven to work is just pulling the plug out of the socket.

author: scrat
author_rank: Frequent Contributor
timestamp: June 10, 2011, 09:38:07 am
content: A remote controlled unplugger? Or just hiring someone to do the job? :-)

author: shadewind
author_rank: Frequent Contributor
timestamp: June 10, 2011, 11:58:07 am
content: Quote from: Trigger on June 10, 2011, 06:49:54 amThat sounds bulky and a lot more complicated than getting some kind of premade relay.

author: PetrosA
author_rank: Frequent Contributor
timestamp: June 10, 2011, 08:46:02 pm
content: I don't think I've ever seen a relay with large enough gaps to prevent lightning from arcing over. The closest thing I've seen would be an automatic transfer switch for a generator, but that's going to cost in the hundreds of dollars.

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2011, 08:58:17 pm
content: Shorting the live neutral to earth as mentioned above is the only way because it ensures there's no potential difference between them, thereby protecting your equipment. It doesn't matter if the lightning current flows up the earth conductor as all the parts inside the equipment will be at the same voltage. This doesn't completely protect you or your house of course, installing a proper lightning conductor is the only way to do that.

author: shadewind
author_rank: Frequent Contributor
timestamp: June 11, 2011, 02:40:17 am
content: That sounds great. I'm not after protection for me or for my house, I just don't want to buy new gear in my studio because I forgot to unplug it. But of course, the first and foremost reason as that it's fun to build stuff.