Is there any reason why I shouldn't use a 12V car-relay to switch mains?

[Monkeh]

Yes, because tape and a 'non-combustible chamber' solve all the evils of using inappropriate parts.

and also solved all the evils if the appropriate parts fail (it happened to me alright) safety is few fold of layers, you dont leave the layer count to 0, usually every material around you is already engineered to be non-combustible to handle appropriate and inapproprite parts. you can do it but make sure a layer of safety surrounding it. what i saw in this thread is... "you cant do it"  i dont agree with that. even dave of taking things apart is not apropriate or batshit things for beginners from the point of view.

Tape and a box aren't safety layers, they're what irresponsible, uncaring imbeciles wrap their stupidity in to make themselves feel good.

If you want to do things properly and safely, you cannot use a 12V relay for 240V. I do not care how many arguments and 'layers of safety' you want to wrap it in, you don't do it.

[SeanB]

Few lights can range from 15W to 3kW, it depends just how many lamps you consider to be room lighting and controlled by the same one switch. 12V relay will do for a while with 15W, but for higher wattage, and after multiple cycles you have no warranty that it will not either arc shut or arc over.

[Mechatrommer]

ok ok now i've seen the light! we should use this...

no further argument needed we should all agree with this (even though i already failed few of those. thank you! for the discussion its very informative

[Lightages]

Telling anyone that a 12 volt relay is safe to use for  240 is not only irresponsible but also criminal IMHO. It is especially so for unattended home automation.  If someone wants to experiment at their own risk  and properly informed,  then that is their problem.

Do not use 12 volts relays for 240. It is as simple and clear as that. A beginner should not be experimenting with safety.

[tggzzz]

Using a 12V DC component with 240V AC is very dangerous. Full stop. Anyone that follows your advice is in danger of damaging themselves and/or their houses.

Encouraging others to do this is completely irresponsible.

If you believe otherwise then clearly you are so dangerously ignorant that you don't even understand the externt of your ignorance.

OK, so you (mechatrommer) have not found any problems when testing a lashup in limited circumstances for a limited time. THAT DOES NOT PROVE IT IS SAFE. It may be accepted practice in Malaysia (but I doubt it), but it most definitely is not elsewhere.

Your statements are similar to telling people that there's no problems walking across a busy road without looking. And then illustrating that it is OK by walking across a small backstreet once.

You really should retract your statements.

Anybody that believes this is dangerous might like to consider leaving a comment on the youtube video, and clicking the "report to moderator" link on the message.

[Andy Watson]

Anybody that believes this is dangerous might like to consider leaving a comment on the youtube video, and clicking the "report to moderator" link on the message.

+1

[Mechatrommer]

well, the burden of proof lies in you, i have done my part. "if i dont see it... i dont believe it", thats the culture i believe. "the culture" of believing mere lips statement from unknown members doesnt blend with me very well. and by following any labels in most electrical equipments (only for certified personnel) "law", then this forum should (1) not exist, (2) validate a person's competency before joining as member or specifically watching the video, because some noobs will like to follow whats done in this forum and risk damaging or killing something. but well, this "free to watch hazardous materials in this forum" may be acceptable to the "more civilized" world, you may make the complaint to youtube hmm. and was that somebody suggesting that me is like suggesting its safe to cross road without looking? who said its safe to cross road without looking? otoh i also have read more and more "view" toward asian/china "culture" in this forum coming in. meh whats this forum got into? more civilized? no offense just trying to be academic

[timb]

Technically, everyone was saying it can't be done and that the contacts would instantly flash over, causing your house to instantly vaporize. He did prove that to be demonstrably false. He also said that a proper mains rated relay should be used.

So, he's not factually wrong, if you want to be pedantic.


Sent from my Smartphone

[Mechatrommer]

well glad someone with competent english tried to clear that up. i said earlier, the reason to not to do it is if you dont have enough knowledge, so try to gain knowledge (or experience) before doing it. and besides my short video, its backep up by facts and science (unless you have something factual to say otherwise). you can google (i forgot what too lazy to find) how much clearence before 240Vac arc, iirc its so tiny to care of or compared to clerarance in common devices or pcb. and i'm also talking about low wattage load, assuming thats what the OP meant. agreed we dont usually engineer things close to its rated capacity, esp if we dont have enough experience and using no-brand-name devices. but lighting up 18W bulb with 30A relay? come on! thats only 18W/240V = 75mA load, thats 400X safety factor. if i'm not mistaken, 30A rating is based on how much the contacts can take before arc deposition problem become "out of spec", granted higher voltage will create longer arc during transient hence faster deposition rate, but what the hell, 400X safety factor should compensate, no? everything will eventually die, even the rated one.

just because you have seen tragedy doesnt means it always apply to other scenarios in not so similar manner, abandon ship of electrocutephobia, use your senses and use by knowledge. i also crynged when seeing video like photoinduction and mikeelectricstuff, but they eventually did it didnt they? but that not necessarily means to justify its not safe or safe, its just because i dont have proper knowledge for it or no guts enough to take the risk, hence i'm not doing it, i want to do it? i better gear up, thats the first thing you should do. something is not safe because its not well understood, correct? once you have the knowledge, you may forget the paperwork, thats only for muggles who are ignorant enough to race for the knowledge, those paperwork is made by those "knowledgable people anyway" for a very specific (well understood) condition to occur... no?

[Simon]

I've not read the whole thread but i am getting reports. Please can we calm the thing down. The definitive answer is: "thought shalt not no anything that the datasheet does not explicitly tell you you can do" Unless your a rocket scientist with kit and knowledge to try your crazy misuse of a part out in a controlled and safe manner. END OF.

And yes in my opinion trying to use a 12V rated relay for mains is a very bad idea. 40ADC on relay contacts is quite something, it has to have been achieved with compromises!

[Mechatrommer]

the problem is... there is no datasheet, there is no mechanical drawing to be more specific, this is pretty much an open question with variety of answers. everybody is like absolutely sure "it cant be done", so i'm providing another variant (based on provided clue by the OP)

[Simon]

the problem is... there is no datasheet, there is no mechanical drawing to be more specific, this is pretty much an open question with variety of answers. everybody is like absolutely sure "it cant be done", so i'm providing another variant

Automotive means 12 or 24V NO HIGHER. The relay will most likely state on it's body that it's for 12 or 24V so there is no case to go higher. If you buy a 240VAC relay it will say on it what the coil voltage is and what the switch voltage is, it's that simple.

[rolycat]

Technically, everyone was saying it can't be done and that the contacts would instantly flash over, causing your house to instantly vaporize. He did prove that to be demonstrably false.

No-one said it couldn't be done, they said that doing it would be extremely unwise and incredibly dangerous.

He also said that a proper mains rated relay should be used.

No, initially he said:

your 12v car relay is perfectly fine for what you have mentioned (just to switch a bulb on and off) no burning house, no coronal arc whatsoever, even the degraded contact. the only reason not to use it is the lack of knowledge and guts to do it. we are at 240Vac here no worry.

So, he's not factually wrong, if you want to be pedantic.

For once I don't want to be pedantic, I just want people to stay alive.

You do not exceed maximum rated values when building circuits for domestic use at mains voltages and currents. Period.

If you want to live dangerously that's your decision. Endangering others is criminally stupid.

[Mechatrommer]

Endangering others is criminally stupid.

define "endangering others".... people drove while drunk its endangering others but they will get the punishment until they did kill someone right? by getting caught (drive while drunk) all they get is a ticket isnt it?, then why do you say selling beer is not endangering others? and whats endangering about my statement you quoted? i did warn them didnt i? about knowledge. how about making campaign about banning photoinduction, mike and dave to be "criminally stupid" posting highly evolved videos? then why do you say they are not endangering? whats fun about this thread is the quote of burning house and persistent 240Vac arc on disconnected relay contacts which i never come accross, please prove me wrong with a very good link or video.

[Simon]

I am looking this thread as clearly it will go nowhere other than a mud slinging match.

As i said further back unless you have the equipment and knowledge to further test a component beyond it's rated specs you are not in a position to recommend exceeding the specification particularly if your talking hazardous voltages which is anything over 50V.

If it's rated for 12V then only at your own personal risk do you put 240V through it.