How to implimate a Total-Lab-Mains-Power-Switch? - Page 1
Forum section: electronics / projects

author: george graves
author_rank: Super Contributor
timestamp: March 13, 2017, 11:11:02 am
content: Recently a buddy of mine suffered a house fire. Total loss. It's looking like it was electrical - but the cause not known yet. He's a fellow "geek" - and so who knows what it was. Since then, I've been thinking more about safety.So the question is - say you have a lab full of equipment - and wanted a "flip this switch when you leave for the night" and all mains powered are shut off. I know you could do it with a circuit breaker at the panel, and have done that when I go on vacation, but I'm not sure circuit breakers are designed for daily use like that.Yea, you could do it with a wall switch tied into the wall outlets. But that seems kinda cheesy, and prone to accidental flick of the switch.And I guess you could use a power strip with a switch, but that would be running everything off of one socket, one power strip dasied chained to others. You see the issue I assume, I hope I don't have to list each and every one.How would you re-wire your lab for a "one switch kills all mains power"?

author: ajb
author_rank: Super Contributor
timestamp: March 13, 2017, 12:47:14 pm
content: Sorry to hear about your friend, that's really rough.Circuit breakers are definitely not intended for daily on/off use.If you have multiple circuits, ideally, you'd wire the whole lab on its own subpanel separate from the main panel. Small panels designed for just such a purpose are not terribly expensive, and can be fed from a suitably rated breaker installed in the main panel. The subpanel can be fitted with whatever combinatrion of conventional/GFCI/AFCI breakers suit your purposes/code requirements. In between you could install a suitably rated disconnect switch.If you have only one circuit, you could skip the subpanel and just install a small disconnect switch in line. You can get thosefairly inexpensively, although I don't know if there may be code implications for installing such equipment on a residential wall. At the very least a local inspector is likely to be surprised to see such a thing.If you want to add some automation to the on/off control, you could use a suitably rated contactor in addition to or in place of the disconnect.All that said, you've only addressed a single room in the house. A lab disconnect may be useful in case you're working on something a bit dodgy, but in normal operation I would expect a lab to be no more likely than any other room in the house to be the source of an electrical fire, if not less. If you're the type of person who has a lab and uses it regularly, I'd suspect that you're likely the sort who will notice when the equipment seems to be misbehaving, or when a cable needs replacing, and address those issues promptly. Versus how many other appliances in your house that get used intermittently, or that are used constantly but only rarely noticed. Coffee makers, heat/AC systems, fans, lamps, even your light switches and receptacles can cause electrical fires. So if you're really concerned with the risk of fire, a better first step is to check that all of your appliances and electrical infrastructure is in good shape. Replace conventional breakers with AFCIs for living spaces (required for new construction in most places). Make sure your dryer vent is clear! Then worry about the lab.

author: tecman
author_rank: Frequent Contributor
timestamp: March 13, 2017, 02:48:07 pm
content: Quote from: ajb on March 13, 2017, 12:47:14 pmBoth NEC and ANSI allow breakers to be used as switches. There are also "switch duty breakers" available which offer longer mechanical life than a standard breaker.For the requirement I would use a contactor (relay) to switch a larger load capacity with a single switch.paul

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 13, 2017, 03:55:44 pm
content: electrical disaster not necessarily excessive current, usually its unattended operation like battery charging, switched on soldering iron while you throw out your garbage bag etc and they blow or falls on combustible material. when you need something unattended, big switch will be of no use. nothing beats "paying attention to safety and precaution" before leaving your room. isolate lipo charging from the rest of the thing, ensure your soldering iron placed in a stable stand, put away unnecessary combustible material from the scene, dont let any exposed live circuit or wire around so your kids or girlfriend will not get hurt things like that.Quote from: george graves on March 13, 2017, 11:11:02 amuse 2 parallel switch then at some practical and convenient distance, remember the "lost in space" where they need 2 persons 2 keys to activate hyperdrive?

author: stevelup
author_rank: Regular Contributor
timestamp: March 13, 2017, 06:33:54 pm
content: I'd look at using a contactor (basically a big relay).You can then also implement an emergency off pushbutton by simply inserting an e-stop button in the contactor coil circuit.

author: mubes
author_rank: Regular Contributor
timestamp: March 13, 2017, 10:06:27 pm
content: My lab is on a totally separate spur from the fusebox, with its own protection etc. There are a couple of circuits in there which are on the main house power (for the router and servers) but the rest, including heating, are on the spur. The sparky who installed it, according to code, put something that looks a bit like a cooker isolation switch at the ingress (to my engineering shame I didn't investigate more closely)...I don't use it daily, but when I'm travelling I certainly feel more comfortable knowing that everything is deffo off...Dave

author: Cerebus
author_rank: Super Contributor
timestamp: March 13, 2017, 10:59:56 pm
content: Quote from: stevelup on March 13, 2017, 06:33:54 pmYup, that's the absolutely standard way (in the UK at least) of doing this sort of thing. One advantage is that it makes it relatively easy to sprinkle extra Emergency Power Off switches around the lab and there's something very comforting having an EPO next to the bench where you can hit it with a knee if you ever do anything silly/unfortunate with both your hands otherwise occupied.We once wired an entire entertainment venue like this, with EPOs by stage, in the sound booth and the manager's office that cut all power to the stage, PA and stage lights, and also turned on all the house lighting simultaneously. Relay logic, ya can't beat it. There was only one 'on' switch and that was key controlled so we could stop visiting bands road crew messing about with electricity until there was 'adult supervision' on the premises.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2017, 12:32:08 am
content: Actually an emergency stop shouldn't be used as a routine method of turning a system off. It should be there for emergency use only.Another thing to consider is that there are other sources of ignition than mains electricity. Batteries can be very dangerous and some cleaning solvents are highly flammable and have an explosive vapour.

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2017, 12:33:49 am
content: I use a nice quality power strip with a covered switch. The importance of the covered switch becomes apparent after you bump it with your foot/elbow a few times and shut off power to the whole bench while you're in the middle of something. Now this may not work for everyone but I've only got one bench of gear so it's adequate for me.The first thing I'd do to prevent electrical fires is go through and get rid of all of those horrid "back stab" receptacles that home builders in the US love where the wires just poke into the back and engage spring loaded contacts. Those things are terrible, the contact patch is tiny and over time the contacts lose their springiness and heat up, oxidize and heat up even more. My grandmother unplugged something in her kitchen once and saw that the contacts inside the outlet were glowing red from the load of the refrigerator on the end of the run. In my house I had one fail and fortunately it burned the wire off and opened the circuit. My other half's mother's place had a close call due to the same thing, space heater on the end of the run in the other bathroom, only caught this because someone happened to go into the bathroom and see smoke.Now I always use good quality Spec grade receptacles and I usually pigtail them and use wire nuts if this is middle of the run, always use the screw terminals, they are much more robust, the better quality receptacles normally don't even have the back stab terminals. This applies to the US anyway, probably Canada, but always check local electrical codes before acting on advice like this. Now days NEC requires tamper resistant receptacles in homes but I don't like those things, they're built cheaply but cost as much as the spec grade commercial stuff.

author: Cerebus
author_rank: Super Contributor
timestamp: March 14, 2017, 01:41:28 am
content: Quote from: Hero999 on March 14, 2017, 12:32:08 amEh? Which is better, an emergency cutoff that is also used regularly and therefore has a high confidence of working in an emergency because it's effectively tested daily, or an emergency cutoff that is only tested in an actual emergency? I know which I'd rather rely on to keepmealive.Everywhere I have ever worked that has used EPOs has required regular testing of EPOs. For things like lathes and printing presses, it has been at start of every shift and for the big 'shuts down the whole facility' EPO it's been monthly or quarterly.Moreover, at least in the UK, it's a requirement to fit switches to moving machinery that fail safely off if the mains power is removed and then restored and I haven't see a single example of these where the EPO circuit and the routine power off circuit are not one and the same thing and frequently for small equipment are the same physical switch. This is normally wired with a contactor and there isn't a simple, fail safe way of doing this that doesn't involve the 'off' switch and EPO switch(es) being connected to the same circuit.

author: Someone
author_rank: Super Contributor
timestamp: March 14, 2017, 02:04:03 am
content: Quote from: Cerebus on March 14, 2017, 01:41:28 amI've worked at many places where the latching red mushroom buttons (with or without key unlock) were the routine method of turning off the space, you're not thinking where the switch is when you use it frequently so when there are problems its just a natural reaction.

author: calexanian
author_rank: Super Contributor
timestamp: March 14, 2017, 06:03:46 am
content: In my home lab.If I were able to get into my wall I would run a dedicated circuit for my test benches and gear each with its respective duplex receptacles running to a standard 2 way light switch conveniently located near the bench and another near the exit. Yes there is the possibility somebody could accidentally turn it on while you are at your bench. There is also a chance your toddlers can walk in on you while giving a report on south Korea on the BBC.In my private work lab.I have everything on the test bench going to a common power strip and use the switch on that.If I were setting up a multi user lab.I would have dedicated contractors and breakers for each position and each position with an on/of switch and a E Stop button that cuts everything at all positions and an additional E stop button at all entrances/exits

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2017, 06:23:58 am
content: Seems a little excessive really, I mean how often do you need to cut the power to the whole bench? If one is worried about fires then the answer is to shut off the power to everything when not in use, if there's no power it can't catch fire. A simple power strip or switch is sufficient IMO.