Off the shelf Safety Relay with Reset  - Page 1
Forum section: electronics / projects

author: LaserSteve
author_rank: Super Contributor
timestamp: September 25, 2015, 04:10:55 pm
content: I have a experimental machine at work where I need to ensure the graduate student stays and monitors the experiment.I'm looking for a stock digital or analog safety relay module where if the student does not hit the button every 30 seconds or 1 minute, the system is shut off. Sort of a Deadman's switch that is a bit liberal.I can easily build this. However the University safety office will prefer a commercial module for liability reasons.. This is to ensure a experiment involving large amounts of flowing liquid nitrogen is monitored and not left un-attended...Most safety controllers are "Deadman" switches, where if the buttons are released the machine stops. In this case the operator needs their hands free to make control adjustments.Suggestions?Steve

author: retrolefty
author_rank: Super Contributor
timestamp: September 25, 2015, 04:36:38 pm
content: I worked on a lot of 'safety shutdown' systems at a refinery. Your idea of a 'active deadman switch' is OK but I think the task really requires more analysis and features. You might also require additional stuff tripped so 'fan out' might be required, and of course there should also probably be a manual emergency trip independent on any active logic devoce (fail-safe of power outage). If you have for instance a 1 min trip, you should also have a warning buzzer at say 45 seconds. You might also want a automated logger to record events (resets, trips) with a time stamp and stored either locally on a SD card or off loaded to a PC application.

author: jitter
author_rank: Frequent Contributor
timestamp: September 25, 2015, 04:40:25 pm
content: Quote from: LaserSteve on September 25, 2015, 04:10:55 pmHow about a time delay relay operated by a momentary switch?

author: Berni
author_rank: Super Contributor
timestamp: September 25, 2015, 05:51:38 pm
content: There are timer relays that are designed for stairway lights (Gives you enough time to get trough the stairway but makes it impossible to forget to turn the light off again). You push a momentary switch and the relay stays on for however long you set the little adjustment pot on it. Often they are designed to fit on DIN rails like circuit breakers.If you wanted to also have a warning lamp or something you could wire up a second one and set it to a slightly shorter time.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 25, 2015, 06:40:18 pm
content: Quote from: LaserSteve on September 25, 2015, 04:10:55 pmWrite up a draft specification of the safety mechanism, get a few quotes from external electrical control panel manufacturing companies and give the quotes to the safety department. You'll find they'll be quite happy with "others" designing it as the liability is then theirs

author: Jeroen3
author_rank: Super Contributor
timestamp: September 25, 2015, 07:04:16 pm
content: I think you can use a zero-voltage trip-coil* attached to some din-rail circuit breakers. You won't be able to close the breaker unless both the timer* and the keyswitch* are closed.You can use any source to trigger the timer. A button, a pir, a remote, a footswitch.*These are designed to break a circuit on power loss. You don't want big tools to start running when the power is turned back on.*You can use a multitimer such as these:http://nl.farnell.com/carlo-gavazzi/dmb51cm24/relay-100h-spdt-240vac-din/dp/1838955*The keyswitch ensures people without key cannot re-enable the system.All can be bought and put in a box for little money.You've said it already. You're in the United States of Lawsuit. Get your papers straight.Also, you're a university, and the machine is already expirimental. You can't put rubber tiles everywhere. Have the students follow economics if they don't want any risk.But I would personally ensure that if I'm working ina roomwith lots of nitrogen that there is a dangerous level nitrogen alarm. It's called suffocation-gas for a reason in dutch.

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2015, 07:25:42 pm
content: Quote from: LaserSteve on September 25, 2015, 04:10:55 pmIf your graduate students aren't capable of understanding why they need to stay then maybe they shouldn't be in the room.Quote from: LaserSteve on September 25, 2015, 04:10:55 pmIf your students are only thinking of smoking their next joint then they might think up ways to defeat the system ("it was only for two minutes!")Quote from: LaserSteve on September 25, 2015, 04:10:55 pmCover your ass. Make somebody else responsible (external consultant?)That's what the university really wants...

author: bookaboo
author_rank: Frequent Contributor
timestamp: September 25, 2015, 07:28:25 pm
content: Something that already does have such safety features are remote controls for cranes. The safety relay is built in the receiver and relies on a constant "IM OK" signal from the transmitter, the transmitter has a red e-stop pushbutton for the operator should they want to stop the crane. Removing the battery, going out of range or a handset fault will have the same effect as pressing the e-stop switch as the "IM OK" signal is lost.What may be useful for you is that the transmitters also have an auto time out feature (adjustable but usually 3 minutes) so that if no switch is pressed the safety relay is turned off automatically. They are not cheap though, decent units like Ikusi are well over £1000 new. You may be able to find something second hand or as the application is not heavy duty one of the lesser brands would do.

author: Rerouter
author_rank: Super Contributor
timestamp: September 25, 2015, 07:58:43 pm
content: You can go cheap, look for an industrial timer module, most have a reset input, so you have the timer set to kill power at however many seconds, and they have to push a button on the reset line within that window,Put it in series with an E-stop relay and you should cover most cases

author: LaserSteve
author_rank: Super Contributor
timestamp: September 25, 2015, 09:37:34 pm
content: Quote from: retrolefty on September 25, 2015, 04:36:38 pmRight now, I just need to prove an operator is there. The extensive analysis is already done by myself, another Engineer and the Director of Safety. It just has to gate two solenoid valves. In graduate research we have a bit of a different concept then industrial safety. Our big problem is distractions with utterly brilliant people. Ie Cell Phone, Professor wants to see them, Youtube, BROWSING FORUMS, talking, that sort of thing. I can remote the control panel quite far away. E-trip and BRS is already there.The main concern is to prevent total freezing of a bulk sample that has to be mixed and reacted at -50'C. The second concern is not having a human present to manually valve off the system if a solenoid valve freezes open or if two PID controllers totally fail. We have concerns about water vapor in the lines, RTD sensors breaking, things like that. Things that sensors won't always catch. The mixing vessel has a blow off top, there are blow out panels, things like that. Its a 10 Liter sample.We know about O2 sensors, we have them and we check them.The general consensus is to make a distracted operator stay on their toes while not being so anal as to require two hands on the switches at all times. And have it be as simple as possible.Cost is not that big of an object, 250-300$ is reasonable.Steve

author: eetech00
author_rank: Regular Contributor
timestamp: September 26, 2015, 12:57:08 am
content: Hmm..How about two buttons? On operated by hand ("clear") and one operated by foot("start")?The start button is closed when the operator presses down on it with their foot. It would trigger a "hello" signal that continues until the operator takes their foot off the button. They then need to press the "clear" button to stop the hello signal.Or maybe a proximity sensor, instead of a foot operated button, to trigger the "hello" signal?

author: ajb
author_rank: Super Contributor
timestamp: September 26, 2015, 03:57:59 am
content: There are some neat-looking programmable timers out there,Automation Directhave a couple that look promising. They're helpful folks, if you give them a call I'm sure they'd be able to help you out.Enforcing a window to the reset button would be particularly effective at combating distraction, since it eliminates the possibility of simply taping the button down. You could do that with two timers. Before the window opens, the dead man's button is wired to the system's stop button and when pressed will shut the system down. When the first one times out, it activates some sort of signal to the operator and switches the button from the system stop to the timer resets, opening the reset window. When the second timer runs out, closing the window, it shuts down the system.

author: retrolefty
author_rank: Super Contributor
timestamp: September 26, 2015, 06:07:04 am
content: Well again it should consider human behavior. Do you expect the grad student operator to sit right at the switch and keep track of time and hit the switch once a min, or possibly he/she won't know when the min is up so will sit and hit it more frequently? Do you expect the operator to always be within reach of the button? You are asking a grad student person to become a button slave?Unless you make it more friendly your better off paying a real button slave minimum wage to sit, never leave the button location, and press every min.I feel there must be a warning alarm so the operator can causally walk from anywhere in the room to the button without having to run. Don't make it people friendly and they will just find a way to defeat it.How painful/costly/time loss to the process is a shutdown/restart?

author: Jeroen3
author_rank: Super Contributor
timestamp: September 26, 2015, 10:41:20 am
content: For me it looks like you're looking for an off-the-shelf product to fix behavioural defects in students.Stop searching, there is no such thing.

author: dom0
author_rank: Super Contributor
timestamp: September 26, 2015, 04:34:04 pm
content: Quote from: Jeroen3 on September 26, 2015, 10:41:20 amSure there is, worked fine for aeons.

author: stevenhoneyman
author_rank: Regular Contributor
timestamp: September 27, 2015, 02:53:39 pm
content: EDIT:Sorry, I read 30 minutes not 30 seconds...!I skim-read most of the above replies so sorry if it's already been mentioned... the device you are looking for already exists. I had one of these on my old 'firestarter' soldering iron just for my own peace of mind:https://energenie4u.co.uk/catalogue/product/ENER001They do a variable length one as well, and i'm sure other similar brands exist. "Rundown plug" was a good search term

author: LaserSteve
author_rank: Super Contributor
timestamp: September 27, 2015, 10:03:58 pm
content: StevenHoneyMan, you may be closer then you thought. For many devices around here, such a module is a very good idea. Thanks...I also have a call into Automation Direct...Thanks,Steve