Help making something safer.

Hi there. So I'm working on a testing fixture for my company and all. It switches 9v5a through 3 different inductors. Step on a foot pedal to let the power flow and all. But I'm trying to pick out a fuse to add to the circuit for an emergency disconnect in case during the test someone accidentally touches the unit being tested. (it shouldn't ever happen, but I'd prefer safer then sorry).

So what should I be looking for in a fuse or circuit breaker to trip off in the case that someone gets shocked?

Something like a ground fault interrupter but for DC circuits. Or, monitor the current and if it exceeds the 5A level have a switch to cut out the source.

CrossRoads: Something like a ground fault interrupter but for DC circuits. Or, monitor the current and if it exceeds the 5A level have a switch to cut out the source.

what I was thinking of doing somethign like the monitoring. Theres already going to be an arduino nano onboard that triggers the relay. Was just wondering if it would be better to use a fuse or somethign though rather then monitoring the amperage.

Well, a fuse takes a relatively long time to blow, and by then humans are already damaged. A GFCI will trip quite quick, as would monitoring the current and shutting off the source by opening a relay or something.

Nikarus:
Hi there. So I’m working on a testing fixture for my company and all. It switches 9v5a through 3 different inductors. Step on a foot pedal to let the power flow and all.
But I’m trying to pick out a fuse to add to the circuit for an emergency disconnect in case during the test someone accidentally touches the unit being tested. (it shouldn’t ever happen, but I’d prefer safer then sorry).

So what should I be looking for in a fuse or circuit breaker to trip off in the case that someone gets shocked?

Well 9 volts doesn’t represent much of a shock hazard. Can you supply a schematic of the complete circuit to let us look for possible hazards?

Lefty

To crossroads. The power itself is controlled by a solid state relay, so it wouldnt be hard to trip that off itself.

And to lefty, theres not really much of a circuit. Just the power supply, the relay, and the inductor. then off of the relay theres a foot pedal, the emergency stop switch and an arduino. If the arduino says its OK, and the ESS is not clicked down, then the foot pedal turns the power on and off. If anything says that the circuit is off though, it kills it. but I had been thinkign of throwing a fuse or circuitbreaker into the main loop in between the relay and the source.

Other then that theres not much else that can be done. Seeing as before these units were tested by setting up the power supply. Shutting the supply off by hand. Attaching alligator clips to the leads at different points on the inductors, then turning the supply back on. I'm fairly certain I'm already jumping ahead in safety just by putting the relay box in there (also dont have to turn the supply off and on repeatedly, what was taking a bit of time for the tester because theres a lot of walking involved).

Now your just gonna go and hook up all the leads with the supply off. Then a multipole switch lets you cycle through the different connection points. Operator stands in 1 spot repeatedly stepping on a pedal rather then running around. Just thought maybe 1 more layer of safety wouldn't hurt.

Nikarus: To crossroads. The power itself is controlled by a solid state relay, so it wouldnt be hard to trip that off itself.

And to lefty, theres not really much of a circuit. Just the power supply, the relay, and the inductor. then off of the relay theres a foot pedal, the emergency stop switch and an arduino. If the arduino says its OK, and the ESS is not clicked down, then the foot pedal turns the power on and off. If anything says that the circuit is off though, it kills it. but I had been thinkign of throwing a fuse or circuitbreaker into the main loop in between the relay and the source.

Other then that theres not much else that can be done. Seeing as before these units were tested by setting up the power supply. Shutting the supply off by hand. Attaching alligator clips to the leads at different points on the inductors, then turning the supply back on. I'm fairly certain I'm already jumping ahead in safety just by putting the relay box in there (also dont have to turn the supply off and on repeatedly, what was taking a bit of time for the tester because theres a lot of walking involved).

Now your just gonna go and hook up all the leads with the supply off. Then a multipole switch lets you cycle through the different connection points. Operator stands in 1 spot repeatedly stepping on a pedal rather then running around. Just thought maybe 1 more layer of safety wouldn't hurt.

Well without a schematic drawing I can make no other comments then that exposure to 9 volts is not a shock hazard and rule number one is to first do no harm.

Lefty

You say 9V into an inductor... so this is AC (or at least pulsed DC) right?

Retroplayer: You say 9V into an inductor... so this is AC (or at least pulsed DC) right?

No its a continuous DC load used for checking the polarity of the unit, which is comprised of several smaller inductors. (hence teh switch to click through and test each one rather then having the operator walking around reconnecting the thing 6 times)

Edit.

Heres a blanket question. So I know you need specifically DC switching relays (at least solid state ones. An AC built one won't work) do you have that discrepancy with circuit breakers?

Nikarus: emergency disconnect in case during the test someone accidentally touches the unit being tested. (it shouldn't ever happen, but I'd prefer safer then sorry).

What hazard are you trying to protect people from?

retrolefty: Well without a schematic drawing I can make no other comments then that exposure to 9 volts is not a shock hazard and rule number one is to first do no harm.

Lefty

Except that his 9 volts is at 5 Amps into an inductor. The back EMF, when switching off that current, [u]could[/u] be a shock hazard. I would go for some sort of ground leakage detector, to detect if there are fingers in contact with the circuit, that would not allow it to be switched OFF if there were.

Current follows the path of least resistance. I don’t think fingers are going to be the path of least resistance here. And reglardless, for it to actually be deadly, the user would need to have both hands on it at the same time, crossing the current through the heart. (BTW, the back EMF would dissipate very quickly and there is hardly any current capability there.) Actually getting a useable high voltage from a coil requires pulsing the coil and storing the charge between pulses.

DC (if there were actually enough voltage over the resistance of the body to pull enough current and if the source were actually able to supply that current in the first place) causes muscle locks and burns generally, but is rarely fatal. Take a camera flash circuit. At around 300V DC stored in the cap, it is enough to burn the bejeezus out of you (over a short sitance of skin) but it isn’t going to kill anyone. Now think of all the circuitry required to get 300V into that cap.

If this is truly a concern, all that would be needed is the off position of the switch to connect a low impedence path to ground to “discharge” the inductors.

Retroplayer,

Are you claiming that a burn ISN'T a shock hazard? It may not be fatal, but it's still a shock and a hazard. I'm sure your local 'Health and Safety' people would disagree with you.

Henry_Best: Retroplayer,

Are you claiming that a burn ISN'T a shock hazard? It may not be fatal, but it's still a shock and a hazard. I'm sure your local 'Health and Safety' people would disagree with you.

Nope. I don't see a statement anything like that in my post. I said burns were more likely with DC, but that there was really nothing in this circuit to get to that level. If burns are really the concern, why not be concerned about the heat generated in those coils by running 5A through them? If anyone is going to get burned, that would be the most likely reason.

Good job reading!

Henry_Best: Retroplayer,

Are you claiming that a burn ISN'T a shock hazard? It may not be fatal, but it's still a shock and a hazard. I'm sure your local 'Health and Safety' people would disagree with you.

I guess it's what definition of shock you want to use.

  1. Wow, I just received an electrical shock and it sure hurt for a moment.
  2. OMG I just received a burn that shocked the heck out of me and it sure hurts. Hope it doesn't blister or get infected.

My premise earlier that a 9vdc circuit does not represent a shock hazard was made on the assumption of talking about electrical shock hazards and if it applied to the OPs application and questions.

Lefty

And to think that kids have put 9V batteries on their tongues for decades and nobody has died or even burned....

Retroplayer: And to think that kids have put 9V batteries on their tongues for decades and nobody has died or even burned....

Back in the 'good old days' in the 50s my father would take me to the auto supply store with him. While he was dealing with the clerk on some car parts questions I would wonder over to the shelves that contained every imaginable sized battery that was ever available at the time, usually all carbon zinc types of all kinds of voltages and Ah ratings. I would touch across both terminals and work my way up the voltage range. I could sometimes just barely feel the 67.5 volt ones but would definitely get a good tingle from the 90vdc ones. ;)

Lefty

Ever hold the ring and tip lines in your fingers when the phone rang? Not very pleasant. Also got myself on the flyback of a TV more than once.

I’ve even gotten shocked with full 120VAC more than once. The worst was a badly installed freezer. The old metal kind that had a self-defrost. Well, my dad put one in the garage and the water would pool up underneath it. When you stood in that and grabbed the door, it bit you pretty good.

But, alas, no burns. The only time I have ever been burned by electricity was while trying to fix a digital camera for my niece and I accidently ran my finger across the flash cap solder joints.

My Brother got burned while installing a car stereo once. His skin was sweaty and the power and ground laid down against his forearm. He ended up with a nice deep cauterized cut. Why he had the wires exposed and working on it while the ignition was on is beyond me.

The point being, there is very little danger here and IMO not worth whatever effort it takes to protect against it. The coils should be enameled wire in the first place, so they are insulated.