"Continuity" switches and safe voltage levels.

I've seen 24VDC on these "Continuity" type switches before from multiple OEMs. You connect 24VDC to the Common of a Sourcing Input module and switch the ground or negative side of the power supply to the input terminal. Or you connect 24VDC to a relay coil and switch the 0VDC/Ground side of the power supply to the relay to turn it on/off. Some have used continuity level sensor modules to do it. Many different ways to do it.

Recently I've had a few cases where operators have reported being "shocked" by these devices. Usually on the more tender parts of a body, like the underside of your arm if it touches it.

I've always considered 24VDC a "Safe" voltage level. But there is some definite pain involved when touched just right or under the right circumstances (humidity, sweating, etc.)

Is there a way to easily limit the current available on these types of devices? I've been trying to figure out what I can do in a parallel or series circuit to control available current on the setup but haven't found anything yet.

(Not 100% sure if current is even my issue here.)

I think the better approach is to determine why someone can come into contact with ANY voltage on the system and fix it so they can't.

I think you need to look into what is actually happening. Touching one lead of a continuity tester, even a megger isn't going to be felt. Holding one side of 24V, and touching anywhere isn't going to be felt (holding one side of a megger and touching the other most certainly will be).

Do they have the tester connected to something when they are getting 'shocked' ? Are they trying to test on energized conductors? Even long unterminated conductors can couple to the AC Line, and that can be felt to ground, if they are also touching that.

I've tested it out myself before where available.

Example:
I have a die in a hydraulic press that uses these "continuity" sensors to make sure a part is correctly loaded (tight tolerances on location)

+24VDC on the DC Common of a digital input card. An electrically isolated small metal pad on the die that the part makes physical contact with. This isolated pad is wired to the DC Input terminal on an the input card. The 0VDC terminal is connected to a second electrically isolated pad and the part when touching both pads completes the circuit to the input.

If I touch the isolated pads with my hand or most parts of my body I feel nothing. But if I place the underside of my arm, a more sensitive spot, against the isolated pads, it feels like a sharp pin prick. (Much like a 9V battery against your skin or your tongue)

Verifying voltages (DC and AC) against the pads and ground show nothing unexpected and the wires for this DC loop are not running near or parallel to anything AC.

That's just one example. I have seen it used in many spots like an extrusion process to make sure the product is continuous and has not broken in two pieces. In all examples dangerous motion and voltages are removed and controlled through proper safety systems but 24VDC input power was never determined to be unsafe, and was left available.

I worked in a facility that encouraged crying under the guise of safety. To alleviate unneeded stress I just popped in a 5Vdc power supply and fired a 3-32Vdc SSR.

Discal said:

I worked in a facility that encouraged crying under the guise of safety. To alleviate unneeded stress I just popped in a 5Vdc power supply and fired a 3-32Vdc SSR.

Click to expand...

I will have to look into 5V options for some of them. I wonder if I'd have a voltage drop issue across our extrusion usage when the product is 200+ feet long.

One should be able to use Ohm's law to calculate current to the SSR. Then, using wire size and the voltage drop calculation in the code book, see if the voltage will drop below 3 VDC to fire the relay. If not, then it will work as long as the resistance doesn't change (I. E. loose connections, corroded wire, etc.)

My question would be: why are they operating Equipment like extrusion machines, presses etc without over-alls and gloves?

I work in a very different industry but our mandatory clothing requirements are long sleeves, high-sensitivity gloves at minimum and safety glasses.

Tell bubba to stop licking the contacts on the chocolate bar extruder

I think you answered your own question. You expect a tingle from a 9vdc battery and you know it's safe. You certainly should expect a stronger tingle from 24vdc. It's safe in that it's not going to stop your heart, especially if it's across a few inches of skin on your arm. It may cause you to jerk and bang into something, so it's not completely harmless. You may be able to cover the edges that are most likely to be touched and minimize the problem.

I would be looking at the earthing of your machine. If you're doing this, I'd say your 0VDC should be earthed, as should your machine.

I've seen an old 24VDC power supply floating at 300VDC above earth (common was 300VDC, 24V was 324VDC). If you wired the "zero" volt wire off that power supply to a machine frame that wasn't connected solidly to earth, that would give you a tingle.

If all part of your machine frame, operator stations, platforms, handrails etc are at the same potential as the 0VDC, you will never get a tingle. Have a look at the faraday cage suits that people wear when maintaining HV transmission lines for an extreme example of this.

Tharon said:

Some have used continuity level sensor modules to do it. Many different ways to do it.

...

Is there a way to easily limit the current available on these types of devices? I've been trying to figure out what I can do in a parallel or series circuit to control available current on the setup but haven't found anything yet.

(Not 100% sure if current is even my issue here.)

Click to expand...

https://www.galco.com/buy/SSAC/LLC44A1A

It's for liquid sensing, but I have seen them used for applications similar to yours and installed them myself, testing them with my own skin. I forget what the voltage is, but the current is limited to less than 1ma.

ASF said:

I would be looking at the earthing of your machine. If you're doing this, I'd say your 0VDC should be earthed, as should your machine.

I've seen an old 24VDC power supply floating at 300VDC above earth (common was 300VDC, 24V was 324VDC). If you wired the "zero" volt wire off that power supply to a machine frame that wasn't connected solidly to earth, that would give you a tingle.

If all part of your machine frame, operator stations, platforms, handrails etc are at the same potential as the 0VDC, you will never get a tingle. Have a look at the faraday cage suits that people wear when maintaining HV transmission lines for an extreme example of this.

Click to expand...

My 0VDC is at ground potential, usually right at the power supply itself with a ground wire from the ground bar to the 0VDC terminal of the supply. They still do get a tingle because if anything "sensitive" touches one of the isolated pads and then any other part of your body is touching a grounded object, it completes the circuit. I've felt it myself when doing repairs. I'm positive it's not a floating 24V power supply causing high voltage potentials.