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#174347 - 02/02/08 10:44 AM 24 volts = safe?  
tech_bit  Offline
New Member
Joined: Jan 2008
Posts: 2

I work as a technician in a tubing mill. Unfortunately there are some occasions when the tube will "bend" or "buckle" in the mill causing it "shoot out" of the stands. This is extremely dangerous.

To minimize the danger, we install limit switches to to trip if the tube bends. The problem with this is it is difficult to tell which direction the tube will bend, and it is possible for the tube to actually destroy the limits without actually tripping them.

Someone came up with the idea to wire a 24 volt transformer-
wire the hot to a 24 volt control relay coil. Ground the neutral side of the xformer and run the neutral wire from the coil to a piece of copper tubing which will "wrap" around the tube.

When the tube bends, this will energize the relay and we can trigger an e-stop.
I like the idea but... I think it's dangerous and it goes against everything I've learned.
To me we have an UNINSULATED ENERGIZED CONDUCTOR mounted at chest level for the operators to adjust.
We are using the grounded nuetral and the equipment grounding neutral interchangeably. It just seems wrong to me on a lot of different levels.
Everyone is looking at me like I'm over-reacting and saying "it's just 24 volts."
I think they are all crazy.
I respect them, and everything else in the plant is done safely, it just seems this one circuit seems to be eluding everyone elses concern.
I don't mind over-reacting to electrical safety, so if I am... good.
But I would also like some feedback or ideas.

Last edited by pauluk; 02/08/08 07:21 AM. Reason: Removed duplicate post

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#174349 - 02/02/08 11:43 AM Re: 24 volts = safe? [Re: tech_bit]  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Assuming you use a class 2 power supply this could be done.

I would look into using non-contact proximity sensors.

Bob Badger
Construction & Maintenance Electrician

#174355 - 02/02/08 06:23 PM Re: 24 volts = safe? [Re: iwire]  
sparkyinak  Offline
Joined: Jul 2007
Posts: 1,327
ditto on the not-contact proximity switches

"Live Awesome!" - Kevin Carosa

#174509 - 02/06/08 12:13 PM Re: 24 volts = safe? [Re: sparkyinak]  
Mike Wescoatt  Offline
Joined: Jun 2001
Posts: 161
Cedar City, Utah
you could use a set of "light curtains". they are LED sensor / receiver pairs in various lengths that are made for e-stop applications. Usually they e-stop when an operator puts arms or hands thru the curtain (like on a press) but you could use 3-4 lengthwise (to form a box) to do the e-stop when a tube breaks it.

Mike Wescoatt

#174515 - 02/06/08 01:22 PM Re: 24 volts = safe? [Re: Mike Wescoatt]  
electech  Offline
Joined: Feb 2002
Posts: 113
Northern Il
To expand on iwire's comment: A class II power supply (UL 1310), or a Limited Power Source (ANSI/UL 60950-1), will ensure the output is limited so as to not create a shock or energy hazard.

The INDOOR (dry) voltage limit for what is considered safe for any user to handle bare-handed per IEC/ANSI/UL 60950-1 or UL 1310 is 42.4 Vp (30Vrms) or 60 Vdc. You have that covered. However, you have not indicated anything to suggest the transformer has adequate isolation from the primary to secondary. In order to be considered safe to handle bare-handed, the output needs to be classified SELV (Safety Extra Low Voltage) to ensure it is safe under a single-fault condition. The SELV designation means there will be double or reinforced insulation between the primary and secondary. Alternatively, it is possible to have only Basic insulation with a grounded secondary IF it can be shown that a fault across the Basic insulation from primary to secondary does not cause the secondary to exceed SELV limits. This is doubtful with only one leg of the secondary grounded and no other protection. There are applications for low voltage transformers that do not meet SELV requirements (these would be ELV and not SELV), so you need to be sure you are not using one of these. Just being "Listed", given no other information, does not mean anything toward this end.

The other issue is does this bare wiring present an "energy hazard" that could, for example, weld a piece of jewelry or a tool an operator may be holding, and cause burns. 100 VA is the limit for UL 1310 or a UL 60950-1 Limited Power Source transformer. Routing the non-grounded output of the transformer through the relay contact prior to where the output is exposed to the operator probably satisfies this, but a Listed 5A fuse on the output guarantees compliance with Class 2 / LPS limits.

Any NRTL Listed UL 1310 wall-wart type class 2 device should meet all requirements. Its just when you start looking at bare transformers that safety starts looking like more of an issue.

#174533 - 02/07/08 12:28 PM Re: 24 volts = safe? [Re: electech]  
tech_bit  Offline
New Member
Joined: Jan 2008
Posts: 2
Thank you very much for your replies.

Thanks so much for the class 2 power supply information, as well as the SELV info.

I just wrote a long explaination of myself, then got freaked out and imagined my boss was reading this right now, with a pink slip in hand...


I'm going to blatently ask this.

The INDOOR (dry) voltage limit for what is considered safe for any user to handle bare-handed per IEC/ANSI/UL 60950-1 or UL 1310 is 42.4 Vp (30Vrms) or 60 Vdc.

What if you cut your hand and you are wet? I think this is my main concern. This is not an uncommon scenario where I work. I think that's what is really bugging me.

Anyway, let me bring up the SELV issue to the folks at work and see what they say.

I really appreciate the information.

Thanks again.

#174534 - 02/07/08 01:19 PM Re: 24 volts = safe? [Re: tech_bit]  
sparkyinak  Offline
Joined: Jul 2007
Posts: 1,327
Do not quote me on this, I just saw recently (don't remember where) the the proverbial less then 50 Volt is consider safe rule, was based on studies and research of how electricity effects the body. Depends on who you talk to, as litte as 6mA (.006 amps) can kill a human. Ohm's Law tells us that the body resistance needs to be less then 8333 Ohms at 50 volts. That is pretty darn low for flesh. I typically use 10K Ohms in equations for body resistance. I'll see if I can find that info again. It may take a while. We are currently getting pulverized by snow then it is suppose to freeze. I just got back from a road trip so I got several feet of snow to remove so I can get some fuel oil delivered.

"Live Awesome!" - Kevin Carosa

#174546 - 02/07/08 10:12 PM Re: 24 volts = safe? [Re: sparkyinak]  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,208
Chesapeake, VA
Yes, when you cut your hand, you are wet. Blood makes the skin a better conductor. In fact, if you jab electrodes into your fingers, you can receive a fatal shock from as little as a 9V battery.

#174551 - 02/07/08 11:38 PM Re: 24 volts = safe? [Re: SteveFehr]  
sparkyinak  Offline
Joined: Jul 2007
Posts: 1,327
Thanks for the post Steve you jogged my brain. The info I mentioned earlier I was going to look for I do believe came from a post in the past or something I saw somewhere on the internet about being shocked to death by meter probes stuck into the flesh.

"Live Awesome!" - Kevin Carosa

#174563 - 02/08/08 07:17 AM Re: 24 volts = safe? [Re: sparkyinak]  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
It's the outer layers of skin which contribute to most of the resistance of the body. Once you pierce below those and go into a deep, open cut, the resistance is much lower.

Voltage and current figures for the electric chair show that once the initial high-voltage jolt has broken down the resistance of the outer skin the head-to-ankle resistance can be as low as 200 ohms!

That's an extreme example obviously, but it shows why medical apparatus has to be designed extremely carefully to avoid leakage currents which would be quite dangerous if applied to open wounds.

Along with proximity sensors, light curtains, etc., another possibility to consider if an actual metallic contact circuit is required would be to have the relay switched by a simple transistor control circuit. The "energized" trigger contact could then be fed by a very-high value bias resistance which would limit the current to a few microamps.

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