Non PLC related, but looking for your input

Hi guys-
sorry this isnt PLC related but i know there is a lot of guys here that could answer this with all their electrical knowledge.

The company i work for has recently required we purchase DC ONLY welders for any work done in a wet/damp environment. I am a bit confused since current is current whether it be AC or DC. I have also heard its easier to "get off" when being shocked if it is AC. Mostly due to the wave going + t0 -.

Just looking for an explination and possibly the correct information. My fear is a safetly manager may have been mislead and has misinformation that DC would be any safer.

agian i appoligize this isnt PLC related but i dont believe there is a welder related Q&A. Thanks in advance.

Look below

OSHA 1910.250 is the regulation for welding.

I have not "yet" read anything stating a DC welder would be less likely to shock than a AC. PPE (personal protective equipment) is worn to protect against injury, the type of PPE depends on situation.

To help prevent shock:
Insulate welder from workpiece and ground using dry insulation. Rubber mat or dry wood.
Wear dry, hole-free gloves. (Change as necessary to keep dry.)
Do not touch electrically "hot" parts or electrode with bare skin or wet clothing.
If wet area and welder cannot be insulated from workpiece with dry insulation, use a semiautomatic, constant-voltage welder or stick welder with voltage reducing device.
Maybe the above is where the DC welders come in
Keep electrode holder and cable insulation in good condition. Do not use if insulation is damaged or missing.

Lincoln Electric offers a free copy of their E205 safety brochure.
http://www.lincolnelectric.com/community/safety/safeform.asp

thanks Ron-
I think the paranoid dept...eh.. i mean the safety dept may have been mislead.

I meant to add the OSHA link, the actual standard is 1910 Subpart Q, 1910.251-1910.255
http://www.osha.gov/pls/oshaweb/owastand.display_standard_group?p_toc_level=1&p_part_number=1910

I havent read anything that directly states it but after doing some reading I can see where a DC unit may offer less of a shock hazard in wet conditions than an AC.

I may be way off base but here is what I think may be part of the reasoning. A DC welder may output less voltage and the welding leads may not have a potential or common to ground. If that is the case a DC welder would offer a good preventive against shock.

Never thought about this before but has my interest so will have to investigate further.

I worked as a welder in an outdoor rail yard in western washington and DC shocked me everytime I grounded myself.

that makes sense, i thing you wouldnt want to ground yourself thus the need for PPE.

One differnce needs to be considered, which is the peak ac verses rms.

A 50Vdc welder is 50V, but a 50Vac peak will be about 71V. Human skin is usally at higher resistance. I wonder if the peak breakdowns human skin and create a low resistance path thereafter. In this sense ac may be more dangerous than dc.

yeah it fries you in only one direction vs both as with AC

I have found it safest just to let someone else do it while I go drink coffee. I don't really care if it is AC or DC.

I do know that a 120 volt ac coffee pot will plug into a 64 volt dc locomotive outlet, and make coffee just fine!

Not sure how that relates to welding, but it is AC / DC.

regards.....casey

I think I remember reading a story, probably entirely apocryphal, about the business battles which were fought in the early days of electrification in the U.S.

Allegedly the two main pioneers, Edison and Westinghouse, were competing strongly, each promoting their own 'brand' of electricity for domestic, commercial and industrial use. One used a DC system, one used an AC system. The DC system was seen as having great advantages for the simplicity of wiring within a building but lost out on transmission issues. The AC was exactly the opposite: less transmission losses, but slightly more complex installation requirements. Of course every opportunity was keenly fought over.

Until the proposal for the introduction of the first electric chair! Neither Edison nor Westinghouse wanted to win this contract. Their 'product' would be indelibly marked as being "the dangerous one" in people's minds. Can you imagine the back-pedalling and evasion that must have gone one to make sure they lost the business?

As I say, whether it's true or not, it's a nice story.

By the way, does anyone know what electric chairs use currently (or should I say, nowadays)?

Regards

Ken.

dc vs ac

60 hz is the problem.

Your heart is controlled by the SA node nerve pulse thing.
Your heart rate is approx. 6 hz

6 and 60 hit it off really well and I'll take a DC burn vs a 60hz shuffle any day.

RE: wet ground etc. This part is VERY PLC related.

If the safety dept is writing an SOP be sure to include in environments like that to ALWAYS require to welder to carry BOTH leads to the point of welding. Simply grabbing the ground clamp and working it past the rust into the I beam 100ft from your stinger is a super NO-NO in wet and rusty environments. Especially near things like cranes etc.

Sure you'll find a path to ground but it is very likely through the VFD's ground lead or a stretch of good liquid-tite.

The electric chair was invented by an employee of Edison's (although many people believe it was Edison himself). However, typical Edison, he couldn't resist killing two birds with one stone (pun not intended!) and used the opportunity to point out how dangerous AC was, in his adverts.

You can read all about it in Wikipedia here

They also have an interesting entry about Nicola Tesla, which is well worth reading.

Personally, given the choice (which you usually aren't!), give me AC any day - at least there's the chance that you'll get thrown off!

Excellent link, Roy. Everything you might have wanted to know - and then some!

Ken.

WayOT: Electric Chair, doomsday machine, and other moral issues

I had a thought of late.
The PLC (and all its associated stuff) is fairly bland. What makes it work is the connection to I/O, as well as software.

So, consider a moral mess if one were asked to consider programming something like an electric chair, or perhaps a doomsday machine, or something of a similar nature.

I've actually worked on U.S.Naval weapon systems that had 'special' capabilities, and struggled with this question. Of course, military is different from civilian.

But, out here in the real world, how would any of us, who do this for a living, react to a moral dividing point when presented in a business environment.

Just a thought as the world evolves.

RMA said:

The electric chair was invented by an employee of Edison's (although many people believe it was Edison himself). However, typical Edison, he couldn't resist killing two birds with one stone (pun not intended!) and used the opportunity to point out how dangerous AC was, in his adverts.

You can read all about it in Wikipedia here

They also have an interesting entry about Nicola Tesla, which is well worth reading.

Personally, given the choice (which you usually aren't!), give me AC any day - at least there's the chance that you'll get thrown off!

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