Need Help Determining DC Electrical Hazards

[Sparketta]

I am trying to determine whether a certain system poses a hazard per OSHA standards- can anyone help me?

The power to the system is provided by 28VDC power supplies, some are 100 amp and some are 250 amp. Some are fed by 120VAC and some by 480VAC. We have been told that when working downstream of the power supplies, that they do not have to be locked out, because they provide less than 50 volts, but I am not sure how OSHA's equation (below) factors in as I do not believe induced voltage applies to DC, and certainly the amperage the power supplies are capable of could be very harmful. Can someone figure this out and explain it?


OSHA says this about determining electrical hazards:

"Hazardous energy means a voltage at which there is sufficient energy to cause injury. If no precautions are taken to protect employees from hazards associated with involuntary reactions from electric shock, a hazard is presumed to exist if the induced voltage is sufficient to pass a current of 1 milliampere through a 500 ohm resistor. (The 500 ohm resistor represents the resistance of an employee. The 1 milliampere current is the threshold of perception.) If employees are protected from injury due to involuntary reactions from electric shock, a hazard is presumed to exist if the resultant current would be more than 6 milliamperes (the let go threshold for women).

Hazardous (induced) voltage means 50 volts (rms) or more."

Thanks for any explanations you can give.

 

[Mike Furlan]

I am trying to determine whether a certain system poses a hazard per OSHA standards- can anyone help me?

The power to the system is provided by 28VDC power supplies, some are 100 amp and some are 250 amp. Some are fed by 120VAC and some by 480VAC. We have been told that when working downstream of the power supplies, that they do not have to be locked out, because they provide less than 50 volts, but I am not sure how OSHA's equation (below) factors in as I do not believe induced voltage applies to DC, and certainly the amperage the power supplies are capable of could be very harmful. Can someone figure this out and explain it?


OSHA says this about determining electrical hazards:

"Hazardous energy means a voltage at which there is sufficient energy to cause injury. If no precautions are taken to protect employees from hazards associated with involuntary reactions from electric shock, a hazard is presumed to exist if the induced voltage is sufficient to pass a current of 1 milliampere through a 500 ohm resistor. (The 500 ohm resistor represents the resistance of an employee. The 1 milliampere current is the threshold of perception.) If employees are protected from injury due to involuntary reactions from electric shock, a hazard is presumed to exist if the resultant current would be more than 6 milliamperes (the let go threshold for women).

Hazardous (induced) voltage means 50 volts (rms) or more."

Thanks for any explanations you can give.

Great question.

I'm replying more to stimulate discussion than to give a definitive answer.

From NFPA 70E, Article 340.5 (3) Results of Voltage. A voltage of 30V rms, or 60 V dc, is considered safe except when the skin is broken the internal body resistance can be as low as 500 ohms so fatalities can occur.

 

[Sparketta]

Great question.

I'm replying more to stimulate discussion than to give a definitive answer.

From NFPA 70E, Article 340.5 (3) Results of Voltage. A voltage of 30V rms, or 60 V dc, is considered safe except when the skin is broken the internal body resistance can be as low as 500 ohms so fatalities can occur.

Thanks for the response, Mike. That is very interesting. I still wonder if the amperage of the system makes any difference, though.

We hook load banks up to the power supplies to validate them and they really will draw 100 or 250 amps.

 

[brian john]

Is there a bttery connected to this system?

Can it be turned off or does it feed a critical load (considered critical by the end user)

 

[eric9822]

28 VDC doesn't pose a shock hazard and is below the 50 volt threshold where variuos NFPA70E sections kick in. The capability for supplying 150 to 200 Amps may IMHO pose a flash/burn hazard that would warrant lockout/tagout or justification of energized work along with an energized work permit system.

An excerpt from NFPA70E Article 130 Working On or Near Live Parts, Section 130.1 Justification for Work

Energized parts that operate at less then 50 volts to ground shall not be required to be de-energized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.

 

[billsnuff]

is this in an an industrial setting where only trained and qualified elec workers are allowed to work on the equipment? This would have some impact on the severity of restrictions re: LOTO

 

[Sparketta]

Is there a bttery connected to this system?

Can it be turned off or does it feed a critical load (considered critical by the end user)

There are no batteries and the system feeds a critical load when it is on, but it can be powered down.

28 VDC doesn't pose a shock hazard and is below the 50 volt threshold where variuos NFPA70E sections kick in. The capability for supplying 150 to 200 Amps may IMHO pose a flash/burn hazard that would warrant lockout/tagout or justification of energized work along with an energized work permit system.

An excerpt from NFPA70E Article 130 Working On or Near Live Parts, Section 130.1 Justification for Work

Energized parts that operate at less then 50 volts to ground shall not be required to be de-energized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.

OSHA says the same thing as the NFPA does about not having to de-energize under 50 volts if there is no increased exposure to burns/arcs.

I had pretty much the same thought you did about the amperage posing a flash/burn hazard.

is this in an an industrial setting where only trained and qualified elec workers are allowed to work on the equipment? This would have some impact on the severity of restrictions re: LOTO

It is an industrial setting, but there are people who work in the area that are not trained/qualified. Also the people working on the system are somewhat trained and have some electrical knowledge, but it is not extensive. They work to explicit step-by-step instructions written by engineers. The workers do not have training to work on energized systems, and the system is de-energized when they work on it, but not locked out (OSHA's take on it in 1910.333 is that if it is not locked out it should be treated as energized). The system is large and is controlled remotely from a console.

 

[nakulak]

clearly there is a flash/burn hazard, so either a system is put in place to provide for correct ppe, or lockout/tagout needs to be implemented (jmho)

 

[dlhoule]

I also work in an industrial setting and I will tell you that I would not consider working on that system without locking it out. One place I worked had 24 Volt generators which were run at about 18V and 20,000 amps. I saw some pretty serious arcing in that system.

In your system I feel a person could very easily receive some nasty burns under the right circumstances. Why not LOTO?

 

[billsnuff]

i did a quick search on the risk assessment steps FWIW.

http://www.hse.gov.uk/pubns/indg163.pdf