more dangerous AC or DC

[coop3339]

I was working with a guy today and we were talking about how solar is always worked on hot in the combiners. I said it was strange that so many non electricians think nothing of working on the live 600-1000V conductors. The guy I was working with said, "it's probably safer than working live in the panel at your house". I said how can that be, your house is only 120V to ground and solar can be 600-1000V. He said, "yea but its the frequency that stops your heart". I do remember hearing that in apprentice school. I think its 50mA AC across your chest can kill you. My question is, does it take less AC amps at 60hz to kill you than DC amps? Or is it 50mA AC or DC?

 

[zbang]

He said, "yea but its the frequency that stops your heart". I do remember hearing that in apprentice school. I think its 50mA AC across your chest can kill you.

Where do they come up with these ideas (your friend)? It's the current, not the frequency, although if it's high enough there will be other effects. One can start with: https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

There was also discussion of ac v dc recently on one of these forums.

 

[Carultch]

Where do they come up with these ideas (your friend)? It's the current, not the frequency, although if it's high enough there will be other effects. One can start with: https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

There was also discussion of ac v dc recently on one of these forums.

I've always understood it to be the ENERGY based on the combination of both the voltage across your body, and the current thru your body, and time in contact with the shock, that makes a circuit contact dangerous. It isn't correct to only focus on the amps, because the volts across your body directly determine the amps.

One reason why you often hear the phrase, "it's not the volts that kills you, it's the amps", is to explain how you can be a "bird on the wire" in contact with extremely high voltage and still be safe. It isn't the voltage level that matters, but rather the voltage difference. Another reason is static electricity discharge. There you can have both high voltage and high current, but for an extremely brief period of time. And thus a much smaller amount of total energy.

All time and all RMS parameters of voltage and current being equal, for otherwise equivalent situations of AC and DC, a shock from AC will be more dangerous for other reasons. Such as the involuntary muscle spasms will cause you to grab onto the wires, rather than let go of the wires as it will for DC.

However, 600 Volts DC verses 240 Volts AC, isn't enough to make the DC safer than the AC. Maybe it is a little safer than working on industrial 277/480Vac, but definitely not household AC voltage.

 

[ActionDave]

I've always understood it to be the ENERGY based on the combination of both the voltage across your body, and the current thru your body, and time in contact with the shock, that makes a circuit contact dangerous. It isn't correct to only focus on the amps, because the volts across your body directly determine the amps......

That is the way I see it. "It's the amps that kill ya" ranks right up there with "A shock from 240V is better than 120V shock because it will knock you away".


When I updated my first aid cert a couple of years ago the instructor said that 60Htz was dangerous because it could send your heart into fibrillation better than a higher or lower frequency. He was an EMT not a doctor but an informed opinion FWIW.

 

[coop3339]

Where do they come up with these ideas (your friend)? It's the current, not the frequency, although if it's high enough there will be other effects. One can start with: https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

There was also discussion of ac v dc recently on one of these forums.

This article seems to suggest that frequency does play a role in fatal electrocution.

http://en.wikipedia.org/wiki/Electric_shock

 

[Dennis Alwon]

Here is another thread on the subject

http://forums.mikeholt.com/showthread.php?t=166162

 

[jaggedben]

Let's keep in mind that there are two hazards when we work with electricity : electric shock and arc flash. AC and PV have someone different characteristics for both, so that's four slightly different hazards. I'm gonna talk about PV, not DC in general, because they aren't always the same either and the question is about PV.

The arc flash from a single PV source circuit is as serious as the other three in my opinion (although the same does NOT apply to combined output circuits which could be hundreds of KW!). It could certainly burn you badly and damage equipment, but by itself it's not explosive and won't alight anything more than couple inches away. If you've ever used a stick welder it's similar; another danger is that if you stare at it, it can blind you. I've heard of solar installers lighting their cigarettes with the two bare ends of wires connected to panels, but I think that only an idiot would do that (i.e. smoke cigarettes :lol. With that said, it's still the sort of thing that could lead to your death if it surprises you and causes you to fall off a roof.

The arc flash from AC is a lot more explosive and can throw molten metal in your eyes from several feet away even if the available fault current is low and a breaker trips quickly. At higher voltages and fault currents AC arc flash can easily cause enough of an explosion to kill you.

As far as electric shock, any of it can kill you if you get unlucky. The higher voltage of PV scares me a little bit more than 240V AC, but I treat them the same.

 

[just the cowboy]

I was taught DC holds you.

I was taught DC holds you.

Such as the involuntary muscle spasms will cause you to grab onto the wires, rather than let go of the wires as it will for DC.

I was taught DC holds you and AC you can break because it does reverse and repel as well as contract, and DC will just make you contract. I saw this once when someone grabbed a large rectifier tube, they contracted their hand so hard it broke the tube (12" tube). Also I was taught that is why AC will throw you across the room because your muscles contract and expand in one cycle and causes your muscles work faster than your brain can control them.

 

[Zee]

solar is always worked on hot in the combiners.

That is what can kill you.

Why is solar always worked on hot?

 

[Carultch]

That is what can kill you.

Why is solar always worked on hot?

Because there is no disconnect that can completely de-energize solar, other than the sun going down at dusk.

You can stop the current by shutting down the inverters, using a load-break disconnect, and then by rocking out the combiner fuses. However, this only stops the current. Open-circuit voltage is still present up to the combiner fuse block inputs.

Voltage can also be present from other combiners still in the on-position, feeding the same inverter. And it can also still be present from the charged capacitors in the inverters, that take time to deenergize to safe levels.

 

[jaggedben]

Because there is no disconnect that can completely de-energize solar, other than the sun going down at dusk.

That only applies to a single source circuit, and does not require working in equipment while hot.

You can stop the current by shutting down the inverters, using a load-break disconnect, and then by rocking out the combiner fuses. However, this only stops the current. Open-circuit voltage is still present up to the combiner fuse block inputs.

Not if you put connectors outside the combiner and disconnect them while working. A lot of combiners come with these whips installed. They should be used. Not only is this safer, on grounded systems it greatly simplifies locating a ground fault in a single source circuit. And makes it faster and safer.

Voltage can also be present from other combiners still in the on-position, feeding the same inverter. And it can also still be present from the charged capacitors in the inverters, that take time to deenergize to safe levels.

This is why 690.16 requires a disconnect on the inverter side of a combiner. Still not an argument as to why anyone should have to work in a hot combiner.

 

[Zee]

s

s

Because there is no disconnect that can completely de-energize solar,......

Yes, I appreciate your response and agree...for a service call. But OP mentions ALWAYS. During install there is no reason to ever work on any part of the DC live. The CB should be all wired up, before PV panels hit the rack.

 

[Zee]

IF a CB is live and cannot be reasonably shut down, a good idea is to remove the GFDI fuse!
If you are lucky you are already working on an ungrounded system where that is unnecessary.

Then V only exists between + and -.
No V between + and ground....... or - and ground.

 

[Carultch]

IF a CB is live and cannot be reasonably shut down, a good idea is to remove the GFDI fuse!
If you are lucky you are already working on an ungrounded system where that is unnecessary.

Then V only exists between + and -.
No V between + and ground....... or - and ground.

Obviously if V exists between + & -, then there's gotta be some voltage between at least one of the polarities and ground.

Is it such that as soon as you touch ground and one of the polarities with your body, that it automatically grounds that polarity with (hopefully) no bad effect on you?

 

[winnie]

It seems to me that proper procedure could eliminate the 'need' to work solar installations 'hot'.

Since solar panels are current limited sources, can't they be safely shunted (shorted) to eliminate the voltage?

The maximum power point of a solar panel has a current that is something like 80% of the short circuit current of that panel. So conductors designed for carrying normal production current should have no problem with a shunt.

-Jon

 

[kwired]

IF a CB is live and cannot be reasonably shut down, a good idea is to remove the GFDI fuse!
If you are lucky you are already working on an ungrounded system where that is unnecessary.

Then V only exists between + and -.
No V between + and ground....... or - and ground.

I have no experience with PV equipment, but I would assume there is no ground reference to the DC output of the panels - if so there is no voltage to anything but the other pole of the source, which reduces shock hazard quite a bit.

Obviously if V exists between + & -, then there's gotta be some voltage between at least one of the polarities and ground.

Is it such that as soon as you touch ground and one of the polarities with your body, that it automatically grounds that polarity with (hopefully) no bad effect on you?

There is no voltage to "ground" on any voltage source unless some point of the system is bonded to "ground" somehow.

Any bi-polar source (simple 2 wire AC or DC source) only has voltage between two terminals of the source until you intentionally "ground" one of those terminals either directly or somewhere else in the system. Once you ground it you then have potential from the other source terminal to ground as well as the "grounded conductor". You can ground either terminal, AC systems don't really matter which terminal you ground, DC systems will just put either the + or the - terminal at ground potential. Then comes multiwire voltage systems which really are not that much different from a grounding perspective - you can still ground any conductor of the system but when applying NEC or other standards they seem to prefer you use the neutral or other conductor that has lowest potential to other system conductors in most instances which does keep voltage to ground as low as possible and helps reduce shock potential. You also can only ground one system conductor or else you have undesired current path or even a short circuit if you try to ground more then one conductor.

 

[Carultch]

It seems to me that proper procedure could eliminate the 'need' to work solar installations 'hot'.

Since solar panels are current limited sources, can't they be safely shunted (shorted) to eliminate the voltage?

The maximum power point of a solar panel has a current that is something like 80% of the short circuit current of that panel. So conductors designed for carrying normal production current should have no problem with a shunt.

-Jon

With proper procedure, you can disconnect downstream components such as combiners, recombiners, and inverters, but the cell level will remain live. No matter what you do, some part of the PV system will always be "hot" when the sun is up.

Shorting the panels to eliminate voltage is not a good idea. Shorting should only be done very briefly, and with proper procedure, for purposes of measuring the short circuit current. It is safer to let the voltage be there, know how to work with it properly, and keep the circuit open. Shorting the panels without proper procedure can cause arcing and fires.

The cabling is designed with a 1.56 safety factor on the factory-tested short circuit current under a standard "1 sun lamp", so that you can short circuit your modules for 20 years, and it still would not be detected by the fuses. In principle, you can short it. It is just that in practice, you have to do it properly and it doesn't really accomplish anything for you in terms of safety.

 

[Zee]

With proper procedure, you can disconnect downstream components such as combiners, recombiners, and inverters, but the cell level will remain live. No matter what you do, some part of the PV system will always be "hot" when the sun is up

But our point is: there is no V at "cell level", until the panels are mounted.
During installation we should
1. have panels go on LAST and
2. have GFDI fuse removed for installation safety (on grounded systems).
All wiring should be laid in, attached, managed, clipped, pulled and terminated.

No V. (if and when V is applied it exists betwen + and -)

Do you see the distinction between an installation in progress and a service call?

 

[jaggedben]

It seems to me that proper procedure could eliminate the 'need' to work solar installations 'hot'.
Since solar panels are current limited sources, can't they be safely shunted (shorted) to eliminate the voltage?

Sounds complicated and unnecessary. It's generally safer and simpler to just disconnect the equipment that needs to be worked on. If it's a small system you can turn off the whole system and disconnect individual source circuits that need to be tested or worked on at the panels connectors. On systems with multiple combiners (i.e. large commercial or utility) there should be a load-break disconnect for each combiner so you can isolate one you need to work on and then follow similar procedures as stated above. (i.e. without having to power down the entire system).

Extra connectors should be installed outside all j-boxes and combiners to facilitate such disconnection and testing. (i.e. at a relatively accessible location, not out on the roof underneath a panel way out of arms reach).

I have no experience with PV equipment, but I would assume there is no ground reference to the DC output of the panels - if so there is no voltage to anything but the other pole of the source, which reduces shock hazard quite a bit.

Most systems used to have one side referenced to ground via a fuse which is supposed to blow in case of a ground fault. That's becoming far less common. Most systems now have no ground reference and use RCD type devices to detect ground faults. I agree these are safer.

 

[jaggedben]

...
2. have GFDI fuse removed for installation safety (on grounded systems).
...

I still think whips outside the j-box is simpler, safer, and faster. Fortunately we are not installing too many grounded inverters anymore so the issue is mostly moot.