more dangerous AC or DC

Status
Not open for further replies.

Zee

Senior Member
Location
CA
I still think whips outside the j-box is simpler, safer, and faster.
It is a superior solution. I agree. I think it' s admirable you go through the trouble of installing them, as they are not required, but help down the line.
Me.....I am cheap, lazy, and fast. Installing whips and extra (MC) plugs = extra work, :roll: I will admit!
( however, now....i may well do so tho...)

It's like all the rapid shutdown and disconnect requirements of today: they will create safer, more manageable DC systems.
(They also add more cost and labor. This cost will become less and less with tech developing.)


Additionally, we may service another installer's system, in which case we may or may not have these MC connector whips handy. Removal of the GFDI fuse can be a literal, LIFE-saver in these cases.


"not installing too many grounded inverters anymore",
yes, and thankfully so! I have seen two shocks of personnel. Both were: body between positive and rail/lay in lug/metal racking. Only because the rail was bonded (thru GFDI fuse) to negative at inverter..... did it cause a shock.

I am installing lots of optimizers with SAFE DC voltage features, and micros recently. With the availability of module-level ,smart tech that shuts it all down to 1 V per panel at the CB, or the V of an individual panel or less...i cannot stomach the thought of "undisconnectable" 600vdc systems any more.:eek:
Unless i save a ton of money! ;)

Also i do avoid the opt.s and micros on steep/ inaccessible roofs. Once scaffolding disappears, i really don't look forward to a module level issue up there...so i keep it simple: mod.s and wire.:sick:
But this is a whole new issue and topic....
 

kwired

Electron manager
Location
NE Nebraska
"not installing too many grounded inverters anymore",
yes, and thankfully so! I have seen two shocks of personnel. Both were: body between positive and rail/lay in lug/metal racking. Only because the rail was bonded (thru GFDI fuse) to negative at inverter..... did it cause a shock.
as it should, assuming that as this is described this is a sort of ground fault detector and is probably similar principle of operation as a GFCI, if so then it doesn't prevent shock when a person comes in direct contact with an ungrounded conductor and a grounded object, it just determines how much current will flow outside normal current path before it interrupts the circuit. If current flows through you, you will get a shock. From what I have read they also appear to be set for equipment protection levels more so then for people protection levels, so you may not even pass enough fault current through a person to make them trip.
 

RICK NAPIER

Senior Member
Location
New Jersey
This chart gives a nice comparison. An American researcher named Charles Dalziel performed experiments with both human and animal subjects to determine the effects of electric currents on the body. A table showing his research data is presented here:
01703x01.png
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
as it should, assuming that as this is described this is a sort of ground fault detector and is probably similar principle of operation as a GFCI, if so then it doesn't prevent shock when a person comes in direct contact with an ungrounded conductor and a grounded object, it just determines how much current will flow outside normal current path before it interrupts the circuit. If current flows through you, you will get a shock. From what I have read they also appear to be set for equipment protection levels more so then for people protection levels, so you may not even pass enough fault current through a person to make them trip.

It's really a fire safety thing. Since you can't protect a PV source circuit from itself with an overcurrent device, you need to have ground-fault detection so that ground-faults will be noticed and serviced before they create an arcing short that sets something alight. At least that's the idea.

As for personnel protection, ungrounded systems don't make arrays safer if there is a fault. But they are safer to deal with when there isn't a fault. And the GFDI on them is better at detecting faults, so that makes it less likely that personnel will get shocked for not knowing what they are dealing with. (It's a bit too good, actually. I had to re-wire a system that had a fault that was passing 0.3 milliamps. :roll:)
 

Carultch

Senior Member
Location
Massachusetts
This chart gives a nice comparison. An American researcher named Charles Dalziel performed experiments with both human and animal subjects to determine the effects of electric currents on the body. A table showing his research data is presented here:
01703x01.png


Is there a difference between the danger levels in the following examples of 50 mAdc thru the human body?
1. 50 mA caused by 5000 Vdc across a "dry" human body with a resistance of 100 kOhms.
2. 50 mA caused by 50 Vdc across a "wet" human body with a resistance of 1 kOhm.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Is there a difference between the danger levels in the following examples of 50 mAdc thru the human body?
1. 50 mA caused by 5000 Vdc across a "dry" human body with a resistance of 100 kOhms.
2. 50 mA caused by 50 Vdc across a "wet" human body with a resistance of 1 kOhm.

Under the reasonable assumption that the difference in resistance is 100% the result of resistance across the dry and fairly good insulating outer layer of skin:

1. The effect of current deep within the body on muscles and nerves, particularly involving the heart, will initially be identical for the two cases.
2. The heat dissipated across the skin resistance will be much higher in the first case than in the second case, which will lead to greater localized burning to the skin and surface tissue in the first case.
3. If the current continues for a significant length of time (minutes? seconds?), the insulation resistance of the skin will decrease to close to the "wet" level or even lower. The result will eventually be 5000ma instead of just 50ma with a constant voltage source of 5000V.

Current kills, but voltage drives current.
 

Carultch

Senior Member
Location
Massachusetts
Under the reasonable assumption that the difference in resistance is 100% the result of resistance across the dry and fairly good insulating outer layer of skin:

1. The effect of current deep within the body on muscles and nerves, particularly involving the heart, will initially be identical for the two cases.
2. The heat dissipated across the skin resistance will be much higher in the first case than in the second case, which will lead to greater localized burning to the skin and surface tissue in the first case.
3. If the current continues for a significant length of time (minutes? seconds?), the insulation resistance of the skin will decrease to close to the "wet" level or even lower. The result will eventually be 5000ma instead of just 50ma with a constant voltage source of 5000V.

Current kills, but voltage drives current.

Nice response, thanks. Makes perfect sense.
 
Status
Not open for further replies.
Top