why do they pick 300V?

G

gene6

Senior Member
Location
NY
Occupation
Electrician
One of the large companies we have a service work contract with has a new updated safety manual that they just had to train me on that basically results in no hot work above 300V unless someone from corporate puts a request in ink, which never happens so no hot work on 480..
Now if I am going to open a 480V panel cover in their facility they are going to schedule a shutdown of that panel, ok fine were all more safe and they have some pretty old panels from the 1960's.
My question to you safety experts is why pick 300V and why is it just based on voltage?
They also have a even older 3-phase 240 system thats like 1200 amp buss duct and I can work on that hot all day long with panel covers off?
 
277 volt +/- 8%?

A lot of industrial lighting to 277v, but voltage is not steady, it's a range. If they picked 280v and then someone went out and measured something that was supposed to be 277 and they got 281, well then it's not "safe". 300v gives some wiggle room?

I'm totally making this up, how's it sound?
 
Odd voltage...........
For us (UK and EU) it is all 230V single phase and 400V three phase.
 
Here is what it says:
ENERGIZED WORK AND THE ENERGIZED WORK PERMIT
• An Energized Electrical Work Permit is required anytime work is performed within or on a energized system over 300V Line to Line or Line to Ground AC or DC regardless of phase or frequency.
• An Energized Electrical Work Permit is not required under the following conditions: testing, troubleshooting and voltage checking at equipment as per the equipment maintenance policy on page 82.
• Measuring voltage, thermography, ultrasound or visual inspections of a system over 300V Line to Line or Line to ground require establishment of a restricted approach boundary by a member of the safety team if one has not been provided.
• An Energized Electrical Work Permit is not required for general housekeeping and non-electrical tasks provided that the established restricted approach boundary is not crossed.
 
Joe.B said:
Yeah, we're the odd ones....

;)
Click to expand...
Yes, I suppose that's true. Some some of you know that I have travelled quite a bit and have experienced different electrical systems. Japan has various various supply voltage systems and both 50Hz and 60HJz.
 
Besoeker3 said:
Yes, I suppose that's true. Some some of you know that I have travelled quite a bit and have experienced different electrical systems. Japan has various various supply voltage systems and both 50Hz and 60HJz.
Click to expand...
Next thing I suppose is we should all be using the metric system too eh?
 
Joe.B said:
Next thing I suppose is we should all be using the metric system too eh?
Click to expand...
A little bit of my ancient history.......
At at primary and secondary school we used old fashioned units - feet and inches. It was not until I started university before we used metric or SI units. System Internarial
 
gene6 said:
Here is what it says:
ENERGIZED WORK AND THE ENERGIZED WORK PERMIT
• An Energized Electrical Work Permit is required anytime work is performed within or on a energized system over 300V Line to Line or Line to Ground AC or DC regardless of phase or frequency.
• An Energized Electrical Work Permit is not required under the following conditions: testing, troubleshooting and voltage checking at equipment as per the equipment maintenance policy on page 82.
• Measuring voltage, thermography, ultrasound or visual inspections of a system over 300V Line to Line or Line to ground require establishment of a restricted approach boundary by a member of the safety team if one has not been provided.
• An Energized Electrical Work Permit is not required for general housekeeping and non-electrical tasks provided that the established restricted approach boundary is not crossed.
Click to expand...
Line to line ?! Line to ground??
So... kinda sorta technicaly it could say
"Nothing above 260v....
A very dumb engineer or a very smart lawyer- or vice versa could argue and still sue...
 
gene6 said:
Here is what it says:
ENERGIZED WORK AND THE ENERGIZED WORK PERMIT
• An Energized Electrical Work Permit is required anytime work is performed within or on a energized system over 300V Line to Line or Line to Ground AC or DC regardless of phase or frequency.
...
Click to expand...
So if I'm an electrician and I know a junction box only contains one hot in a 277V line-ground circuit, it sure seems like I shouldn't need a permit. But if I'm a smart lawyer after someone got hurt I can argue that the wiring *system* is still 480V line-line.
 
50-300V shows up in a few places in NFPA 70E, I do not know why the 300V was chosen, I know the 50V limit goes back to the Bell Telephone days and many telcom systems to this day are 48V and thus under the 50V.
1737187483611.png
 
Interesting info, I ran into a friend who is a retired lineman and he said the 300 volts probably comes from the NESC going way back, going back as far as he knew '300 volts or less' was used in the NESC in several tables for things like clearances of overhead lines and also clearances underground like gas pipes.
 
zbang said:
We already are- volts, watts, seconds, liters (of soda), grams (many things), etc etc. About the only thing left of the so-called Imperial measurements are length and capacIity, but we've already thrown over a few, like furlongs, pecks, and stones.
Click to expand...
Well, I am an electrical engineer and I am British so we use SI (metric) units - mostly. But there are a few oddities. Our cars are right hand-drive. Go to a pub for a beer and you will get a pint.
 
There's nothing in NFPA 70E that says an energized work permit is not required below 300 V. That's a purely site-specific dividing line. The owner is free to develop their own safety rules, although they may regret it later.
 
D. Castor said:
There's nothing in NFPA 70E that says an energized work permit is not required below 300 V. That's a purely site-specific dividing line.
Click to expand...
Regardless of that one hot work policy or whatever it is, it is used in many other places as a dividing line like in article 230 for service drop clearances, and possibly article 225.
There is a also 300V insulation class for wires etc.
The question is why and when did 300V become a dividing line in all these various standards?
 
300V covers the maximum that a 277V circuit can deliver plus the +5% that is allowable in distribution systems (so 291V). For the purposes of 70E, it has to do with the amount of potential incident energy that can be in a fault on that system. It’s the same reason why the AIC values in circuit breakers can be higher at 240V than they are at 480V: lower voltage means lower energy in the incident, less potential damage.

A line needed to be drawn somewhere…
 
You must log in or register to reply here.
Top