Neutrals, connecting all branch circuits neutrals together

[iwire]

So while distance is important, it's only half the answer. The other half is the amount of energy at the origin.

Well yeah....kind of self evident don't you think?

 

[tallgirl]

Well yeah....kind of self evident don't you think?

I dunno. You wrote "only distance is what protects us". Perhaps you meant "only distance and several other thing I've not mentioned is what protects us"?

 

[George Stolz]

Distance may not protect you.

Have you encountered a phenomenon that failed to dissipate by the square of the distance from the origin?

The statement "Distance may not protect you" is pretty vague and misleading, IMO. That sounds like you're saying "unsafe at any distance", which is false.

This definutely looks like a "green grass/blue sky arguement" going on here.

Electromagnetic fields -- and "arc flash" is an electromagnetic field -- obey inverse square law behavior.

I don't think an arc flash is an electromagnetic field. I believe it is an arc between two points of very high temperature. I don't hear about people's belt buckles getting sucked into an arc flash, but I do hear about people getting burned from them.

 

[Bea]

Not sure about question but if neutral conductors are part of a multi branch circuit then they must connected together NEC 300.14. I would agree with Trevor if the ciruits are individual circuits this would create a parallel path and be a violation NEC 310.4.

 

[tallgirl]

Have you encountered a phenomenon that failed to dissipate by the square of the distance from the origin?

Depends on the phenomenon

The statement "Distance may not protect you" is pretty vague and misleading, IMO. That sounds like you're saying "unsafe at any distance", which is false.

This definutely looks like a "green grass/blue sky arguement" going on here.

Oh, probably so. I have a lot of respect for Bob, but sometimes he's a bit terse (and I'm a bit verbose ...) and I'm not sure what all he's saying.

Anyway, if you know the energy at point A, the only way to know the amount of energy at point B is to know how far A is from the origin, and the strength of the field there.

For example, if the amount of energy at A is 40 calories per square centimeter (class 4 PPE), you can't know if you have to move 10 feet or ten miles (or 100 miles ...) to reduce that to 20 calories per square centimeter.

On top of that, shielding can help. So, distance isn't the only thing going.

I don't think an arc flash is an electromagnetic field. I believe it is an arc between two points of very high temperature. I don't hear about people's belt buckles getting sucked into an arc flash, but I do hear about people getting burned from them.

Right, but light is still electromagnetic in nature. It's just a different kind from all the other kinds.

Electromagnetism

Light is an electron's way of saying "Here I am!" The voltage potential between the two arcing points ionizes the air between them, stripping electrons off the gas molecules in the process, and those electrons tell us about it by emitting photons, which we see as "light".

 

[winnie]

Several points:

Arc-flash describes the tremendous amount of energy released during a short circuit event through the air. This energy is going to be released in the form of direct thermal energy, mechanical concussive force, sound, and radiant energy across the spectrum, from radio frequency through ultra-violet, and even x-rays if the voltage is high enough. Currents can be high enough that the magnetic effects will place significant mechanical stresses on structures.

I don't believe that any PPE would help if you were actually _in_ the arc. Outside of the arc, PPE can help and is required in situations where there is an arc flash hazard. So I believe that it is strictly the case that in addition to PPE, some minimal distance is required for protection from an arc-flash.

The radiant energy is electromagnetic radiation, radio, IR, light, UV, even some X-rays if the voltage is high enough. While it is true that the radiant energy follows the inverse-square law, there is an important feature of the inverse square law to consider: the inverse square law only strictly applies to _point sources_, and is approximately true when you are far away from real sources. But when you are close to the source (where your distance from the source is of the same magnitude or smaller than the physical extent of the source), then intensity falls of more slowly than inverse square. If you had a 'wall of light', then near the wall the light flux density would remain roughly constant as the distance changes, and would start looking inverse square as the distance become much greater than the dimensions of the wall.

I don't know the linear dimensions of arc-flash events that calculations are required for, so I don't know just how applicable the inverse-square law is in determining the cal/cm2 values that the PPE must protect from. Certainly at large distance, the inverse-square law will hold; at enough of a distance an arc-flash is probably pretty darn cool to watch, as long as no-one was hurt in the making of it.

-Jon

 

[infinity]

Not sure about question but if neutral conductors are part of a multi branch circuit then they must connected together NEC 300.14. I would agree with Trevor if the ciruits are individual circuits this would create a parallel path and be a violation NEC 310.4.

Are you saying that two sets of MWBC's in the same EMT homerun would have their neutrals connected at a downstream location? Wouldn't that put them in parallel?