Re: 680.25 Intent
I have no fact to offer. But I do perceive a much higher danger from shock, when you are dealing with pools and hot tubs. If any equipment is incorrectly installed, or if any equipment suffers an internal failure, it may well create a shock hazard. But I think a pool (or hot tub) is an especially dangerous environment, because there will be persons immersed within the water, and because the water is a good conductor.
Think of it this way: If you are using a food mixer, and if it has a small short circuit (i.e., one that does not, by itself, trip the breaker), you have at least two chances to live through it: (1) The shock may be so slight that all you feel is a tingle, and you quickly let go, or (2) The shock knocks you down, and your hand is forced (by your falling body) to let go of the mixer. But if you are swimming or soaking, you will have nothing to let go of. Also, a small tingle may be enough to prevent you from swimming back to the edge of a pool, and you can drown, even if the current wasn't enough to harm you by itself.
When you deal with bad things that might happen and look for things to do about them, there are two separate considerations. One is the probability of the failure (or the accident), and the other is the consequences of the failure (or the accident). As an example, and AFCI breaker can never influence the probability that an arc fault may be created by a bed post rubbing against a lamp cord. But the AFCI can eliminate any adverse consequences, by recognizing the arc fault and by tripping the circuit, before it can cause a fire.
In the case of the special (and somewhat strange) pool rules, I think they are intended to reduce the probability of a failure. If the feeder is in a conduit, it becomes far less likely that a person digging up ground for a new garden area might accidentally nick the feeder. I think the exception is there to recognize that people install pools long after houses have been built. If there is an existing sub-panel (i.e., not the main), and if it is presently fed without the benefit of a conduit, and if you wish to use it to power a sub-sub-panel for the pool equipment, it would be unreasonable to require the owner to run a new feeder in conduit to the first sub-panel.
As to the insulated ground conductor, here again I can only guess. But my guess would be along the same lines. When pulling an EGC through the conduit, it is less likely that the copper will suffer damage (i.e., during the pull), if it is insulated. That is not to say that it is dangerous to use bare copper wire. I am merely saying that this reduces the probability of a failure, by however slight an amount, for essentially a nominal additional cost.
[ September 22, 2005, 04:20 PM: Message edited by: charlie b ]
