scsinet
Active Member
Originally Posted by 1boatnut
http:///forum/post/3261957
GFI's are not effective in certain circumstances.
They work by measuring the difference between the electrical current going INTO an appliance and the current going OUT of the appliance. This assumes that the person being shocked is grounded. If you were ungrounded and touched the hot and neutral wires at the same time, there would be no drop in current detected, so the GFI would not activate. Then, you would be at the mercy of the fuses or circuit breakers, which may or may not stop the current before its too late!
You're absolutely correct.
However, bear in mind that the most serious of electrical shock scenarios is a ground fault scenario. A hot to neutral fault is statistically less serious. The reason for this is that in the majority of ground fault scenarios, the grounded surface being contacted is physically separated from the source of the current, generally meaning another part of the body, such as the other arm or the feet. This causes the current to flow through the torso, where it can interfere with heart function. A hot to neutral scenario is usually more contained. A 120v shock traveling from one part of your hand to another is very unlikely to cause any permanent harm... an electrical burn, while potentially serious, is usually the worst you can expect and is not life threatening.
Your point reiterates why a grounding probe can enhance the safety of the setup. By grounding the water column, you virtually ensure that you are going to trip a GFI.
Consider the following scenarios:
1. A heater breaks in the tank, with ungrounded water and a GFI. More than likely, the GFI won't trip, because as you so aptly stated, with no fault to ground, it won't sense anything amiss. However, that heater's hot and neutral internals are exposed to water. Assuming that insufficient current flows between these two parts to trip the circuit breaker (unlikely), you likely won't get much more than a tingle in the water because the current is going to follow the shortest path through the water (straight line) from point a to point B. If you somehow stuck your hand into the space in the water between the exposed hot and neutral, you'd certainly get popped pretty good.
2. A heater breaks in exactly the same scenario with a GFI and a grounded water column. GFI pops more or less instantly.
3. Two pieces of equipement fail in opposite sides of the tank, one exposing a hot lead and one a neutral to the water. Current will flow through the length of the tank between them, more than likely killing all livestock and hurting like hell when you stick your hand in.
4. Same circumstance as #3 with a GFI and a grounded water column. GFI pops more or less instantly.
Anyhoo, my point is that with a GFI and a no probe, you still get a greatly enhanced degree of life safety. With a GFI AND a probe, you get even more, but probably more along the lines of property safety, not life safety.
No protection device can protect against ALL failure situations, but that fact does not justify neglecting to use them.