shock in the shower
Stumped me. I had a customer complain about receiving tingling shock while in the shower in a pier and beam house.
Could not duplicate shock or measure current. I guessed shower head to tub drain.
Grounded another rod to cold water,hot water, gas line,freon lines and neutral. All were already buried in dirt under house and in yard.
Stll tingle shock.
Finally saw SE cable with a broken neutral conductor at pole.
Qyestion, with all this grounding, how can I measure this dangerious condition and how do verify piping is a 0 volt potential.
The problem with a floating neutral at the SE is the neutral voltage depends on the loads that are energized. Picture this - nothing on in the house except two 100W light bulbs, one on either hot conductor. The two loads exactly balance each other out and the neutral conductor will be (theoretically) at zero volts. But turn on something else and unbalance the neutral current and the neutral voltage will rise above zero. As things get turned on/off the voltage could be there one second and gone the next. Makes measuring problematic.
Then consider that the waste piping may be non-metallic or not bonded and that's where the voltage difference comes in (if the waste were bonded same as cold/hot water, then there could be no differential voltage between drain and shower head to cause a shock).
Keep in mind that earth ground is not a perfect conductor, could be several 10's of ohms, and will allow an unbalanced load to raise the voltage up significantly above zero (with a broken neutral at the Service Entrance), so you really can't verify your zero volt potential reliably under all conditions, the neutral needs to be intact. I have certainly seen bad neutrals before but this is the first case like yours I have heard of. Interesting! (and scary...)
Time to call out the utility post haste to fix the SE drop!
Piping is supposed to be bonded so it is at the same potential as ground. the ground source is the neutral back to the pole or pad transformer. If you EVER have amperage on a water line you need to look into the cause and that will usually be a failure in the neutral
connection at the weather head or at the pole.
How to measure? Turn on a buch of 120 volt appliances on the same side of the buss. You want to increase the amps on one side of the buss and do not give the other buss amps to balance out. A portable electric heater is good.
Then check for amps on your ground electrode conductor that goes to the water line.
In a perfect world the ground electrode conductor is tied to the same spot as the neutral in the panel, but the route to the transformer (pole or pad) is the least resistance. The water line is a safety for ground fault and lightning strike.
I have seen the same problem several times.
It all started when I was under the house and got zapped squeezing by gas and sewer pipe in the crawl space. Since the cast iron sewer was laying on the dirt, I assummed the gas pipe was hot. Maybe it was the other way around.
I understand the no neutral circuit. I am suprised that ground wasn't made from cold water to sewer, but now that I think. garbage disposal, ac coil drain, clothes washer are all isolated by some PVC. Bath tub has no connection to tub spout.
Yup, that would be a good test.
Originally Posted by wolfdog
What a great question!
Originally Posted by timjimbob
Ground rods, electrodes, and other piping have nothing to do with what that person was experiencing.
We use what is called a grounded neutral system. The ground rods are used to drain off any lightning hits to the ground.
You correctly noticed the problem: an open neutral. This condition allowed for a difference of potential between the house and the transformer secondary center tap. That's where the neutral finally ends up. When the neutral opened at the pole, the ground rods formed a high impedance path back to the utility transformer, instead of the low impedance path that would be provided by the neutral. With the neutral connected to the transformer and then bonded to the grounding of the building you would not have any difference of potential and no tingling or shock feeling.
This also means the single phase voltages would have been lower because of the higher impedance between the ungrounded conductors and the center tap of the utility transformer which defines the 120 volt values. The 240 volt circuits are fed from the entire length of the transformer secondary winding.
The neutral does a very good job of completing the circuit back to the transformer, and the ground rods and the earth do a terrible job of completing that circuit.
The bonding is there to keep everything at the potential of the neutral, not the potential of the grounding electrodes. When that is as intended, the equipment grounding conductors can assist the overcurrent devices in opening.
I see the point.
I was suprised, though, that as thouroughly as everything was bonded together there was a potential difference between a shower head and bath tub with cast iron plumbing. I guess it was the tub that was at the higher voltage potential and shower head was at ground potential.
Not necessarily higher or lower, just different, because the entire house was trying to send neutral current back to the transformer on the pole, which ALSO has a ground rod at or near the transformer. The earth provides about 60% of the neutral current between the grid and the generating station, but you don't want it between a building and the step-down transformer, that's for sure.
Originally Posted by timjimbob
Had an open neutral one time and the only high impedence path was a metal service pipe to the tin on the wall, it was glowing red it was so hot!
With an open SE neutral, the unbalanced current will travel to the grounding system, which is bonded to any metal water pipes as well as supplemental rods. The voltage will be equal to ground(assuming it is properly grounded), but can still give you a little tingle depending on the voltage difference between the house ground and the ground at the transformer
Originally Posted by bkmichael65
...which can be pretty significant.