Came across one the other day that has me scratching my head a bit. customer has a spa on back porch, inside screen enclosure. customer they get a tingle when standing barefoot on concrete floor and touch water. went to troubleshoot and sure enough while kneeling on floor and touch water i got a tingle i would equate to licking a 9 volt battery. weir part is the spa is gfi protected and bonded, so i was thinking backfeed off another circuit. but even with the main to the house turned off it was still there. is it feasible it could be induction from the overhead transfomer lines im guess are 25-30 feet away diagonally?
induced voltage
- Thread starter deec
- Start date
- Status
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
A tingle indicates a voltage difference. Either the water or the concrete is being energized, relative to earth (true ground).
With the main disco open, the source must be external. I would suspect a utility neutral issue before I would induction.
With the main disco open, the source must be external. I would suspect a utility neutral issue before I would induction.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The situation that you describe is caused by current flowing on grounded and grounding conductors. There are many possible sources for this current, and we've had several threads on this or related topics recently.
The most likely source of this current is elevated neutral voltage on the utility side of things, caused by loading of their primary circuits. This sort of current could also be caused by a failed service neutral elevating neutral voltage, or a failed service neutral at a neighbor. Failure of the concentric neutral on an underground cable is yet another potential source of these currents. Since the neutral and grounding electrodes form conductive 'loops' with the soil, then induction from nearby HV lines are also a possibility.
The issue is that one bit of supposedly grounded metal (in this case the EGC connected to the hot tub) being at different voltage than another grounded conductor (wet concrete sitting on soil), with the customer coming between the two.
The only _fix_ is to get rid of the source of the current, however sometimes this is not possible.
The practical solution is additional bonding, which forces all conductive materials that the customer may come in contact with to the _same_ elevated voltage. If everything is at the same voltage, there is no chance of a shock (one of the Mike Holt videos has Mike himself swimming in a pool which was intentionally energized to 120V). The downside of such bonding is that it moves the 'shock zone' to the edge of the bonded area. If you have a region of particular concern (say the region immediately surrounding a hot tub or swimming pool), this sort of bonding is generally _required_.
The slab around the hot tub should be electrically connected to the hot tub system, so that everything is at the same voltage, no matter the source of the voltage. Note that if you do this then at the edge of the slab someone with bare feet might feel a tingle stepping from slab to grass. Hopefully the bonding would have reduced the 'step potential' to below the threshold of being sensed.
It is worth while searching for the source of this current, to see if it is a problem that can be fixed. At the same time the equipotential bond around the hot tub should be created or repaired.
-Jon
The most likely source of this current is elevated neutral voltage on the utility side of things, caused by loading of their primary circuits. This sort of current could also be caused by a failed service neutral elevating neutral voltage, or a failed service neutral at a neighbor. Failure of the concentric neutral on an underground cable is yet another potential source of these currents. Since the neutral and grounding electrodes form conductive 'loops' with the soil, then induction from nearby HV lines are also a possibility.
The issue is that one bit of supposedly grounded metal (in this case the EGC connected to the hot tub) being at different voltage than another grounded conductor (wet concrete sitting on soil), with the customer coming between the two.
The only _fix_ is to get rid of the source of the current, however sometimes this is not possible.
The practical solution is additional bonding, which forces all conductive materials that the customer may come in contact with to the _same_ elevated voltage. If everything is at the same voltage, there is no chance of a shock (one of the Mike Holt videos has Mike himself swimming in a pool which was intentionally energized to 120V). The downside of such bonding is that it moves the 'shock zone' to the edge of the bonded area. If you have a region of particular concern (say the region immediately surrounding a hot tub or swimming pool), this sort of bonding is generally _required_.
The slab around the hot tub should be electrically connected to the hot tub system, so that everything is at the same voltage, no matter the source of the voltage. Note that if you do this then at the edge of the slab someone with bare feet might feel a tingle stepping from slab to grass. Hopefully the bonding would have reduced the 'step potential' to below the threshold of being sensed.
It is worth while searching for the source of this current, to see if it is a problem that can be fixed. At the same time the equipotential bond around the hot tub should be created or repaired.
-Jon
- Status