how to test for stray electrical currents in marina water at docks
- Thread starter smallfish
- Start date
- Status
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
There is no stray voltage in the water as it is at earth potential. Stray currents are introduced by items brought in/near the water.
One of the more common problems is the fact we use grounded conductors for carrying current, whether it be 120 volt circuit with a grounded neutral or 7200 volt utility distribution again with a grounded neutral conductor. When current flows through a conductor there is voltage drop, might be very little but no conductor has a perfect zero resistance. Now the fact we bond all non current carrying parts to the grounded conductor means any voltage drop on that conductor will be present when you reference a true ground point (the water).
Electrocutions involving boat hoists or other dock items even though GFCI protection was used is almost always because the grounded conductor along with the equipment grounding conductor that is bonded to it are seeing a rise in voltage over true ground. Such rise can come from premises wiring or even from the POCO's neutral on their primary distribution - they all tie together effectively making them one conductor but tied to different systems.
It is easy to test for voltage, connect a meter between metal object in suspect and the water. finding exactly where any measured voltage is coming from is a bigger challenge.
One of the more common problems is the fact we use grounded conductors for carrying current, whether it be 120 volt circuit with a grounded neutral or 7200 volt utility distribution again with a grounded neutral conductor. When current flows through a conductor there is voltage drop, might be very little but no conductor has a perfect zero resistance. Now the fact we bond all non current carrying parts to the grounded conductor means any voltage drop on that conductor will be present when you reference a true ground point (the water).
Electrocutions involving boat hoists or other dock items even though GFCI protection was used is almost always because the grounded conductor along with the equipment grounding conductor that is bonded to it are seeing a rise in voltage over true ground. Such rise can come from premises wiring or even from the POCO's neutral on their primary distribution - they all tie together effectively making them one conductor but tied to different systems.
It is easy to test for voltage, connect a meter between metal object in suspect and the water. finding exactly where any measured voltage is coming from is a bigger challenge.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
180728-2005 EDT
smallfish:
Different waters have different conductivities. Distilled water exposed to air may be around 200,000 ohm-cm. City water will be much lower. Sea water very much lower.
With a 60 year old probe that I don't know the calibration of anymore I measure 35,000 ohms on distilled water. On Ann Arbor water I read 150 ohms. My guess at the calibration constant would make these measurements 350,000 ohm-cm, and 1500 ohm-cm. I don't know Detroit water conductivity, nor the river.
Our local river and lake waters are likely somewhat in the range of 150 to 15,000 ohm-cm. Thus, moderately conductive, but nothing like sea water at possibly 30 ohm-cm.
Current flow will appear as a voltage drop between two points.
To make measurements assume current is 60 Hz. Use a digital voltmeter with at least 0.000,1 resolution. The meter needs an input series capacitor to remove any DC component. A Fluke 27 or 87 will work.
Create two probes. These could be screwdrivers. Create a 3 or 4 ft insulated handle for one.
Pick some point to be your voltage reference. Possibly a spot in earth adjacent to the water. With the other probe go around and measure anything you want including points in the water. Be careful if you really expect problems. Typically in my backyard I read 10s of to several hundred mV over a 12 ft distance. Direction of current is along a line between the probes where voltage is a maximum.
When trying to determine direction keep the probe spacing about constant.
Create a map of your area and record data on it.
Do you have a known problem? What is it and where?
.
smallfish:
Different waters have different conductivities. Distilled water exposed to air may be around 200,000 ohm-cm. City water will be much lower. Sea water very much lower.
With a 60 year old probe that I don't know the calibration of anymore I measure 35,000 ohms on distilled water. On Ann Arbor water I read 150 ohms. My guess at the calibration constant would make these measurements 350,000 ohm-cm, and 1500 ohm-cm. I don't know Detroit water conductivity, nor the river.
Our local river and lake waters are likely somewhat in the range of 150 to 15,000 ohm-cm. Thus, moderately conductive, but nothing like sea water at possibly 30 ohm-cm.
Current flow will appear as a voltage drop between two points.
To make measurements assume current is 60 Hz. Use a digital voltmeter with at least 0.000,1 resolution. The meter needs an input series capacitor to remove any DC component. A Fluke 27 or 87 will work.
Create two probes. These could be screwdrivers. Create a 3 or 4 ft insulated handle for one.
Pick some point to be your voltage reference. Possibly a spot in earth adjacent to the water. With the other probe go around and measure anything you want including points in the water. Be careful if you really expect problems. Typically in my backyard I read 10s of to several hundred mV over a 12 ft distance. Direction of current is along a line between the probes where voltage is a maximum.
When trying to determine direction keep the probe spacing about constant.
Create a map of your area and record data on it.
Do you have a known problem? What is it and where?
.
- Status