Thank you for your question regarding stray voltages on the ground wire of a landscape lighting system. It is our pleasure to help.
Many outdoor lighting systems now use 24-volt DC (Direct Current), called Low-Voltage Landscape Lights. These systems typically have a control box mounted somewhere on an exterior wall, which will house the lighting control systems and a 120V AC to 24V DC transformer. The reason these low-voltage systems are popular today is twofold: First the low-voltage systems use high-intensity white LED (Light Emitting Diode) bulbs that save a lot money in electrical costs. Secondly, low-voltage systems are safer for pets and children to play around. If your landscape lighting system is a 24-volt DC system and you are measuring 23-volts DC on the ground wire, this could indicate a crossed line somewhere in the system, and may indicate a short, but probably not. Remember, DC systems like this are 2-wire systems, one wire being the hot and the other being the return. Often they label the return line as the “Ground” wire, but it should be more accurately thought of as a “Neutral” wire, because it is current carrying. As such, there technically is no ground, which is often called “a floating ground”.
In low-voltage lighting systems less than 30 volts and less than 25 amps (like many typical landscape lighting systems), the DC source consists of an ‘isolating’ power supply. Because of this ‘isolating’ transformer, the voltages are effectively floating relative to the line voltage and ground. This means that you may read 0VDC (zero) on the “hot” wire and -24VDC (negative 24 volts) on the return, or you could read +24VDC on the hot wire, and 0VDC on the return. Either reading would be accurate.
To quote the National Electrical Code: “NEC 411.5 (A) Grounding. Secondary circuits shall not be grounded.”
For a solidly grounded 120-volt AC (Alternating Current) system with a 3-wire configuration (hot, neutral and ground), a ground wire with 23-volts AC indicates a serious problem. Typically it is only one or two things. Either there is an unintentional bond between the neutral and ground wire somewhere in the system, or there is an electrical fault occurring. In either case, the required GFCI (Ground Fault Circuit Interrupter) should be tripping and killing the circuit the second you apply power to the system. But assuming you have temporarily bypassed the GFCI for troubleshooting purposes, let us make sure we understand how you are measuring this stray ground voltage.
If you are using a standard voltage (RMS) meter, you will have two (2) probes, usually one red and one black; it doesn’t matter which color is used in this case. The voltage readings are taken by placing the probes across one of the following places in the electrical circuit:
a. Hot wire to Neutral wire: this should read 120-volts AC
b. Neutral wire to Ground wire: this should read less than 1-volt AC
c. Hot wire to Chassis: This should read 120-volts AC
d. Neutral wire to Chassis: this should read less than 1-volt AC
e. Ground wire to Chassis: The green ground wire should be physically bonded to the chassis and as such you should read 0-volts
Now it sounds from your email that you may have added the following scenario’s to your voltage measurement in that you have literally thrust one of the leads from the volt-meter into the soil of the native earth near the landscape box:
f. Hot wire to soil
g. Neutral wire to soil
h. Ground wire to soil: is this your scenario?
If either f, g, or h, above are your situation, then what you have done is actually turned one of the probes on your volt-meter into a very short grounding electrode and you are relying on the earth as the return path for the current. You would need to understand the resistance-to-ground of the probe-to-earth contact, and the soil resistivity of earth back to the main electrical service. For any of the 3 above (f, g, h) this is NOT the proper way to measure voltage in this or any circuit.
That said, if “scenario h” is measuring 23-volts, than we would expect that scenario b & d would have voltages higher than 1-volt as well. Is this your case?
If “scenarios a, b, c, d, and e” are all within the parameters listed above, and you are still reading 23-volts in “scenario h”, then it is likely you are detecting stray currents propagating naturally across the surface of the earth, which are coming up from the soil through your meter and into your landscape lighting system. We call this a Ground Potential Rise or GPR. These stray earth voltages (GPR) could be coming from a variety of nearby sources. Would there happen to be a high-voltage tower or a utility substation nearby? This could explain the voltages (GPR) you are reading.
We hope you find this information useful and we would really like to hear back from you in regards to this issue, as we find it quite interesting. Please feel free to give one of our engineers a call and we will be glad to discuss your issue with you free of charge.
The Engineering Team at E&S Grounding Soltuions