If you’re concerned about the persistently high voltage on the ground wire, you’ll want to know what’s causing it.
The ground wire is a safety measure. It should not normally have a voltage when referenced with the live or neutral wires. If it does, a problem can cause an electrical fault or malfunction. So you will need to identify the cause to deal with it.
There are various possible causes, but if you notice any component or important part of the circuit is loose, corroded, rusted, damp, wet, or covered with dust, it may be the cause. Technically, the voltage on the ground wire is caused, for instance, by stray voltage, a corroded connection, a short circuit, a voltage drop, tied wires, or shared neutrals.
I will explain these causes in detail and tell you how you can check for voltage on the ground wire, especially the N-G reading.
Possible Causes of Voltage on the Ground Wire
There are several possible causes of voltage on the ground wire.
I’ve listed some of the most common ones below. I’ve divided them into wire-, conduit-, ground-rod-, electrical-box-related, and other causes.
- Loose wires
- Worn-out insulation, exposing the wiring
- Shared neutral
- Voltage drop (wiring is too long)
- Neutral and ground wires are tied together
- Corroded conduit
- Broken conduit
- Loose ground rod or ground rod connection
- Corroded ground rod or ground rod connection
Electrical box-related causes
- Damp areas in the electrical box
- Accumulation of dust around electrical equipment
- Loose or missing covers on electrical boxes (making the wiring exposed)
- Missing ground hole in power outlets
- Insulated wires wrapped around metal (electrically conductive) pipes
- Unbalanced electrical load
- Stray voltage
As you can see, if any component or important part of the circuit is loose, corroded, rusted, damp, wet, or covered with dust, it can cause a voltage on the ground wire.
Ground Wire Voltage
I will now explain some of the causes of voltage on the ground (or earth) wire in more detail.
In an AC circuit, the live/hot and neutral wires provide a path for electrical current to flow. Current is supplied to a load/appliance via the live wire and returns to the main panel through the neutral wire.
In case of an electrical fault or malfunction, the third earth, or ground, wire is only for having an alternative route for the electricity as a safety measure. It normally has a voltage of 0 volts as a reference point for the live and neutral wires. (Allen, 2000)
The voltage across neutral and ground is an important reading to check for this. This is called the N-G (or N-E) voltage and should be close to or equal to zero. If it isn’t, i.e., if you get a high N-G reading, it could be due to one or more of the following reasons:
Cause 1: Corroded Connection
I mentioned above that corrosion in either the wire, conduit, ground rod, outlet, or electrical box, is a common reason for voltage on the ground.
Therefore, you should always look for signs of corrosion if you have this problem.
Cause 2: Stray Voltage
Stray voltage occurs when electrical charge leaks from the live/hot wire.
This current leakage, multiplied by the ground wire resistance, causes high voltage potentials. If the stray voltage is substantially high, you must locate the source of the leakage. (Allen, 2000)
Cause 3: Short Circuit
The situation of stray voltage becomes even more serious if a short circuit occurs, if two wires touch, or if someone or something touches the live wire and the ground line.
Cause 4: Voltage Drop
A voltage drop can also cause the voltage on the ground.
The voltage level drops if the wire is too long. The voltage level will be reduced if the electrical cable between the appliance or power outlet and the circuit breaker panel is too long. When this happens, there is a greater chance of voltage on the ground.
The voltage drop across the ground wire should not normally exceed 30 mV (millivolts). If it does, you may need to repair the ground wire.
Cause 5: Tied Wires
If the neutral and ground wires are closely tied together (as some people wrongly do), it can lead to electricity energizing the ground wire.
Cause 6: Shared Neutral
Shared neutral wires are not uncommon in a commercial setting with a 3-phase electrical supply. It reduces the amount of wiring but is at the risk of voltage on the ground when the system is unbalanced and a high volume of current flows through a neutral wire.
Checking for Voltage on the Ground Wire
To know whether there is the voltage on the ground wire, we would have to test it.
I am using a voltmeter or multimeter to check for voltage on the ground wire.
Set the AC voltage range to within 200 volts (or more if it’s a 240-volt supply). If testing the wires, place one probe on the neutral wire and the other on the ground wire. If testing an outlet, put the probes into their respective slots.
Check the reading. If there is voltage (a non-zero reading), the voltage is on the ground wire.
Here is another method you can use. Follow these steps in order:
Step 1: Set the voltmeter or multimeter to AC with the cutoff set at the highest possible value. Make sure the leads are in good condition.
Step 2: On the voltmeter or multimeter, connect the red lead to the positive (+) port and the black lead to the port marked ‘COM.’
Step 3: Insert the ends of these leads into the hot/live and neutral slots (red in the hot/live slot and black in the neutral slot). Note the reading on the voltmeter or multimeter. In the screenshot below, the voltage is close to the supply voltage of 240 volts.
Step 4: Now connect the red probe to the earth/ground slot/connector and take another reading.
After doing the above, the final voltage reading should be no more than ~5 volts (but not zero, as otherwise, there is no grounding/earthing).
Step 5: Now connect the black probe to the neutral and the red probe to the ground slot.
If you live in the USA, you should get these results to confirm that the ground has no voltage (permitting a 5% deviation):
- Live/Hot with Neutral = ~ 120 volts
- Live/Hot with Ground/Earth = ~120 volts
- Neutral with Ground/Earth = ~0 volts
High Voltage Level
If you find a small voltage on the ground wire, around 2 volts or less, you don’t need to worry about it. Only if it’s more than that, especially if it’s very high or close to the supply voltage, should you be worried.
A high voltage level is a danger to people and can cause problems in all connected equipment. Stray voltage can also harm animals if they accidentally touch a grounded object, such as a metal fence. A short circuit may occur if the wires touch, which can be a fire hazard.
A good ground is especially important for reference voltage sensors. The bad ground would cause less voltage and lead to issues with equipment that relies on accurate voltage levels. (Erjavec & Thompson, 2014)
(1) Neal Allen. Network maintenance and troubleshooting guide: Networking technology series. Cisco Press. 2000.
(2) Jack Erjavec and Rob Thompson. Automotive technology: A systems approach, p. 715. Cengage Learning. 2014.
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