How to measure ground resistance with a multimeter
Content
Electric shocks often occur in our homes and workplaces.
Fortunately, we stay alive more often than not. Why is this? Well, grounding in electrical systems plays an important role.
To make sure your electrical system is always safe, you need to run ground resistance tests to make sure it's in optimal condition.
However, not everyone knows how to do this, and we will help you.
This article provides information on how to measure ground resistance, including what grounding really is and how multiple diagnoses can be made with a multimeter.
Let's get started.
What is ground resistance?
Equipment ground resistance is the level of resistance or obstruction that the voltage in the equipment encounters when it goes to ground.
It simply measures how easily voltage travels from the device's components to ground.
This is an important indicator for measuring the level of protection that electrical equipment provides to people.
Why is ground resistance important?
Protective grounding intentionally creates a connection between the components of the device and ground through the ground block.
This grounding unit is called a loop and is made of electrodes such as steel or more preferably copper.
Components connected to a grounding device are not normally energized when the device is energized.
For example, it can be equipment with a metal case. This metal case is usually connected to a grounding device.
In the event of an insulation breakdown, grounding helps direct the voltage from that metal body to ground, where it is dissipated.
Where there is no ground, voltage builds up in the metal case. If someone touches the metal case, all voltage will be grounded through it, which can be fatal for that person.
In addition, even where there is grounding, problems can arise. Electricity usually travels through the channel with the least resistance.
Fortunately, humans have an average resistance of 1000 when wet and 10,000 when dry, which is a lot.
However, if your ground unit's resistance is not much lower than this value, then it is useless.
Grounding helps, and that is why you need to repeatedly measure the ground resistance of the device and always ensure it is optimal.
The multimeter proves to be a handy tool for diagnosing multiple electrical equipment.
How to measure ground resistance with a multimeter
To measure ground resistance, you'll need a long wire, a digital multimeter, and ground metal. Disconnect your equipment from any power source, set your multimeter to ohms, and connect your device's ground component to the ground rod using a long piece of wire. Place the probes on the wire and ground block of your device to take a reading..
This process entails a lot more, and we'll dive into the details.
There are three methods for testing equipment earth resistance.
In this post, we will talk about the two-line method, since the multimeter is only effective here.
The two line method entails measuring the grounding device in your equipment against ground metal.
This metal is a rod that is in the ground and fastened with a clamp. You would expect the resistance of this to be extremely low.
Step 1: Get a piece of wire
A piece of wire is used for testing, especially if the ground rod is far from the equipment grounding device.
This can be seen in the case of objects that are difficult to move, such as washing machines.
You also strip the insulation from both ends of the wire to ensure perfect contact between the multimeter probes for accurate diagnosis.
If you can move the equipment to a ground rod, then no wire is needed.
Step 2: Disconnect the equipment from the power source
Electrical current must not flow through the circuit of the resistance measuring equipment.
You want to take precautions when running tests, so make sure there are no active connections.
Step 3: Set the scale of the multimeter
To test ground, you set the multimeter to ohms. Resistance is measured in ohms and is indicated by the omega (ohm) symbol on the multimeter.
You also need to make sure the multimeter is set to below 100 ohms if it has multiple options.
Step 4: Place the multimeter leads
This is where you test and take measurements.
Place the multimeter's black test lead on the ground block of your equipment, and the red test lead on the bare end of the wire.
If you didn't use a piece of wire, attach the red test lead directly to the ground rod.
Step 5: Evaluate Your Measurements
Once all this is done, your multimeter is expected to give a reading. Generally, if the resistance is good, you should expect readings below 5.0 ohms.
A resistance value of 5.0 ohms is the recommended resistance value from the US National Electrical Code and can be relied upon for safety worldwide.
The resistance you get varies from equipment to equipment and some are even below 1 ohm.
The ohm reading you get with the two-line method is the sum of the resistance of the grounding device and the resistance of the ground rod.
That's it, but there's more to your home. You can also check if the outlet is properly grounded by following these steps;
- Set the multimeter to AC voltage (200 VAC).
- Verify that power is running through the circuit by checking the hot and neutral outlets for approximately 120V or 240V. Once confirmed, proceed to the next step.
- Place the black test lead into a grounded outlet and the red test lead into a hot outlet.
- If the multimeter reads the same range of 120V or 240V, the outlet is properly grounded. If it doesn't, there's a problem.
- Place the black test lead into a grounded outlet and the red test lead into a neutral outlet. With proper grounding, no problem, you would expect to see a zero value.
This video shows exactly how to check the grounding in outlets.
Conclusion
Measuring ground resistance with a multimeter is not the most difficult thing. You simply get readings in ohms by connecting a ground rod to a grounding device in your equipment, or by checking outlets for correct voltage readings.
Obviously this doesn't make it the most reliable method, but it's easiest to do with a multimeter.
F.A.Q.
How many ohms is a good ground?
Any resistance value below 5.0 ohms is a good ground. This value is taken from a US National Electrical Code provision that can be relied upon for safety worldwide.
What are the three most common methods for determining ground resistance?
There are three methods for determining ground resistance: two-line, three-line, and four-line. The four-line method is the most accurate of all.
What is the maximum allowable earth resistance?
The maximum allowable resistance to ground is 5.0 ohms. By the time of NEC, this value drops to 2.0 ohms for small substations, 1.0 ohms for large power plants, and 0.5 ohms for large power plants.
How to check ground continuity?
To test for ground continuity at an outlet, set the tester to AC voltage, insert the red test lead into a hot outlet, and the black test lead into a ground outlet. If you get at least 120V, there is continuity.
Do neutral and ground have continuity?
If the outlet is connected to wires in the wall, there will be continuity between the neutral and ground outlets. The same is not expected if the socket is idle.
