Electrical ground is a familiar term around the house, but not everybody understands what a grounded circuit really is. Likewise, you may not know what happens when a system ground is not properly grounded to the earth, or a neutral (or return) is not properly “bonded” to the ground.
Start by considering the point of origin at the transfer station. There, a high voltage is generated or received from a power plant to be routed through the utility company’s electrical grid. This power will then be regulated and distributed to homes, sometimes after traveling thousands of miles or kilometers in electrical energy cables.
To maintain steady levels and minimize voltage fluctuations along the grid, the earth is used as the common point of reference, where the voltage is actually 0 volts or minimal.
Without the ground reference point, any electrical equipment could suffer irregular and damaging voltages. Not having a ground would also hamper the operation of protective devices such as circuit breakers which couldn’t trip from unwanted fault currents for lack of a path back to the source, which is necessary for overcurrent protection (tripping).
In simpler terms, a short-circuited appliance that is not properly grounded will be “electrified,” with the possibility of shocking or electrocuting anyone that touches it, since this someone would be providing the appliance with a return path with lesser resistance.
If the appliance or device is properly grounded, that fault current flows through and right back to the ground bus, where there’s practically 0 Ohms resistance (a lesser resistance return path).
1. How to Check a Faulty Circuit
Getting an electric shock while using a tool or an appliance is a sure indicator that you have defective wiring somewhere between that particular outlet and the electric panel.
Although this is not necessarily always caused by a faulty ground, there's a good possibility it is. This problem can stem from a broken or loose wire, a bad connection in a push-in connection, a stripped terminal screw, or even at the bus bar inside the panel.
Another possibility for appliances or equipment powered through an extension cord is the common occurrence of the ground prong (GND) at the male end being removed, severed, internally damaged (which you can check by measuring the continuity between the prong end and the half-round socket at the female end of the cord).
Finally, one must never overlook the possibility that the ground connection could be faulty inside the appliance itself.
With that out of the way, the faster and easier way to determine if the outlet is properly grounded is by plugging an outlet tester into it or using a multimeter.
Outlet or receptacle testers sell for under $20, and although they won't detect every possible problem that can occur within a circuit, they can often help you determine quite accurately where electrical problems lie between.
You can trace the issue to a point between the electric panel and the last outlet at the end of the wired circuit by following the next few steps.
Step 1.1 - While the outlet is live, insert the receptacle tester in the outlet and look at which of the three lamps are lit. If both yellow lamps are glowing, the circuit is working properly.
1) If only one of the two yellow lamps is lit, that indicates a bad connection to the ground or to the neutral line. The tester should tell you which.
2) If one of the two lamps is dim, it shows a poor ground or poor neutral connection, indicating that even though there is some semblance of connection, something is not right.
If you don’t have an outlet tester, you can use a multimeter set on AC Volts for 110-120 volts and take measurements between the smaller slot (Hot) of the receptacle and the half-round socket (GND). If you get 0 volts or minimal reading, the problem might still be that the ground is open at another location closer to the electric panel.
If you get a normal reading, proceed to Step 1.4.
Step 1.2 - If you get a faulty condition in the previous step, you now have to find out at which location the problem arises along the circuit. You can follow the circuit’s path by first shutting off the circuit breaker dedicated to that specific circuit.
Step 1.3 - Using the outlet tester or a simple lamp, check all the outlets without any power going back to the panel and mark everyone with a piece of tape. With that done you can now switch the circuit breaker back on.
Step 1.4 - Going back to the outlet farthest from the electric panel, repeat Step 1.1 for every outlet as you move towards the panel until you get a good reading on your tester. This could bring you, however, to the outlet closest to the electric panel in which case you’ll need to proceed to Step 3.
From the last outlet that gave a good voltage reading, you can now go back to the previous outlet downline that was just tested and start your quest to find the source of the problem.
Safety Concern: It’s important to realize here that if your testing confirms that a particular outlet is wired and operates properly, it only applies to the section of the circuit from the electric panel up to that particular outlet, and that any non-compliant outlet with a faulty GROUND past that point leaves that section of the circuit at risk.
As shown in Fig. 2, outlet #2 and every other outlet downline on the circuit do not have ground protection creating a serious electrocution hazard.
Fig. 3 shows the broken ground wire before reaching outlet #1, leaving every outlet on that circuit unsafe to use.
Step 1.5 - With the power still on, remove the cover plate from the defective outlet mentioned last in Step 1.4.
Step 1.6 - A multimeter is required for the following steps. With the multimeter set on AC Volts, hold one probe firmly inside the smaller slot (Hot) of the outlet while using the other probe...
1st - on the metal of the electrical box
2nd - inside the half-moon ground socket of the receptacle.
0 volts or minimal reading at both locations indicates faulty ground.
0 volts or minimal on the ground socket and 110-120 volts on the box indicate a non-grounded outlet with the box grounded (probably just a broken or loose Ground wire at the outlet’s terminal).
110-120 volts on the ground socket and 0 volts or minimal on the electrical box indicate a non-grounded electrical box with the outlet otherwise properly hooked up.
For any of the 3 previous results, proceed directly to Step 1.7.
A voltage reading between 110-120 volts across the Hot and the ground socket and also across the Hot and the metal box is a good indication of a sound ground connection from the outlet and electrical box to the electric panel.
It doesn’t prove however that the outlet itself is not improperly wired as in Figure 4 for example, with a Bootleg ground (see section 4).
Note: The circuit could also be wired with Armored Electrical Cable such as AC, MC, Greenfield, and BX, where the metal jacket is used to provide a positive ground connection between all electrical boxes within the circuit.
Step 1.7 - Before going further the circuit breaker must be switched Off at the panel and the outlets checked for 0 volts or minimal reading.
Step 1.8 - Remove the receptacle from its box and pull it out for a visual check of the wiring. Check that the ground wire is intact and properly secured to the outlet’s GND terminal screw.
Also, make sure that it is somehow physically connected to the electrical box. Make sure that there is no bootleg connection to the neutral line (common in older constructions—see description below).
Step 1.9 - If everything checks out OK, you’ll have to assume that the ground connection is broken between the electric box for this outlet and the previous outlet towards the electric panel.
Open and pull out the receptacle and repeat Step 1.8. If the ground wire running between the two outlets is well connected at both ends, the problem could be from wiring that got damaged or severed during some construction work at the house.
2. How to Check for Bad Wiring
Step 2.1 - With the power still switched Off, short out the ground terminal with the neutral terminal at one of the outlets with a wire across their terminals.
Step 2.2 - Return to the outlet at the other end of this cable and disconnect both the ground and the neutral wires from everything else.
Step 2.3 - Adjust your multimeter for resistance or continuity and check the reading which should be 0 Ohms. This step verifies both the ground and the neutral wires.
The cable will need replacement if you get an infinity reading or any amount of resistance between the 2 wires. Such a situation could bring on serious alterations and require the services of a professional.
3. Problem Over the Whole Circuit
If the problem is over the whole circuit as determined in Step 1.4, the electric panel may need to be opened to check the connections inside the panel.
Step 3.1 - Get a good battery-powered trouble light and set it up to light up the panel.
Step 3.2 - Turn Off the main breaker for the electric panel.
Step 3.3 - Remove the front cover to the electric panel.
Step 3.4 - Follow the wire connected to the dedicated circuit’s breaker back to where the cable comes into the panel. You may need long-nose pliers to move wires around.
Step 3.5 - With the right cable identified, pick up the bare ground wire with the long-nose pliers and follow the wire right down to the ground bus bar making sure that it is not broken and is firmly secured to the bus.
Step 3.6 - If you didn't find anything wrong inside the panel, before re-closing it you could do the same continuity test as in Steps 2.1 to 2.3 by removing the ground wire from the bus and the neutral wire from the neutral bus and holding them together with a wire clamp.
Step 3.7 - You can then move to the next electrical box on that circuit and remove the ground wire and the neutral wire of the same cable from their respective terminals and measure for continuity across them.
There could be a problem along that section of cable but if not, you can re-attach the four-wire ends to their proper terminals and re-install the panel cover.
4. The Bootleg Ground and Its Origin
Prior to the 1960s, wiring, circuits, electrical tools, and appliances were not grounded, making it dangerous should a device experience a short circuit causing its entire body (chassis) to become electrified and possibly hurt or electrocute anyone touching it.
The addition of a ground wire in the 1960s became a necessity to provide the easiest possible path for fault current to return to a Ground bus properly connected to earth ground, at the same time tripping a protective safety device or breaker in this case.
So in the 1960s, the 2-prong outlets were replaced by a 3-prong receptacle, the third slot being semi-circular in shape to accommodate the ground prong.
As people upgraded their older homes to 3-prong outlets so they wouldn’t need to use adapters, many contractors and DIYers failed to install the proper ground wire from the electrical panel.
Instead, they cheat or “bootleg” the outlets by connecting the ground terminal of the 3-prong outlets to the Neutral terminal (Fig. 4), thus fooling the outlet tester with the proper voltage readings between the Hot and the Neutral, but also between the Hot and the GND, even though there's no connection to the ground bus.
This problem can also be compounded with a Hot and GND Reverse as shown in Fig. 5 where the subsequent outlet passed the Bootleg and gets furthermore miswired, with the Hot and Neutral interchanged at the outlet and the GND terminal connected to the Hotwire creating a “Reverse Bootleg.”
This last problem however can easily be detected with the tester resulting with Hot & GND Reverse.