Symptom
A tingle or shock from an enclosure, a conduit, or a motor frame; a fault that trips nothing and instead energizes metal; a surge protector that has not protected anything; GFCI trips with no apparent cause; voltage measured between an enclosure and nearby grounded metal; noise on instruments served from the panel.
The short answer
Missing Equipment Ground
The equipment grounding conductor bonds every enclosure and metal part that could become energized back to the source, so that a fault to the enclosure produces enough current to open the breaker at once and the enclosure never stays energized. When it is missing, broken, or interrupted by a painted surface, a nonmetallic section, or a bad conduit joint, a fault leaves the enclosure at line voltage until someone touches it, surge protective devices have no path to divert into, and instrument references float. Test with the power on for voltage between the enclosure and a known ground, then with the power off and locked out for low resistance between the enclosure and the source ground. A ground rod at the equipment is not an equipment ground; its resistance is far too high to trip a breaker.
Key points
- The equipment ground is the fault current path. Without it a fault energizes metal instead of tripping a breaker.
- A ground rod at the equipment is not an equipment ground; at 25 ohms it passes 5 amps at 120 V and trips nothing.
- Test energized for voltage enclosure to ground, then de-energized for resistance enclosure to source ground, below one ohm.
- The usual breaks: paint under a lug, a nonmetallic conduit section without a conductor, a corroded conduit joint, a bonding jumper never installed.
- Surge protection and instrument grounding both depend on the same conductor; fixing it fixes three problems.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Conduit used as the grounding conductor with a broken path | A section of nonmetallic conduit, a loose or corroded coupling, a set screw fitting that never bit. Measure resistance across each joint or from the enclosure to the source ground. |
| Ground conductor terminated on paint | Look under the lug. The enclosure ground stud or bus must contact bare metal, with paint removed and a star washer or a listed stud. |
| Bonding jumper not installed at a nonmetallic enclosure, hub, or concentric knockout | Inspect the entries. Concentric and eccentric knockouts and nonmetallic boxes need a bonding jumper or bushing to carry the ground through. |
| Feeder to a remote panel run without a ground conductor | Count the conductors in the feeder. Two hots or a hot and neutral with a rod at the panel is a missing ground. |
| Ground rod relied on as the only ground | Measure the rod resistance or the fault loop impedance. Anything above a fraction of an ohm cannot clear a fault. |
| Ground conductor cut or disconnected during work | Continuity from the enclosure to the source ground; a conductor removed to stop a noise problem is a frequent finding. |
| Motor frame not bonded, or bonded through a flexible conduit | Flexible metal conduit and liquidtight are not a reliable ground; a separate ground conductor to the motor terminal box is needed. |
| Neutral used as the ground at a receptacle or panel | A neutral-to-ground jumper downstream of the service masquerades as a ground and puts neutral current on the enclosure. |
What the equipment ground does
The equipment grounding conductor is the deliberate, low-impedance path from every enclosure, raceway, and motor frame back to the source, where it is bonded to the neutral at the main bonding jumper. When an energized conductor touches the enclosure, current flows through that path back to the source, and it is large, hundreds or thousands of amps, because the path is a few tenths of an ohm. The breaker opens in a cycle or two and the enclosure is never at a dangerous voltage for longer than that. That is its job. The earth itself is not part of it: the ground rod at the service connects the system to earth for lightning and utility reasons, and its resistance is far too high to clear a fault.
I_fault = V / Z_loop
- I_fault = the current that flows during a fault to the enclosure
- V = the line-to-ground voltage, 120 V or 277 V for most control panels
- Z_loop = the impedance of the path from the source through the fault and back along the equipment ground; a few tenths of an ohm with a proper ground, 25 ohms or more through a ground rod
At 120 V through a 0.2 ohm path the fault current is 600 A and a 20 A breaker trips instantly. Through a 25 ohm ground rod it is under 5 A, the breaker never trips, and the enclosure sits at nearly 120 V indefinitely.
How it shows up
| Observation | What it means |
|---|---|
| Tingle or shock from metal | Metal is energized, by a fault or by leakage, and has no path to clear it |
| Fault inside the panel did not trip the breaker; enclosure was hot | No effective fault current path |
| Surge protector shows failed, or storms keep killing equipment | The surge protector had no ground to divert into, or a long, high-impedance one |
| GFCI trips on a circuit feeding the panel | Leakage current is finding a path that is not the ground conductor |
| Instrument noise, offsets, and ground loops that are hard to explain | The panel reference is floating or is reached through a long impedance |
| Voltage between the enclosure and a grounded conduit or water pipe | The enclosure is not at ground potential |
Testing
- 1
Energized voltage check
With the meter on AC volts, measure from the enclosure to a reference that is known to be grounded: a metallic water pipe, a building steel column, the ground bus in the source panel via a long lead. A grounded enclosure reads near zero. Volts means it is not bonded, and there is leakage or a fault.
- 2
De-energize and lock out
Everything that feeds the enclosure. Verify absence of voltage.
- 3
Resistance to the source ground
Measure resistance from the enclosure to the ground bus at the panel that feeds it, with a long test lead or a low-resistance ohmmeter. Under one ohm is expected, usually well under. Higher means a poor path; open means none.
- 4
Walk the path
From the enclosure back to the source: the ground stud and whether it is on bare metal, the ground conductor and where it lands, each conduit joint if conduit is the path, each junction box and the bonding at its knockouts, any nonmetallic section and the conductor that should cross it.
- 5
Check the source
At the panel feeding the equipment, the ground bus must be bonded to the enclosure and carried back to the service, where the main bonding jumper ties it to the neutral. A subpanel with no ground bus, or with the grounds on the neutral bar, is a fault upstream of the one you were chasing.
- 6
Fix and retest
Install or repair the conductor, remove paint under lugs, add bonding jumpers, replace the conduit joint. Then repeat the resistance test and the energized voltage check.
The ground rod is not the answer
A ground rod at a remote panel is often installed with good intentions and no ground conductor in the feeder. The rod connects the panel to earth; it does not connect it to the source. Earth resistance from the rod is typically 25 ohms or more, and the fault current through it is a few amps, well below any breaker trip point. The panel becomes a piece of energized metal with a wire to the dirt. The feeder must carry a grounding conductor sized for the breaker that protects it, and the rod is a supplement for lightning at most.
Motors and drives
A motor frame is bonded by a ground conductor in the motor circuit, landed in the terminal box, not by the flexible conduit that carries the leads. On drive-fed motors, the ground conductor also carries the high-frequency leakage current from the motor cable capacitance, and a missing or long one lets that current find its way through bearings, structures, and instrument grounds instead. Drives also need the ground for their own protective functions; a drive with a missing ground may report ground faults that are not there or miss ones that are.
Frequently asked questions
- The panel is fed through metal conduit. Is that not the ground?
- Metallic conduit with proper fittings is a permitted equipment grounding conductor, and it works until a joint corrodes, a coupling loosens, or someone replaces a section with nonmetallic. Many designers run a separate ground conductor inside the conduit anyway for that reason. If conduit is the ground, every joint is part of the fault path and has to be tested as such.
- How low should the resistance to the source ground be?
- Well under one ohm for a panel fed within a building, often under a tenth. What matters is that the fault current through it is enough to trip the breaker quickly, and that requires the impedance of the whole loop to be low. A value over an ohm means a bad connection somewhere in the path.
- Can I use a plug-in outlet tester?
- It will show an open ground at a receptacle, which is useful. It cannot tell a good ground from a poor one, and it cannot see a neutral-to-ground jumper masquerading as a ground. Follow up any finding with the resistance test.
- Why does the surge protector care?
- A surge protective device works by diverting surge current to ground. With no ground, or a long high-impedance one, the surge has nowhere to go and the protected equipment takes it. The ground conductor to the surge protector should be short and direct; a missing one means the device is decoration.
Related topics
- High Neutral-to-Ground VoltageWhat a neutral-to-ground voltage reading means, the values that are normal, and the causes of a high one: a long or heavily loaded neutral, harmonic current, a loose or open neutral, a missing service bond, or an extra bond downstream, and telling them apart.
- Ground Loop SymptomsHow a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
- Circuit Breakers in Control PanelsMiniature, molded-case, and supplementary protectors: what each is listed to do, how a trip curve is read, where the NEC and UL 508A set the rules, and how breaker choice drives the short-circuit current rating of the whole panel.
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
- Drive Trips on Ground FaultA ground fault trip on a variable frequency drive: how the drive detects it, the motor and cable insulation faults behind most trips, the submersible cable problem, the capacitive leakage on long cables and filters that produces nuisance trips, wiring errors, and the insulation test that decides it.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.